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Xing et al. Microstructures 2023;3:2023031 https://dx.doi.org/10.20517/microstructures.2023.11 Page 13 of 35

bonds between various polar groups on gelatin and GO, and hydrophobic interactions between the
hydrophobic carbon backbone of GO and hydrophobic amino acid side chains on gelatin. These multiple
interactions may contribute to the formation of highly ordered 2D laminar membranes with finely tuned
2D nanochannels for nanofluidic transport.

In summary, the structural properties of nanochannels in nanochannel films based on 2D materials,
including layer thickness, stacking order, defects and grain boundaries, surface functionalization,
embedding of additives or polymers, and nanosheet alignment and orientation, largely affect the overall
filtration performance of the membrane. Controlling these factors can tune the interlayer spacing, pore size
distribution, surface interactions, and flow dynamics within nanochannels, leading to membranes with
higher selectivity, permeability, and fouling resistance. The choice of synthesis and assembly method
depends on the desired material, application, scalability, and desired quality of 2D nanosheets. Each method
has its advantages and limitations. Further studies to understand and optimize these structural properties
will facilitate the development of more efficient and tailored nanochannel membranes for various separation
applications.

Characterization methods
Characterization techniques are essential for understanding the properties of 2D materials. Various
techniques allow for a thorough examination of these materials. XRD is used to determine the crystal
structure and orientation of 2D materials. It involves directing X-rays onto the material and measuring the
resulting diffraction pattern. XRD can provide information about lattice parameters, crystal symmetry, and
the presence of specific crystalline phases. In addition, the layer spacing is calculated using XRD data
according to Bragg’s law. Optical spectroscopy techniques such as UV-Vis absorption spectroscopy and
photoluminescence spectroscopy are used to study the optical properties of 2D materials. They can reveal
information about the material band gap, exciton properties, and light-matter interactions. X-ray
photoelectron spectroscopy (XPS) is used to analyze the chemical composition and electronic states of 2D
materials. It involves irradiating the surface of a material with X-rays and measuring the energy of the
emitted electrons. XPS can provide information about elemental composition, chemical bonding, and the
presence of impurities or functional groups. Raman spectroscopy is used to analyze the vibrational modes of
2D materials. It involves shining a laser on the material and measuring the scattering spectrum. Raman
spectroscopy provides insight into the crystal structure, composition, and strain of material and identifies
different types of 2D materials. Atomic Force Microscopy (AFM) is a powerful technique for imaging the
morphology and surface properties of 2D materials. It uses a sharp probe to scan the entire surface of a
material, detecting force changes between the probe and the sample. AFM can provide information about
the height, roughness, and mechanical properties of a material at high spatial resolution. Scanning electron
microscopy (SEM) provides high-resolution images of the surface/sectional morphology of 2D materials. It
uses a focused electron beam to scan the surface of a material, generating detailed images that reveal
features such as the size, shape, and arrangement of thin sheets or layers of material. Transmission electron
microscopy (TEM) is used to study the internal structure and atomic-level features of 2D materials. It
involves the transmission of an electron beam through a thin sample to image the atomic structure and
defects of the material. TEM can provide information about crystal structure, grain boundaries, stacking
sequences, and even individual atomic arrangements.

Among other things, these characterization techniques allow researchers to gain a comprehensive
understanding of the structural, morphological, chemical, and optical properties of 2D materials. By
combining multiple techniques, a complete characterization of 2D materials can be achieved, facilitating
their optimization and utilization in a variety of applications. The development of advanced
characterization techniques, such as cryoelectron microscopy and in-situ Raman, will provide researchers