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Dang et al. Chem Synth 2023;3:14 https://dx.doi.org/10.20517/cs.2022.33 Page 5 of 20

to fabricate. It is reported that 2D Se nanomaterials have relatively complex structures based on crystal
search computations. Besides the hexagonal and trigonal structure, two other stable structures including 1T-
[63]
MoS -like structure (T-Se) and square structure (S-Se) may exist [Figure 1J] .
2
SYNTHESIS STRATEGIES AND FABRICATION TECHNIQUES OF SELENIUM
NANOMATERIALS
The synthesis strategies and fabrication techniques greatly dictate the morphologies and structures of the
resulting Se. So far, numerous protocols for synthesizing Se nanomaterials, such as chemical reduction,
chemical vapor transfer synthesis, and thermally drawn-assisted synthesis, have been reported [Table 1].
The following section analyzes some representative synthesis and fabrication strategies of Se nanomaterials,
including nanospheres, nanoparticles, nanotubes, nanowires, and nanosheets.

0D Se nanomaterial
Due to the low cost, biocompatibility, and outstanding electrical properties, 0D Se nanomaterials have been
brought into focus in chemical and material communities. Chemical reduction is one of the most common
strategies for the synthesis of 0D Se. This approach involves reducing Se salts with various reducing
chemicals. Se nanoparticles with a hollow structure synthesized by using A (Pr), sodium selenite, and
[52]
mercaptothion as a template, Se sources, and reducing agents have been reported [Figure 2A] . TEM
characterization revealed the hollow structures at the edges and centers. However, the obtained Se
nanoparticles contain impurities, transferring these nanoscale materials is complicated, and breaking the
serious aggregation remains challenging. All these issues restrict the practical application of these
nanoparticles. The emergence of solvothermal synthesis successfully solves the problems, resulting in
[64]
nanoparticles of high purity, uniform morphologies, and slight aggregation [Figure 2B] . Additionally, this
method is facile to enable super-small nanoparticles. One example shown in Figure 2B (ii) shows
nanoparticles with radii of 3-8 nm. In addition, laser ablation of solid bulky Se at the solid-liquid interface
could also be harnessed to produce Se nanoparticles [Figure 2C] . With this “top-down” strategy, colloidal
[65]
Se nanoparticles with sizes of below 100 nm are fabricated in pure water. Such Se nanoparticles are expected
to be useful for biological applications. However, some issues that impede the practical application of 0D Se
are also reported in these studies. For example, the collision and aggregation of Se nanoparticles frequently
occurred, which easily led to serious surface oxidation and degradation. The research on oxidation and
degradation will be at the center of the solvothermal synthesis of 0D Se nanomaterials.

1D Se nanomaterials
1D Se nanomaterials such as nanowires, nanorods, nanotubes, and nanobelts have elicited significant
interest because of their outstanding optical and electrical properties. Two strategies, namely the “top-
down” and “bottom-up” approaches, have been generally employed for the synthesis of 1D formats.
Specifically, the “top-down” strategy relies on selective removal of material from a solid bulky Se in a
subtractive manner, while the “bottom-up” strategy utilizes individual atoms as the building blocks to
realize structure assembling in an additive fashion [66,67] .

1D Se nanomaterials are commonly fabricated with the “bottom-up” approach because the atomic
composition, size, and shape can be tailorable and controllable. This approach typically involves the
decomposition of Se-based compound via chemical reactions either at room or an elevated temperature
[Figure 3A]. The resulting nanowires are highly uniform, large-area and of high aspect ratio
[Figure 3A (i)] . Their lateral dimensions are controllable depending on the process parameters
[68]
[Figure 3A (ii)]. However, Se nanowires prepared by this approach can be easily contaminated by the used
chemical reagents. In addition, the relatively poor crystallinity and high density of defects further frustrate