K-edge X-ray Absorption Spectroscopy Spectra Research Articles

Operando Soft X-ray Absorption of LaMn1-x Co x O3 Perovskites for CO Oxidation.

We employed operando soft X-ray absorption spectroscopy (XAS) to monitor the changes in the valence states and spin properties of LaMn1-x Co x O3 catalysts subjected to a mixture of CO and O2 at ambient pressure. Guided by simulations based on charge transfer multiplet theory, we quantitatively analyze the Mn and Co 2p XAS as well as the oxygen K-edge XAS spectra during the reaction process. The Mn sites are particularly sensitive to the catalytic reaction, displaying dynamics in their oxidation state. When Co doping is introduced (x ≤ 0.5), Mn oxidizes from Mn2+ to Mn3+ and Mn4+, while Co largely maintains a valence state of Co2+. In the case of LaCoO3, we identify high-spin and low-spin Co3+ species combined with Co2+. Our investigation underscores the importance to consider the spin and valence states of catalyst materials under operando conditions.

Probing Isolated Water Molecules in Aqueous Acetonitrile Solutions Using Oxygen K-Edge X-ray Absorption Spectroscopy.

Oxygen K-edge X-ray absorption spectroscopy (XAS) of an aqueous acetonitrile solution exhibited a sharp peak at approximately 537 eV, which was similar to that of water vapor and was not observed in liquid water. The inner-shell spectra of isolated water molecules and water clusters of different sizes surrounded by acetonitrile molecules were obtained by extracting these water structures from the liquid structures of aqueous acetonitrile solutions, as calculated using molecular dynamics simulations. The sharp peak profiles of the O K-edge XAS spectra were derived not from water clusters but from isolated water molecules surrounded by acetonitrile molecules. The present study proposes that isolated water molecules are easily formed in aqueous acetonitrile solutions and that the electronic structures of the isolated water molecules can be analyzed using O K-edge XAS spectra, which separates the contributions of small water clusters.

Coupled Experimental-Theoretical Characterization of a Carbon Electrode in Vanadium Redox Flow Batteries using X-ray Absorption Spectroscopy.

Vanadium redox flow batteries (VRFBs) have emerged as promising solutions for stationary grid energy storage due to their high efficiency, scalability, safety, near room-temperature operation conditions, and the ability to independently size power and energy capacities. The performance of VRFBs heavily relies on the redox couple reactions of V2+/V3+ and VO2+/VO2+ on carbon electrodes. Therefore, a thorough understanding of the surface functionality of carbon electrodes and their propensity for degradation during electrochemical cycles is crucial for designing VRFBs with extended lifespans. In this study, we present a coupled experimental-theoretical approach based on carbon K edge X-ray absorption spectroscopy (XAS) to characterize carbon electrodes prepared under different conditions and identify relevant functional groups that contribute to unique spectroscopic features. Atomic models were created to represent functional groups, such as hydroxyl, carboxyl, methyl, and aldehyde, bonded to carbon atoms in either sp2 or sp3 environments. The interactions between functionalized carbon and various solvated vanadium complexes were modeled using density functional theory. A library of carbon K-edge XAS spectra was generated for distinct carbon atoms in different functional groups, both before and after interacting with solvated vanadium complexes. We demonstrate how these simulated spectra can be used to deconvolve ex situ experimental spectra measured from carbon electrodes and to track changes in the electrode composition following immersion in different electrolytes or extended cycling within a functional VRFB. By doing so, we identify the active species present on the carbon electrodes, which play a crucial role in determining their electrochemical performance.

Quantitative in situ synchrotron X-ray analysis of the ALD/MLD growth of transition metal dichalcogenide TiS2 ultrathin films.

We present the results of a full quantitative analysis of X-ray absorption spectroscopy (XAS) performed in situ during the growth of ultrathin titanium disulfide (TiS2) films via an innovative two-step process, i.e. atomic layer deposition/molecular layer deposition (ALD/MLD) followed by annealing. This growth strategy aims at separating the growth process from the crystallization process by first creating an amorphous Ti-thiolate that is converted later to crystalline TiS2via thermal annealing. The simultaneous analysis of Ti and S K-edge XAS spectra, exploiting the insights from density functional theory calculations, allows us to shed light on the chemical and structural mechanisms underlying the main steps of growth. The nature of the bonding at the base of the interface creation with the SiO2 substrate is disclosed in this study. Evidence of a progressive incorporation of S in the amorphous Ti-thiolate is given. Finally, it is shown that the annealing step plays a critical role since the transformation of the Ti-thiolate into nanocrystalline TiS2 and the loss of S are simultaneously induced, validating the two-step synthesis approach, which entails distinct growth and crystallization steps. These observations contribute to a deeper understanding of the bonding mechanism at the interface and provide insights for future research in this field and the generation of ultra-thin layered materials.

Operando High-Temperature X-Ray Absorption Spectroscopy of LiNiO2 Cathode Material during Synthesis

The increasing demand for higher energy density in batteries for automotive applications has led to a trend toward increasing the Ni content in layered oxide cathode materials to increase their reversible capacity. However, LiNiO2, which has the highest Ni content, shows a faster capacity drop during charge-discharge cycling, which is attributed to its inherent structural instability in both the pristine and delithiated states. Therefore, to address the former issue and to better understand the appropriate synthesis conditions for LiNiO2, we have studied the changes in the local structure of LiNiO2 during the synthesis process using operando analysis techniques. Recently, high temperature in situ X-ray diffraction during LiNiO2 synthesis was reported by Bianchini [1] and Weber [2], which confirmed the loss of lithium and oxygen and the migration of Ni to Li layers above 700 °C. To further understand the mechanism of Li/Ni mixing and oxygen loss, operando high-temperature X-ray absorption spectroscopy (XAS) was performed during the synthesis of LiNiO2 to clarify the electrical structure of Ni at high temperatures. Pre-heated Li x NiO2 (x = 0.76, 1.01, 1.26, and 1.51) precursors were prepared by heating LiOH·H2O and Ni(OH)2 at 480 °C for 24 h in air. The precursor was pelletized and set in a quartz sample holder. Operando high-temperature Ni K-edge XAS spectra of the precursor were obtained at BL9A and BL12C in Photon Factory (PF), High Energy Accelerator Research Organization (KEK) by a transmission mode. The operando spectra of all samples except Li/Ni = 0.76 revealed a clear edge shift to the lower energy side corresponding to nickel reduction above 700 °C. The Li/Ni = 0.76 sample has a lithium-deficient composition and Ni is not oxidized to Ni3+ below 700 °C, so the Li/Ni = 0.76 sample has almost no Ni K-edge energy shift above 700 °C. These results suggest that Ni in LiNiO2 is reduced above 700 °C even in an oxygen atmosphere, regardless of whether the Li/Ni ratio is greater than 1. The Ni reduction above 700 °C is probably the cause of the Li/Ni mixture.Reference[1] M. Bianchini, J. Janek, et al., Angew. Chem., Int. Ed., 58, 10434 (2019).[2] R. Weber, J. R. Dahn, et al., J. Electrochem. Soc., 167, 100501 (2020).

Exploring the impact of ions on oxygen K-edge X-ray absorption spectroscopy in NaCl solution using the GW-Bethe-Salpeter-equation approach.

X-ray absorption spectroscopy (XAS) is a powerful experimental tool to probe the local structure in materials with the core hole excitations. Here, the oxygen K-edge XAS spectra of the NaCl solution and pure water are computed by using a recently developed GW-Bethe-Salpeter equation approach, based on configurations modeled by path-integral molecular dynamics with the deep-learning technique. The neural network is trained on abinitio data obtained with strongly constrained and appropriately normed density functional theory. The observed changes in the XAS features of the NaCl solution, compared to those of pure water, are in good agreement between experimental and theoretical results. We provided detailed explanations for these spectral changes that occur when NaCl is solvated in pure water. Specifically, the presence of solvating ion pairs leads to localization of electron-hole excitons. Our theoretical XAS results support the theory that the effects of the solvating ions on the H-bond network are mainly confined within the first hydration shell of ions, however beyond the shell the arrangement of water molecules remains to be comparable to that observed in pure water.

Probing Substituent Effect on Nickel-Sulfur Bond Covalency in Ni(II)-Dithiophosphinate Complexes by Sulfur K-Edge XAS and DFT Calculations.

Complexes of Ni(II) with a series of aryl or alkyl substituent dithiophosphinic acids were characterized by crystallographic structure, sulfur K-edge X-ray absorption spectroscopy (XAS), and density functional theory (DFT). In these complexes, Ni(II) coordinates with four sulfur atoms from two dithiophosphinate anions form a well-defined square-planar structure. Despite the minor differences in the geometry parameters among the complexes, the electronic structure is affected significantly by the substituent group attached to dithiophosphinic acid. In particular, the addition of ortho-CF3 group to the aryl ring constrains the orientation of the aryl ring and enhances the conjugation between the aryl ring and the coordinating core. Sulfur K-edge XAS spectra help further reveal the electronic structure of the complexes. Both the pre-edge feature and rising-edge feature provide abundant information on the molecular orbitals and show a distinctive effect of the substituent groups on the electronic structure of the complexes, which is supposedly relevant to the ligand's performance in Ln(III)/An(III) separation.

Visualization of air-induced oxidation in single crystalline LiFe0.6Mn0.4PO4 nanowires with carbon sheath using soft X-ray spectromicroscopy

Single crystalline LiFe0.6Mn0.4PO4 (LFMP) nanowires with carbon sheath were analyzed by scanning transmission X-ray microscopy (STXM) with a spatial resolution of around 130 nm. The pinpoint Fe L-edge X-ray absorption spectroscopy (XAS) spectra in the STXM images revealed the natural oxidation of Fe2+ to Fe3+ near the tip of the nanowire by air exposure. The pinpoint O K-edge XAS spectra simultaneously changed with the Fe L-edge XAS in the STXM mapping because of the hybridization between the O 2p and Fe 3d orbitals. On the other hand, the Mn L-edge XAS spectra showed an almost uniform Mn2+ state all over the nanowires, suggesting that the Mn2+ state is stable against the natural oxidation in contrast to the Fe2+ state. We have thus demonstrated that STXM is a useful technique to visualize oxidized areas of materials and to reveal detailed information about their electronic structure.

Simulated sulfur K-edge X-ray absorption spectroscopy database of lithium thiophosphate solid electrolytes

X-ray absorption spectroscopy (XAS) is a premier technique for materials characterization, providing key information about the local chemical environment of the absorber atom. In this work, we develop a database of sulfur K-edge XAS spectra of crystalline and amorphous lithium thiophosphate materials based on the atomic structures reported in Chem. Mater., 34, 6702 (2022). The XAS database is based on simulations using the excited electron and core-hole pseudopotential approach implemented in the Vienna Ab initio Simulation Package. Our database contains 2681 S K-edge XAS spectra for 66 crystalline and glassy structure models, making it the largest collection of first-principles computational XAS spectra for glass/ceramic lithium thiophosphates to date. This database can be used to correlate S spectral features with distinct S species based on their local coordination and short-range ordering in sulfide-based solid electrolytes. The data is openly distributed via the Materials Cloud, allowing researchers to access it for free and use it for further analysis, such as spectral fingerprinting, matching with experiments, and developing machine learning models.

Detailed Fe 3d electronic structures of Prussian blue analogs studied by X-ray absorption spectroscopy

Herein, we studied the detailed electronic structures of Prussian blue analogs with hexacyanoferrate around Fe L- and K absorption edges using room temperature X-ray absorption spectroscopy (XAS). Fe is trivalent for all the synthesized samples. By L3-edge XAS measurements, the excitation from Fe 2p3/2 core level to the 3d (t2g↓), 3d (eg↓), 3d (eg↑) states, and the π*, and σ* states of the Fe–CN molecular orbitals are identified with 3d (t2g↑)3 3d (t2g↓)2 configuration. The same excitation from the 1 s core level is identified using Fe K-edge XAS spectra. The relationship between lattice constants and the energy difference between 3d (t2g↓) to π* and σ* transitions from the Fe 2p3/2 core level suggests that the change in the lattice constant influences the electronic structure near the Fe atom.

Chemical-state distributions in charged LiCoO2 cathode particles visualized by soft X-ray spectromicroscopy

Lithium-ion deintercalation/intercalation during charge/discharge processes is one of the essential reactions that occur in the layered cathodes of lithium-ion batteries, and the performance of the cathode can be expressed as the sum of the reactions that occur in the local area of the individual cathode particles. In this study, the spatial distributions of the chemical states present in prototypical layered LiCoO2 cathode particles were determined at different charging conditions using scanning transmission X-ray microscopy (STXM) with a spatial resolution of approximately 100 nm. The Co L3- and O K-edge X-ray absorption spectroscopy (XAS) spectra, extracted from the same area of the corresponding STXM images, at the initial state as well as after charging to 4.5 V demonstrate the spatial distribution of the chemical state changes depending on individual particles. In addition to the Co L3-edge XAS spectra, the O K-edge XAS spectra of the initial and charged LiCoO2 particles are different, indicating that both the Co and O sites participate in charge compensation during the charging process possibly through the hybridization between the Co 3d and O 2p orbitals. Furthermore, the element maps of both the Co and O sites, derived from the STXM stack images, reveal the spatial distribution of the chemical states inside individual particles after charging to 4.5 V. The element mapping analysis suggests that inhomogeneous reactions occur on the active particles and confirm the existence of non-active particles. The results of this study demonstrate that an STXM-based spatially resolved electronic structural analysis method is useful for understanding the charging and discharging of battery materials.

Different polymorphs of Y doped HfO2 epitaxial thin films: Insights into structural, electronic and optical properties

We report the investigation of local structure, electronic and optical properties of different polymorphs of (YO1.5)x(HfO2)1-x (x = 0, 0.05, 0.10, 0.15, and 0.20) epitaxial thin films on YSZ(100) substrate prepared by pulsed laser deposition technique. The X-ray diffraction results confirm that the deposited films are epitaxial and crystal structure changes from monoclinic to orthorhombic to cubic/tetragonal as Y concentration increases in HfO2 thin films. Spectroscopic identification of phase transformation in these thin films was investigated using Fourier transform infrared and X-ray absorption spectroscopy (XAS). Effect of crystal structure on Hf-O & Hf-Hf bond distances and disorder were investigated by extended X-ray absorption fine structure. O K-edge XAS spectra show profound changes in eg and t2g peaks for different HfO2 polymorphs. X-ray photoelectron spectroscopic studies confirm that different chemical environments are present in different polymorphs of HfO2 thin film. The photoluminescence measurements validate the wide band spectral emission in wavelength region of 400–550 nm for all polymorphs and photoluminescence areal intensity shows strong dependence on phase and local structure disorder. This work will be helpful in understanding the basic material properties and spectroscopic identification of different HfO2 phases of high application relevance in semiconductor industry.

Uncovering temperature-tempted coordination of inclusions within ultra-high-strength-steel via in-situ spectro-microscopy

Despite the common challenge of investigating non-metallic inclusions within ultra-high-strength-steel (UHSS) at sub-micrometer scale via conventional methods, probing nitride inclusions at elevated temperatures is vital for guiding steel’ performance. Herein, an in-situ spectro-microscopic determination using advanced Synchrotron X-ray absorption spectroscopy (XAS) coupled with photoelectron emission microscopy (PEEM) is employed to explore the local structure and electronic properties of selective h-boron nitride (h-BN) containing inclusions (A1 and A2) embedded within steel matrix. While the variation in the relative intensity of π∗/σ∗ excitonic peaks at spatially different locations refers to the polarization and or thickness effects. Several minute features observed in the 192–195 eV energy range show oxygen (O) substituted nitrogen (N) defects (ON,2N,3N), which are more dominant in A2 inclusion. The observed dominance further explains the relatively high intense π∗ peak in A2 due to increased localization. Weak shoulder on the left side of π∗ peak in both room and high-temperature XAS spectra is ascribed to the interaction between h-BN and the local environment, such as Ca-based inclusion or steel matrix. Defects are commonly found in h-BN, and precise identification of the same is vital as they affect the overall physical, chemical, and mechanical properties. Moreover, significant changes in high-temperature B K-edge XAS spectra, such as relative intensity of π∗/σ∗ excitonic peaks at the same location and reduced intensity of defects, suggest the adjusting nature of BN inclusion, complicating their precise prediction and control towards clean steel production.

X-ray absorption spectra of aqueous cellobiose: Experiment and theory

The hydration structure of cellulose is very important for understanding the hydrolysis of cellulose at the molecular level. In this paper, we report a joint experimental and theoretical study on x-ray absorption spectroscopy (XAS) of aqueous cellobiose, a disaccharide unit of cellulose. In the experimental part, high resolution measurements of the carbon K-edge XAS spectra were taken. In the theoretical part, ab initio molecular dynamics simulations and ensemble calculations of electronic excited states were performed to obtain the continuous XAS spectra. The XAS spectra were found to have three characteristic peaks at 289.3, 290.7, and 293.6eV, each representing the absorption by carbon atoms of the alcohol group, the hemiacetal group, and both of these functional groups. It was found that the peak heights in the spectrum change considerably over the temperature range of 25-60 °C, which is a reflection of the number of hydrogen bonds between cellobiose and water. We suggest that this spectral change could be useful information for identifying the hydration of cellulose in various environments.

Geometry of Pentaphenylantimony in Solution: Support for a Trigonal Bipyramidal Assignment from X-ray Absorption Spectroscopy and Vibrational Spectroscopic Data.

Pentaphenylantimony (SbPh5) has been previously crystallized in either a square pyramidal or trigonal bipyramidal geometry. Investigation of the solution-state structure of SbPh5 has been hampered by the extreme fluxionality of this compound, but previous vibrational spectroscopic studies concluded that it maintains a square pyramidal geometry in solution. This non-VSEPR-compliant geometry, which is also assumed by BiPh5 in the solid state, stands in contrast to the trigonal bipyramidal geometries of PPh5 and AsPh5. A range of phenomena have been invoked to explain this discrepancy, most notably, the increased importance of relativistic effects as group 15 is descended. We present crystallographic, spectroscopic, and computational data revealing that SbPh5 in fact assumes the VSEPR-compliant trigonal bipyramidal geometry in solution. In particular, Sb X-ray absorption spectroscopy (XAS) was used to obtain geometry-sensitive spectra that do not suffer from the slow spectroscopic time scale that has prevented NMR studies from elucidating the structure of this fluxional molecule. Sb K-edge and LIII-edge XAS spectra of crystalline solids featuring SbPh5 in either a square pyramidal (nonsolvate) or trigonal bipyramidal (cyclohexane hemisolvate or THF hemisolvate) form were compared to spectra of SbPh5 in solution. The solution-state spectra agree with those from solids containing trigonal bipyramidal SbPh5. The most diagnostic spectroscopic feature was the distribution of intensity in the Sb LIII pre-edge features. These distributions were rationalized using time-dependent density functional theory calculations that take into account spin-orbit coupling. Our use of Sb XAS not only resolves a long-standing physical inorganic question but also demonstrates more widely the utility of XAS in establishing the structures of fluxional main-group compounds. This conclusion was further supported by solid- and solution-state Raman data. Finally, we note that the present high-resolution diffraction data allow τ for nonsolvated SbPh5 to be revised to 0.216.

X-Ray Absorption Spectroscopy Study to Elucidate the Redox Reaction Mechanism of Li4Mn2O5 upon Delithiation

Li-ion batteries are one of the most promising energy storage sources because of excellent electrochemical performance and has dominated the field of portable devices for the past 20 years. It has also been widely used in vehicle electrification and grid storage. Thus, the demand of Li-ion batteries with high energy density increased dramatically. The electrochemical behavior of current Li-ion batteries mainly relies on the transition metal oxide cathodes1,2,3. Mn-based cathode materials have become highly desirable owing to the low cost and high earth abundance. Recently, a nanostructured disordered rocksalt-type Li4Mn2O5 was synthesized, which displays the highest reported capacity till date up to 355 mAh/g with relative reversibility as compared to other contemporary Li-Mn-O electrodes4,5. In spite of such intriguing electrochemical performance, the detailed charge compensation mechanism associated with (de)lithiation is still unclear. Previous study predicted that the electrochemistry of Li4Mn2O5 was driven by Oxygen/Mn redox. Further study in Mn via X-ray absorption spectroscopy (XAS) revealed that Mn4+/Mn5+ redox couple contributes a small partial capacity in the redox mechanism, 4,5 indicating the significant involvement of oxygen during cycling. However, the mechanism of oxygen participation in the electrochemistry was not studied. In pursuit of understanding the detailed electrochemical mechanism, in this study, X-ray absorption spectroscopic study was performed in both Mn and O K-edge at various states of charge-discharge cycle. XAS measurements were conducted at different charged and discharged states of Li4Mn2O5 cycled between 4.8V and 1.2V voltage window. The obtained Mn K-edge XAS spectra clearly unravels the change of oxidation state of Mn upon charging and discharging, with a quantitative analysis linking the oxidation state and absorption energy via integral method. As a complementary probe to Mn K-edge XAS, O K-edge XAS unambiguously demonstrates the involvement of oxygen upon cycling and uncovers the dominant contribution of oxygen to the electrochemical performance at high voltage charging. This finding could shed light on the design of Oxygen-involved Mn-based cathodes with high capacity and reversibility.

Local atomic structure analysis around Mg atom doped in GaN by X-ray absorption spectroscopy and spectrum simulations.

The identification of the incorporated site of magnesium (Mg) and hydrogen (H) required for p-type formation in gallium nitride (GaN) power devices has been demonstrated by X-ray absorption spectroscopy (XAS). In this study, the fluorescence line of Mg with 3 × 1019 atoms cm-3 was successfully separated from that of Ga using a superconducting tunnel junction array detector with high sensitivity and high energy resolution, and consequently the Mg K-edge XAS spectra of such dilute samples were obtained. The site of Mg atoms incorporated into the GaN lattice was identified as the Ga substitutional site bycomparing the experimental XAS spectrum with the simulated spectra calculated by density functional theory. In addition, the presence or absence of H around Mg can be determined through distinctive characteristics expected from the spectrum simulations.

Bulk O2 formation and Mg displacement explain O-redox in Na0.67Mn0.72Mg0.28O2

Summary O-redox in compounds with Li on the transition-metal layers (TML) has recently been attributed to the formation of molecular O2 on charge, trapped in the lattice. Here, we show that a similar process occurs for P2-Na0.67[Mn0.72Mg0.28]O2, which contains Mg2+ on the TML. The molecular O2 is identified by high-resolution RIXS and quantified by magnetometry, showing that it equates to the charge passed. This O2 is trapped in voids that are formed by Mg2+ out-of-plane displacement and Mn4+ in-plane disordering and is then reduced on discharge associated with a large voltage hysteresis. In contrast to compounds containing Li+ in the TML, in which the honeycomb ordering and the high-voltage plateau are irreversibly lost after the first cycle, in P2-Na0.67[Mn0.72Mg0.28]O2, the plateau reappears partially on the second charge due to the partial reversibility of Mn in-plane and Mg out-of-plane migration and the local reformation of the honeycomb ordering.

X-ray absorption spectroscopy examination of Cr, Co, and Cu binding on fluorescent carbon dots

Fluorescent carbon dots (CDs) synthesized from tartaric acid and triethylenetetramine showed potential for the detection of Cr6+, Co2+ and Cu2+ in aqueous solution, with the most effective sensing being based on fluorescence quenching for Co2+. A good linear relationship between the concentration of Co2+ and the relative fluorescent intensity was obtained at 1.7–167.7 μM (R2 = 0.9993) with a limit of detection at 0.94 μM. X-ray absorption spectroscopy (XAS) was used to examine the absorbed transition metal ions on the CDs. Measurements were carried out at Beamline 8, Synchrotron Light Research Institute, Nakhon Ratchasima, Thailand. Cr, Co, and Cu K-edges were measured in various standard compounds containing different oxidation states. An energy shift of 12.49 eV in the absorption of the Co K-edge was observed from elemental Co0 to Co2+, 3+, while shifts of 17.96 eV and 5.45 eV were observed for elemental Cr0 to Cr6+ and elemental Cu0 to Cu2+, respectively. A small decrease in edge energy of the Cr K-edge XAS spectra compared to standard compounds with the LCF results suggested two forms of Cr6+ found in Cr sorption on carbon dots while a mix of Cu1+ and Cu2+ was found in Cu sorption on carbon dots. Interestingly, an increase in edge energy of Co K-edge was observed for Co sorption on carbon dots compared to +2 oxidation state compounds, suggesting that metal species transform to a higher positive charge during binding to CDs. Cobalt species might participate in the formation of a chemical bond with functional groups existing on the surface of CDs. Fourier transform infrared (FTIR) spectra confirmed the results from XAS.

Defect induced ferromagnetic ordering in epitaxial Zn0.95Mn0.05O films on sapphire (0 0 0 1)

We report the influence of Mn dopant on magnetic properties of Zn0.95Mn0.05O (ZMO)/Al2O3(0 0 0 1) hetero-epitaxial systems grown by using pulsed-laser deposition. The room temperature (RT) intrinsic ferromagnetic (FM) ordering verified by superconducting quantum interference device magnetometer and x-ray magnetic circular dichroism spectrum of Mn L2,3 edges is ascribed to the substitutional Mn atoms in the Zn site of ZnO. Mn in ZMO has a tetrahedral local symmetry instead of the octahedral symmetry of MnO, after verifying the absence of the Mn-related impurities or clusters in ZMO epitaxial film by Mn K-edge and Zn K-edge x-ray absorption spectroscopy spectrum, as well as the analysis of long-range structural ordering on Renninger scan of forbidden (0 0 0 5) reflection in x-ray diffraction, transmission electron microscopy and Raman spectrum. Comparison of x-ray absorption spectra of ZMO with those of ZnO epilayers at O K-, Zn K-, and L3-edges indicates that the substitution of the Zn site with Mn enhances the charge-transfer (CT) transition and the presence of Zn vacancies (VZn) also dominate the photoluminescence (PL) spectrum, implying that the formation of numerous VZn defects plays an important role in activating FM interactions. The strong CT effect and the existence of high-density VZn suggest that the intrinsic RT FM ordering of insulating ZMO is a result of the formation of the bound magnetic polarons (BMPs) that interact with each other via intermediate magnetic impurities.