The phase purity of molybdenum disulfide (MoS₂) is one of the core indicators determining its lubrication performance and chemical stability. In the quality control system for molybdenum disulfide, X-ray diffraction analysis (XRD) can directly identify phase composition from the crystal structure level, detect impurity phases, and evaluate crystallinity, making it an indispensable structural characterization tool. Unlike chemical element analysis that focuses on compositional content, XRD addresses the crystallographic form in which substances exist — the same element may form different oxide phases such as MoO₂ or MoO₃, each with markedly different tribological behavior. By systematically analyzing the position, intensity, and full width at half maximum (FWHM) of characteristic diffraction peaks, one can quantitatively evaluate the proportion of the target 2H phase in molybdenum disulfide powder, eliminate impurity phase interference, and provide structural-level evidence for batch release. This article focuses on the application of XRD in phase analysis of molybdenum disulfide, introducing testing principles, standard methods, and quality control points.

Basic Principles of XRD Phase Analysis

X-ray diffraction is based on the Bragg equation (nλ = 2d sinθ), which describes the diffraction condition. When X-rays irradiate a crystal, diffraction peaks occur when the interplanar spacing d and the incident wavelength λ satisfy a specific geometric relationship. Molybdenum disulfide predominantly exists as the 2H polytype, belonging to the hexagonal crystal system with space group P63/mmc. Its characteristic diffraction peaks appear at approximately 14.4° for the (002) plane, 32.7° for the (100) plane, 39.6° for the (103) plane, and 58.6° for the (110) plane (Cu Kα radiation, λ = 1.5406 Å). The (002) diffraction peak has the highest intensity, directly reflecting the periodic stacking of the layered structure, and serves as the primary identifier for the MoS₂ phase. If molybdenum trioxide (MoO₃) impurity is present in the powder, its orthorhombic system (space group Pbnm) characteristic peaks appear at approximately 12.8°, 23.4°, and 27.3°, which do not overlap with the MoS₂ main phase peaks, facilitating differentiation.

Standard Methods and Testing Parameters

In terms of methodology, XRD phase analysis can reference GB/T 23413-2009 "Nano-powder — X-ray diffraction analysis method" and ISO 22235:2018 "Nanotechnologies — Characterization of molybdenum disulfide nanopowders." The former specifies sample preparation methods, scanning parameters, and data processing procedures for nano-powder XRD testing, while the latter provides a systematic framework specifically for characterization of molybdenum disulfide nanopowders. In practical testing, a Cu Kα target (λ = 1.5406 Å) is typically used with an operating voltage of 40 kV and current of 40 mA, scanning range 2θ = 5°–80°, step size 0.02°, and counting time of 1–2 seconds per step. For sample preparation, the powder is uniformly spread in the groove of a glass slide and compacted while avoiding preferred orientation. According to GB/T 23413-2009 requirements, the test report should include the diffraction pattern, characteristic peak d-values and relative intensities, phase identification results, and crystallinity assessment.

Phase Purity Assessment and Impurity Identification

Common impurity phases in molybdenum disulfide powder include molybdenum trioxide (MoO₃), molybdenum dioxide (MoO₂), and elemental molybdenum, all of which can significantly affect the tribological performance of the product. MoO₃ exhibits catalytic oxidation activity at elevated temperatures and may accelerate degradation of base oils in lubrication systems; MoO₂ has higher hardness and friction coefficient than MoS₂, which diminishes the friction-reducing effect. By comparing the diffraction pattern of the test sample with PDF cards (such as MoS₂ corresponding to ICDD PDF No. 37-1492, MoO₃ corresponding to ICDD PDF No. 05-0508) for peak position and intensity matching, one can qualitatively identify each phase and roughly estimate their content proportions. When the diffraction pattern shows only characteristic peaks of the MoS₂ 2H phase without identifiable impurity peaks, the phase purity is generally considered to meet the requirements for high-quality applications. For applications requiring higher sensitivity, Rietveld full-pattern refinement can be combined for quantitative analysis of phase contents, with detection limits below 1%.

Crystallinity and Crystallite Size Analysis

In addition to phase identification, XRD can also provide crystallinity and crystallite size information. The full width at half maximum (FWHM) of diffraction peaks is inversely related to crystallite size. Based on the Scherrer formula (D = Kλ / β cosθ, where K is approximately 0.89), the average crystallite size can be estimated. For high-purity molybdenum disulfide, the FWHM of the (002) peak is typically in the range of 0.3°–0.6°, corresponding to crystallite sizes of approximately 15–30 nm. A significantly increased FWHM may indicate increased lattice defects or a higher degree of nanosizing; a low (002) peak intensity with notable broadening may reflect insufficient crystallinity or the presence of amorphous phases. This structural information is closely related to the friction coefficient and wear life of molybdenum disulfide, and incorporating XRD structural parameters into ex-factory inspection helps establish a complete quality traceability chain from crystal structure to application performance.

Conclusion

X-ray diffraction analysis provides the capability to examine product quality from the atomic arrangement level for phase purity identification and crystal structure characterization of molybdenum disulfide. Based on the methodological framework established by GB/T 23413-2009 and ISO 22235:2018, combined with PDF card comparison and Rietveld refinement, accurate identification of the MoS₂ 2H main phase, sensitive detection of impurity phases, and quantitative assessment of crystallinity can be achieved. Incorporating XRD structural parameters into routine ex-factory inspection, complementing chemical composition analysis and particle size detection, helps ensure the reliability and stability of molybdenum disulfide in applications such as lubrication and friction materials from the source.