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The dramatic growth of the world's population in recent decades has affected the global economy, politics and industry. To address some of these issues, nanotechnologies are being developed worldwide. They offer the opportunity to produce materials and structures with unique properties that can be tailored to meet the requirements of specific applications, e.g., low energy consumption and high efficiency, which are key to solving growing environmental problems. One of the reasons why nanomaterials have very interesting and unique properties (compared to their bulk counterparts) is their huge surface area compared to their volume. Moreover, by reducing the size of an object to its structural units (atoms and molecules), the properties of the material may change substantially, as they are subsequently governed by quantum laws. Many advanced techniques have been proposed for the production of nanomaterials. Here, we focus on two of them: electrospinning and atomic layer deposition.
The main point of this paper
Electrospinning technology:
Electrospinning is a low-cost fiber production method in which charged filaments in a polymer solution are stretched into fibers of different diameters by means of a high electrostatic voltage.
The technology is capable of preparing high aspect ratio (HAR) structures, which are widely used in filtration systems, catalysis, energy harvesting, and biomedical engineering.
Polyvinyl alcohol (PVA) is one of the most widely used electrostatic spinning polymers because it is easy to process and environmentally friendly.
Atomic Layer Deposition (ALD) Technology:
ALD is a vapor phase deposition technique that produces thin films with excellent uniformity and precise thickness control.
ALD provides excellent uniformity in high aspect ratio structures, as well as step coverage, precise angstrom thickness control, continuous pinhole-free layers, and adjustable film composition.
ALD technology has a wide range of industrial applications in a variety of fields such as microelectronics, protective coatings, energy storage and conversion, etc.
Combining the advantages of electrospinning and ALD:
Combining electrospinning and ALD reveals the advantages of both technologies and creates smart devices for a variety of green technology applications
This innovative technique for preparing HAR structures opens up a large number of applications such as catalysis, solar cells, sensors, batteries, etc.
Preparation of hollow ZnO/ALO bilayer submicron fibers:
Hollow ZnO/ALO bilayer submicron fibers were prepared on glass substrates by a two-step method combining electrospinning and ALD
First, the ALO film was coated on the surface of submicron PVA fibers by low-temperature ALD method, and then the polymer core was burned off by high-temperature annealing
By depositing ZnO via thermal ALD, this two-step ALD process provided better quality of the top ZnO film and prevented the Kirkendall effect
New method presented:
The article proposes a new method combining electrostatic spinning and ALD for growing bilayer fibers.
PVA fiber preparation:
Polyvinyl alcohol (PVA) fibers were prepared by electrospinning.
ALD Deposition of Al2O3:
Thin layers of Al2O3 were deposited on the surface of PVA fibers using ALD at low temperatures.
High temperature annealing:
The fiber structure undergoes high temperature annealing to burn the PVA core and form a hollow fiber structure.
DSC Analysis:
The PVA mat is analyzed using Differential Scanning Calorimetry (DSC) to establish the proper annealing regime.
Hollow fiber formation:
Hollow fibers were obtained after burning the PVA core, and these hollow fibers were subsequently covered by an ALD-deposited ZnO layer.
ZnO film quality:
This method enables the preparation of ZnO films with good crystallinity and stoichiometric properties
Electrospinning combined with atomic layer deposition is a powerful technique for synthesizing HAR structures. We successfully obtained submicron ZnO (170 nm)/ALO (23 nm) hollow fibers with an inner cavity diameter of 512 nm on glass substrates. In our non-in situ ALD process, the combustion and outward diffusion of the polymer occur only through the amorphous ALO film. We can then deposit the top ZnO film at temperatures high enough to obtain the desired preferred crystallographic orientation. By using this sequence of processes, the Kirkendall effect and significant deterioration or even destruction of the fibers that can occur when the polymer core is burned in a high temperature process can be prevented. This technique, although more complex than previously proposed, produced hollow fibers of better quality. The results show that the ZnO films deposited on the prepared ALO fibers have better crystallinity and fewer defects under the same process conditions
