# Characterizations of Interfacial Solar Water Evaporation
## 2 Intrinsic Properties
### Scanning Electron Microscopy  
Scanning electron microscopy (SEM) is often used to study the morphology and size of nanomaterials. A very thin electron beam is generated and accelerated in the electronic gun and then focused on the sample to excite secondary electrons, Auger electrons, backscattered electrons, characteristic X-rays, etc., on the sample surface. The number of secondary electrons is sensitive to the surface structure of the sample. The secondary electrons are collected by the detector and converted into optical signals, which are then converted into electrical signals by the electro-multiplier tube and amplifier to display scanning images. The surface information of the samples, including the morphology and crystallinity can be characterized by SEM images clearly.  
[PERSON] et al. [18] claimed that a novel photothermal membrane based on polypyrrole (PPy)-coated stainless steel (SS) mesh may self-heal because of its hydrophobicity. They created the membrane by first depositing PPy, which is a polymeric photothermal material, onto an SS mesh substrate. They then modified the PPy coating by using fluoroalkyl silane to produce the desired hydrophobicity. They used SEM to clarify the original unaltered SS mesh and mesh coated with PPy via electropolymerization for 1 h. The original mesh had a smooth wire surface, an average pore size of 26.8 m, and a knitting wire diameter of 24.5 m. The PPy-coated mesh demonstrated microscale surface features (Figure 5a). [PERSON] et al. [19] reported on a variety of TiO\({}_{\text{x}}\) nanomaterials in diverse hues (white, gray, blue, and black) for application as a solar-thermal observer for water desalination, which they made by reducing P25 TiO\({}_{2}\) with Mg. The as-prepared samples had particle sizes of around 24 nm, which was the same as the purest P25 TiO\({}_{2}\) powder. Generating TiO\({}_{\text{x}}\) lowered the O/Ti ratio, but the sizes and morphologies did not change much, which suggests that  
Figure 4: XPS measurements. (**a**,**b**) The bi-layered hybrid biofoam. Reproduced with permission from ref. [17], Copyright 2019, Wiley-VCH. (**c**,**d**) SnSe@SnO\({}_{2}\) structure. Reproduced with permission from ref. [8], Copyright 2018, The Royal Society of Chemistry.  
the Mg reduction procedure and subsequent HCl wash did not modify the TiO\({}_{2}\) structure (Figure 5b). [PERSON] et al. [20] showed that bamboo charcoal (BC) is a very effective photothermal material for interfacial solar evaporation thanks to a number of distinctive features. SEM showed that the porous quality of bamboo was carried over to BC, which had numerous irregular pores and grooves. There were several microcavities and a few tiny pores on the upper surface, which facilitated light absorption and steam overflow by redirecting and reflecting sunlight several times. Figure 5c shows that the x-y and y-z directions had polygonal holes as well as regularly combined holes (three-hole). All sections of the vertical growth direction (x-y direction) and part of the growth direction (y-z direction) were submerged in water during the experiment. Capillary forces may have moved water from the BC's bottom to the top surface through micropores and nanopores, which would ensure enough water is available for solar evaporation.