X-ray diffraction (XRD) analysis of molybdenum disulfide (MoS2) is the core method for evaluating its crystal structure purity, phase composition, and crystalline quality. In solid lubricant applications, the 2H phase content of MoS2 directly determines lubrication performance, and XRD can precisely identify the ratio of 2H to 1T phases, providing critical data for quality control and product selection. This article systematically introduces the principles of MoS2 XRD analysis, key diffraction peak characteristics, and their applications in industrial quality control.

XRD Analysis Principles and MoS2 Crystal Structure

XRD operates based on Bragg's Law: nλ = 2d sinθ, where λ is the incident X-ray wavelength (typically 1.5406 Å for Cu Kα radiation), d is the interplanar spacing, and θ is the diffraction angle. Different crystal structures produce characteristic diffraction patterns that can be identified by comparison with standard reference cards.

MoS2 exists in two primary crystal phases: 2H and 1T. The 2H phase belongs to the hexagonal crystal system, space group P6₃/mmc, with lattice parameters a = 3.16 Å and c = 12.30 Å, featuring an ABAB stacking arrangement of S-Mo-S layers. The 1T phase belongs to the trigonal crystal system, space group P3̅m1, with an ABCABC stacking arrangement. In lubrication applications, the 2H phase exhibits superior lubrication performance due to weak interlayer van der Waals forces and low shear strength; the 1T phase, while metallic, has a higher friction coefficient and is unsuitable as a solid lubricant.

Key Diffraction Peak Characteristics and Phase Identification

According to the International Centre for Diffraction Data (ICDD) standard card PDF#37-1492, the main diffraction peaks of 2H-MoS2 include:

• **(002) plane**: 2θ ≈ 14.4°, the strongest characteristic peak, reflecting the interlayer spacing c/2 ≈ 6.15 Å, serving as the core indicator for evaluating the structural integrity of MoS2 layered structure. Higher peak intensity and narrower full width at half maximum (FWHM) indicate better crystallinity and more complete layered structure
• **(100) plane**: 2θ ≈ 32.6°, reflecting the atomic arrangement periodicity along the a-axis
• **(103) plane**: 2θ ≈ 39.5°, diagnostic for distinguishing 2H from 1T phases. The 2H phase exhibits a clear diffraction peak here, while the 1T phase is absent or extremely weak at this angle
• **(105) plane**: 2θ ≈ 49.8°
• **(110) plane**: 2θ ≈ 58.4°

The identification characteristic of 1T-MoS2 is the shift of the (002) peak to approximately 9.5° (interlayer spacing increased to about 9.3 Å), with the absence of the (103) peak. Detection of a diffraction peak near 9.5° in industrial-grade MoS2 indicates 1T phase impurity, requiring assessment of whether its content affects application performance.

Key Applications of XRD in Quality Control

### Crystallinity Assessment

The crystallite size can be calculated from the FWHM of the (002) peak using the Scherrer equation: D = Kλ / (β cosθ), where K ≈ 0.9 and β is the FWHM in radians. High-purity 2H-MoS2 typically exhibits a (002) peak FWHM of less than 0.3°, corresponding to crystallite sizes above 50 nm. Excessive FWHM may indicate grain refinement or increased lattice defects, affecting transfer film formation capability.

### Phase Purity Quantification

The Rietveld full-pattern fitting method or Reference Intensity Ratio (RIR) method can quantitatively determine the ratio of 2H to 1T phases. Industrial-grade high-purity MoS2 requires a 2H phase content of ≥95%, while high-end applications demand ≥98%. Some studies use the intensity ratio I(002)/I(103) as a semi-quantitative indicator, which typically ranges from 3-5 for pure 2H phase.

### Impurity Phase Detection

XRD can effectively identify common impurity phases in MoS2, including MoO3 (characteristic peak at 2θ ≈ 27.3°), MoO2 (2θ ≈ 26.1°), and FeS2 (2θ ≈ 33.0°). MoS2 produced by physical flotation typically shows undetectable or extremely weak MoO3 impurity peaks due to the absence of chemical reagents; acid-leached products may exhibit residual sulfate impurity peaks if washing is insufficient.

Industrial XRD Testing Standards and Operational Guidelines

The Chinese national standard GB/T 23274-2009 specifies chemical analysis methods for molybdenum disulfide, with XRD used for qualitative phase analysis. The internationally accepted standard references ASTM D3610. Key operational considerations include:

Sample preparation employs back-loading or side-loading methods to avoid preferred orientation causing abnormal enhancement of the (002) peak intensity. The scanning range is typically set to 5°-70° (2θ), with a step size of 0.02° and 1-2 seconds per step. For quantitative analysis, the internal standard method (using α-Al2O3 or Si) is recommended, with an addition of 10-20 wt% of the sample mass.

For testing frequency, at least one XRD scan per batch is recommended, with additional quantitative phase analysis for critical customer batches. Test data should be archived to establish a batch quality traceability database.

Engineering Interpretation of XRD Results

XRD pattern interpretation should be combined with specific application scenarios. For powder metallurgy applications, MoS2 requires high (002) peak intensity and symmetric peak shape to ensure complete layered structure for self-lubrication. For plastic modification applications with finer D50 (1-5 μm), the XRD peak width may slightly increase due to crystallite size effects, but the (002) peak position should not shift. For grease additive applications, attention should be paid to whether impurity phases affect the colloidal stability of the grease.

High-purity MoS2 produced by physical flotation typically exhibits a strong and sharp (002) peak, no MoO3 impurity peaks, and a clearly identifiable (103) peak. This is directly related to its chemical residue-free production process—no acid radical ions are introduced, preventing lattice distortion and impurity phase formation.

As a fundamental quality control tool for MoS2, XRD analysis provides value by revealing product quality differences at the crystal structure level. For purchasers, requesting XRD patterns and key peak parameters for each batch is an effective approach to establishing a robust supply chain quality management system.

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