Micro/nano-motors play an important role in energy, environment, and biomedicines. Nanobottles attract great attention due to their distinct advantages of a large cavity, high specific surface area, bionic streamline structure, and chemotactic motion.
Since the invention of the steam engine, various motors with macro size, shape and composition have brought tremendous changes to industry and daily life. To have a better understanding of our environments, human beings have also started to explore the micro-world. As nanotechnology advances, different geometries of micro/nano-motors, such as helical, tubular, Janus sphere, nanowire, and other biomimetic shapes have been developed to meet the requirement for practical applications. Among them, nanobottles have a large cavity, high specific surface area and bionic streamline structure, as well as the characteristics of chemotactic motion of perceived gradient, and adjust direction trend gradient, showing great potential for energy, environmental, and biomedical applications. However, the construction of nanobottles is challenged because of difficulties in forming open structures and inadaptability of macroscopic principles. Various strategies have been proposed to manufacture such nanobottles.
2. Fabrication Methods
2.1. Soft-Template Method
Hydrothermal method refers to that in a closed system under high temperature and high pressure to grow crystals in aqueous solution or water vapor. This method has been broadly used in preparation of nanobottles. To control the nanobottle geometry, the hydrothermal method contains hard template and soft template. Soft-template method is carried out through self-assembly between core templates (e.g., organic surfactants, block copolymers, nanoemulsion drops) and precursor molecules to form shape confinement through weak noncovalent bonds (e.g., hydrogen bonds, Van der Waals forces, electrovalent bonds) 
. Due to the chemical interaction between core templates and precursor molecules, this method relies on self-assembly, nucleation, and growth process, and can be used to control the geometry parameter. These are the major steps to fabricate carbon nanobottle with ribose as the carbon source (as a carbon precursor under hydrothermal condition), oleic acid as the core, and poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) as surfactant 
. With temperature increasing, core nanoparticles aggregate and sedimentate to form nanoemulsion. As the reaction is progressed, carbon precursor polymerizes at the emulsion interface, and the polyethylene oxide blocks in surfactant become more hydrophobic to generate swelling of nanoemulsion. The tensile stress is caused by volume expansion of nanoemulsion and eventually leads to polymer shell splitting when the pressure exceeds the critical pressure. At the final stage, continuous filling process leads to the outflow of nanoemulsion to form a new template surface. Precursor continues to aggregate on the surface of the new template to form asymmetric carbon nanobottle
Soft templates generally in thermodynamics unstable liquid/gas are formed with high deformation ability, which can easily affected by various external parameters (e.g., PH value, additives concentration, solvent and temperature). The soft templates can be easily removed through evaporation or calcination without complicate template removal process, but the uniformity of nanoparticles is often affected. Furthermore, it is necessary to use a large amount of organic substances (e.g., ionic liquids) as reaction medium, which is easy to pollute the environment and not conducive to mass production.
2.2. Swelling-Induced Method
Swelling-induction method relies on swelling polymer microspheres with organic solvent, making organic solvent diffuse from the interior of nanoparticles and then quenching with ethanol to obtain the opening structure 
. The main steps with polystyrene microspheres serving as the starting point 
. After coating the surface of polystyrene microspheres with SiO2
thin shell, introduce tetrahydrofuran/water emulsion to swell the polystyrene spheres. After absorbing tetrahydrofuran, the swollen polystyrene creates pressure in the shell. When the pressure surpasses a threshold to poke a hole in the shell, the swollen polystyrene will be squeezed out through the opening and reduce the pressure in the shell to ensure only one hole is created. After being quenched with ethanol to from Janus structure, the swollen extent and the size of Janus particle can be adjusted through increasing the volumetric percentage of solute in tetrahydrofuran/water emulsion. When volume percentage of solvent is less than lower limit, the pressure caused by swelling will not be enough to break the SiO2
shell; while the volume percentage is above the upper limit, dissolution of polystyrene has occurred in shell. Increasing the volume percentage of tetrahydrofuran to 20% causes more polystyrene to protrude through the hole. After quenching through ethanol, the extruded polystyrene spheres have a diameter of 280 nm. When volume percentage of tetrahydrofuran is increased to 30%, the diameter of polystyrene spheres is increased to 333 nm. Further increasing the volume percentage of tetrahydrofuran to 50% would lead to polystyrene being partially dissolved in SiO2
shell. In the pure tetrahydrofuran solution, polystyrene swells to obtain SiO2
During swelling induction, the sizes, shapes and structure of nanobottles can be tailored by controlling swell extent through different types or percentage of solvents. In this process, each swollen particle can maintains the spherical shape by quenching the swelling with ethanol. Different from prior methods that rely on Janus particles, this method is based on commercial polystyrene nanospheres, and the hole is automatically punctured through swelling, making it possible to fabricate uniform nanobottles on a large scale.
2.3. Wet-Chemistry Method
Wet chemistry is a general method to synthesize nanobottles by controlling reaction dynamics and thermodynamic parameters to tune the sizes and shapes of nanobottle 
. Asymmetric Au nanobottle can be prepared with such method 
. Firstly, lead acetate and thioacetamide are heat-treated in the solution of cetyltrimethyl ammonium bromide to obtain PbS nanooctahedron with uniform edge lengths and slightly truncated vertex. The PbS nanooctahedron is used as sacrifice template, which can determine the scale parameters of Au nanobottles, and then disperse sulfide nanooctahedron into the growth solution. Au preferentially deposits at one vertex of each octahedron with the resultant core, then grow along four adjacent facets to produce Janus Au/PbS nanostructure. Finally, added weak acids dissolve and remove sulfide components in Au/PbS Janus nanostructures to produce the Au nanobottle. The opening size (d) and the diameter in the direction perpendicular to its symmetrical axis (D) can be adjusted by such a method, which influences the plasma properties of Au nanobottles. Keeping the opening size constant, the cavity volume and overall size of Au nanobottles will expand with the increase in edge length of PbS nanooctahedron. Furthermore, the opening size can be adjusted through controlling the ratio of Au/PbS. With the increase in Au/PbS ratio, the opening size of Au nanobottle first increases, and then it decreases after reaching the maximum, while the overall size and the cavity volume continue to increase in the Au nanobottle.
In wet-chemistry method, the edge length of PbS nanooctahedron and Au/Pb ratio are crucial factors during Au overgrowth process to adjusting sizes of the Au nanobottles. Moreover, there are three advantages to choosing PbS as sacrificial template for synthesizing Au nanobottles. Firstly, PbS nanocrystals of various shapes can be easily prepared by wet-chemistry method. Secondly, PbS has Fermi level higher than Au, which facilitate the selective deposition of Au on specific surfaces of PbS nanocrystals. Thirdly, PbS is readily dissolved in weak acids.