# Characterizations of Interfacial Solar Water Evaporation
## 2 Intrinsic Properties
### X-ray Diffraction  
X-ray diffraction (XRD) is widely used to analyze the composition and crystallographic structure of materials. When a metal target is bombarded by an electron beam, an electron in the inner layer of an atom shifts to an outer layer, which decreases the atomic energy level. The excess energy is radiated in the form of photons, which can be used to characterize the energy level difference. When a sample is irradiated by X-rays, the coherent scattering produced by particles generates the interference effect, which enhances or weakens the X-ray intensity depending on the diffraction conditions. [PERSON]'s formula can be applied:  
\[2\text{d}\sin\theta=\text{n}\,\lambda, \tag{1}\]  
where d is the distance between crystal planes, \(\theta\) is the angle between the corresponding crystal plane and the incident X-ray, \(\lambda\) is the X-ray wavelength, and n is the diffraction order. An X-ray diffractometer obtains the XRD pattern by measuring the angle between the incident X-ray and crystal plane and the corresponding diffraction intensity. The peak position can then be compared to standard powder diffraction files (PDFs) to determine the crystalline structure.  
[PERSON] et al. [5] successfully produced MXene ceramic nanosheets and used XRD to characterize them. They found that the strongest peaks of Ti\({}_{3}\)AlC\({}_{2}\) at 39\({}^{\circ}\) vanished from the XRD of freeze-dried Ti\({}_{3}\)C\({}_{2}\) nanosheets due to exfoliation. Peaks between 20\({}^{\circ}\) and 40\({}^{\circ}\) were still visible, which indicated that the stacked Ti\({}_{3}\)C\({}_{2}\) MXene layers still exhibited periodicity (Figure 1a). [PERSON] et al. [6] described a novel photothermal material: quasi-metallic WO\({}_{2.9}\) nanorods created by moderate hydrogenation of WO\({}_{3}\). They used XRD to examine the degree of crystallization of two samples. The diffraction peaks of the virgin WO\({}_{3}\) sample matched monoclinic WO\({}_{3}\) crystallites (PDF No. 71-2141). Moderate hydrogenation resulted in an observably altered XRD pattern with strong peaks indexed to monoclinic WO\({}_{2.9}\) (PDF No. 73-2182), indicating a high degree of crystallinity in the final product (Figure 1b). [PERSON] et al. [7] described a metal-organic framework (MOF)-based hierarchical structure (MHS) that concurrently exhibited high solar absorption and superhydrophilic and superoleophobic surface characteristics underwater. The XRD pattern indicated that the MHS had a crystalline structure comprising Cu-CAT-1 nanorods. They also observed peaks for Cu(OH)\({}_{2}\) and Cu, which indicated that the Cu mesh and Cu(OH)\({}_{2}\) nanowire backbone were still present (Figure 1c). [PERSON] et al. [8] reported a SnSe@SnO\({}_{2}\) core-shell nanocomposite for the generation of clean water. In the absence of any diffraction peak corresponding to SnO\({}_{2}\), the XRD pattern of SnSe@SnO\({}_{2}\) was almost identical to that of SnSe nanoparticles, with just one peak corresponding to the orthorhombic structure of SnSe (PDF No. 48-1224). Their observations indicated the low content (i.e., thinness) and amorphous structure of the SnO\({}_{2}\) shell.