ZnO Nanofiber Fabrication Technique

Abstract

Zinc oxide (ZnO) nanofibers have gained significant attention due to their unique properties and potential applications in various fields, including electronics, optoelectronics, sensors, energy storage, and biomedical devices. This chapter presents an overview of different fabrication techniques employed for the synthesis of ZnO nanofibers. It discusses both template-based and template-free methods, highlighting their advantages, limitations, and the resulting morphological and structural characteristics of the fabricated nanofibers. Electrospinning is a crucial nanofiber fabrication technique utilized in electronics design and software to create high-performance materials with tailored properties for applications such as sensors, energy storage devices, and electronic components. Furthermore, the chapter provides insights into the influence of process parameters on the growth mechanism and properties of ZnO nanofibers. The goal is to provide readers with a comprehensive understanding of the various techniques available for fabricating ZnO nanofibers and to guide them in selecting an appropriate method for their specific applications.

1. Introduction

1.1 Overview of ZnO nanofibers

“There’s plenty of room at the bottom”, said Richard Feynman in 1959. The fabrication and characterization of such rooms (nano structures) have imbibed in the mind of each and every researchers today. Nanotechnology has emerged as an exciting research field in the last decade. Recently, nanostructures have attracted extensive research interests because of their unique properties. They have made innovations in nanotechnology, particularly nano sensor devices. Nano structures include nano particles, nano films, nano ribbon, nano flower, nano cluster, nanofibers, etc., all of them are frontier materials and gain dominant importance in nanotechnology. Figure 1 shows the various nano and microscale illustrations.

Figure 1.

Nano-micro scale illustrations.

Currently, nanofibers occupy a prominent position in the realm of nanotechnology due to their exceptional attributes encompassing reduced density, exceedingly high surface area in relation to weight, significant pore volume, and adjustable pore size. These distinctive characteristics render nonwoven nanofibers highly suitable for a wide array of applications. Moreover, these nanofibers exhibit the ability to imitate the structure, chemical composition, and other properties of fiber matrices. Consequently, nanofibers play a pivotal role in various fields such as biomedicine, skin tissue engineering, bone regeneration, wound healing, vascular grafts, and drug delivery systems.

Metal oxide nanofibers are attractive components for nanometer scale electronic devices like LED, Photodiodes, varistors, solar cells, etc., the metal oxides ZnO, SnO₂ and CuO have exceptional blend of interesting properties such as non-toxicity, good electrical properties, high luminous transmittance, excellent substrate adherence, hardness, optical and piezoelectric behaviors, stability in atmosphere and it low cost.

Electrospinning emerges as the most promising technique in the realm of nanotechnology, primarily owing to its simplicity, cost-effectiveness, high productivity, reproducibility, and potential for industrial scalability. This method involves applying a high voltage electric field to extract ultra-thin fibers from a stream of polymeric fluid (solution or melt) delivered through a nozzle measuring in millimeters [1]. The electrospinning process relies on several processing parameters, including solution properties (such as viscosity, surface tension, and conductivity) and other factors (like electric field strength, solution flow rate, needle diameter, and distance between the needle tip and grounded collector). Consequently, modifying any or all of these parameters directly affects the size, shape, and morphology of the resulting nanofibers. By exerting control over these parameters, it becomes possible to produce precisely defined fibers tailored for specific applications. Additionally, ensuring control over the structural characteristics of electrospun fibers—such as fiber diameter, porosity, volume ratio, surface morphology, and mechanical properties—is crucial for biomedical applications.

1.2 Significance of fabrication techniques

The fabrication technique used to produce nanofibers plays a crucial role in determining their properties and functionality. The significance of nanofiber fabrication techniques can be summarized as follows:

Control over Morphology: Nanofiber fabrication techniques allow precise control over the morphology of the resulting fibers, including diameter, length, and surface area. This control is essential for tailoring the physical, chemical, and mechanical properties of nanofibers to meet specific application requirements.

High Aspect Ratio: Nanofibers possess an extremely high aspect ratio, with lengths several orders of magnitude greater than their diameters. Fabrication techniques enable the production of nanofibers with uniform diameters and lengths, resulting in a high aspect ratio, which is advantageous for various applications such as filtration, tissue engineering scaffolds, and reinforcement in composite materials.

Large Surface Area: Nanofibers have an inherently large surface area-to-volume ratio due to their small size and high porosity. Fabrication techniques can further enhance the surface area by creating specific morphologies such as porous, hollow, or structured nanofibers. The increased surface area provides more active sites for chemical reactions, adsorption, and interaction with the surrounding environment, making nanofibers suitable for sensing, catalysis, and energy storage applications.

Tunable Properties: Nanofiber fabrication techniques offer the ability to tune the properties of nanofibers by incorporating dopants, additives, or functional materials during the synthesis process. This allows tailoring of the optical, electrical, magnetic, and mechanical properties of nanofibers for specific applications such as optoelectronics, sensors, and energy devices.

Integration and Hybridization: Nanofiber fabrication techniques enable the integration of different materials and the formation of hybrid structures. By incorporating multiple components into nanofibers, it is possible to create multifunctional materials with enhanced properties and synergistic effects. This opens up opportunities for developing advanced devices, such as flexible electronics, wearable sensors, and drug delivery systems.

Scalability and Reproducibility: Fabrication techniques for nanofibers can be scaled up to produce large quantities of fibers with consistent quality and reproducibility. This is crucial for commercial applications where batch-to-batch consistency and production scalability are essential.

Versatility: Nanofiber fabrication techniques are versatile and can be adapted to various materials, including polymers, metals, oxides, and carbon-based materials. This versatility allows the fabrication of nanofibers with diverse compositions and structures, expanding their application potential across different fields.

2. Template-based fabrication techniques

2.1 Electrospinning

2.1.1 Introduction to the electrospinning method

Electrospinning is a simple, proficient and versatile method to produce polymer based nanofibers for numerous applications. In recent times, this method has garnered considerable interest across various domains, despite its invention dating back to 1934 by Anton [1]. Typically, a fundamental electrospinning configuration consists of a high voltage power supply, a syringe needle connected to the power supply, and a counter-electrode collector, as depicted in the schematic diagram shown in Figure 2. In the process of electrospinning, a significantly high electrostatic voltage is applied to the polymer solution, leading to a substantial electrification of the solution droplet located at the tip of the needle [2, 3]. As a result, the solution droplet at the needle tip receives electric forces, drawing itself towards the opposite collector electrode, thus forming into a conical shape known as “Taylor cone” [4].

Figure 2.

Schematic diagram of electro-spin coating unit.

When the electric force surpasses the surface tension of the polymer solution, the solution is released from the tip of the “Taylor cone” and forms a polymer jet. This charged jet is then stretched into a fine filament by the strong electric force. By evaporating the solvent within the filament, dry fibers are deposited onto the collector electrode. The electrospinning process and the properties of the fibers are influenced by numerous factors, including the characteristics of the polymer materials (such as polymer structure, molecular weight, and solubility), the properties of the solvent (such as boiling point and dielectric properties), solution properties (such as viscosity, concentration, conductivity, and surface tension), operating conditions (such as applied voltage, collecting distance, and flow rate), and the surrounding environment (such as temperature, gas environment, and humidity). Electrospun nanofibers possess several unique traits, including a high surface-to-mass ratio, excellent interconnectivity of pores within a high porosity structure, flexibility combined with reasonable strength, a wide selection of polymer materials to choose from, the capability to incorporate other materials (such as chemicals, polymers, biomaterials, and nanoparticles) into the nanofibers through electrospinning, and the ability to control secondary structures of the nanofibers to create core/sheath structures, side-by-side structures, hollow nanofibers, and nanofibers with porous structures [5]. These characteristics make electrospun nanofibers suitable for various applications, including filtration, affinity membranes, metal ion recovery, tissue engineering scaffolds, controlled release systems, catalyst and enzyme carriers, sensors, and energy storage [6].

2.2 Software design for nanofiber fabrication

The role of software design is crucial in the development of a software system, as it enables developers to create various models that serve as blueprints for the implementation solution. These models can be thoroughly analyzed and evaluated to determine their ability to meet the specified requirements. Additionally, alternative solutions and trade-offs can be explored and assessed. Ultimately, these models are used to plan subsequent development activities and serve as the foundation for coding and testing processes.

As per the definition provided by the Institute of Electrical and Electronics Engineers (IEEE), design encompasses two aspects: it is both the process of defining the system or component’s architecture, components, interfaces, and other characteristics, as well as the outcome of this process. When viewed as a process, software design is the activity within the software development cycle where software requirements are carefully analyzed to generate a description of the internal structure and organization of the system. This description serves as the basis for constructing the system. Specifically, the software design result should outline the system’s architecture, including how it is decomposed and organized into components, and it should define the interfaces between these components. Furthermore, it should provide sufficient detail about these components to facilitate their construction.

Software design encompasses two activities that occur between software requirements analysis and software coding and testing. The first activity is software architectural design, also known as top-level design, which involves describing the overall structure and organization of the system and identifying its various components. The second activity is software detailed design, where each component is sufficiently described to enable coding.

The software design process typically follows a series of steps that begin with stating the basic program requirements and gradually adding more detail. This progression typically involves the following stages where it starts with gathering the requirements and specifications. The program’s desired operation is initially described at a general level and then in more detail. The next step is gathering the flow of steps, decisions, and loops which is planned, often using diagrams, to indicate dependencies between different subtasks. This stage defines the sequence of operations and the data communication requirements. Next the format of variable types for passing data between functions is determined to design function interfaces. The program’s structure and components are designed in a top-down and/or bottom-up design. These two design paradigms help organize the overall structure and individual modules, respectively. Once a plan for the program’s appearance has been established, the focus shifts to implementation. The coding process often reveals flaws in the original design or suggests improvements and additional features. Therefore, design and implementation tend to be iterative rather than a linear progression. The final stage involves testing and debugging. Non-trivial programs inevitably contain errors when initially written, so thorough testing is necessary before implementing them in real-time applications. Debugging is performed to identify and rectify any issues.

2.2.1 Mechanism of nanofibers fabrication

The electrospinning technique utilizes a high electric field to generate extremely fine polymeric fibers, ranging in diameter from a few nanometers to a few micrometers. The mechanism behind electrospinning nanofibers is based on the intricate electro-physical interactions between the polymer solution and electrostatic forces. In this process, a high-voltage electric field is established between the injection needle and the collecting screen, facilitated by a power supply and electrodes. As the polymer solution is slowly extruded from the syringe, a semi-spherical droplet of the solution forms at the needle’s tip. With the increasing voltage, the charged polymer droplet elongates into a cone shape, accumulating surface charge over time. Once the surface charge surpasses the surface tension of the polymer droplet, a polymer jet is initiated. The solvent within the polymer jet carries the material as it travels towards the collecting screen, further increasing the surface charge on the jet [7]. This rise in surface charge induces instability in the polymer jet as it interacts with the electric field. To counteract this instability, the polymer jet undergoes geometric division, initially forming two jets and subsequently splitting into numerous additional jets as the process continues. The spinning force generated by the electrostatic force on the continuously dividing polymer droplets ultimately leads to the formation of nanofibers. These nanofibers are deposited layer-by-layer on a metal target plate, creating a non-woven nanofibrous mat [8].

Throughout the electrospinning process, both extrinsic and intrinsic parameters are known to impact the structural morphology of the nanofibers [9]. Extrinsic parameters, including environmental humidity, temperature, as well as intrinsic parameters such as applied voltage, working distance, conductivity, and viscosity of the polymer solution, must be optimized to achieve uniform nanofiber production. Within the resulting nanofibrous mat, two main structures are commonly observed: a uniform, continuous fibrous structure or a structure containing beads. The relative abundance of these two structures depends on the contributions of various parameters during the electrospinning process.

2.2.2 Block diagram of nanofibers generator

For the past few decades several researchers have developed electrospinning nanofibers generator with larger unit size and least automation techniques. The design introduced microcontroller for the design of high voltage power supply as well as electrospinning spinneret unit. These new design brought cost reduction, unit portability and automation techniques. This technique has many advantages such as ease of operation and controlling the fiber diameter also functionalizing nanofibers through adding functional agents to the electrospinning solution. Hardware and software designs are required to develop the effective automation.

Figure 3 illustrates a block diagram presenting the configuration of an electrospinning nanofibers generator. This custom-designed generator comprises three main modules: (1) High voltage power supply unit, (2) Spinneret or Plunger unit, and (3) Nanofibers collector unit. The high voltage power supply unit, assisted by a microcontroller, is designed and constructed to provide a variable direct current (DC) source necessary for the production of nanofibers. The spinneret-solution feeding unit is composed of a syringe with a stainless steel needle ranging in size from 0.1 mm to 0.3 mm. It is fitted with a syringe holder and connected to a stepper motor. The stepper motor, controlled by a microcontroller unit, generates precise forward and reverse mechanical forces to move the syringe piston accordingly. This motion facilitates the formation of the Taylor cone, from which aligned nanofibers are fabricated and deposited onto the substrate, securely placed in the metal collector holder. Two types of collectors are employed: flat and drum types.

Figure 3.

Block diagram of electrospinning nanofibers generator.

2.2.3 Design and construction of nanofibers generator modules

The nanofibers generator consists of three modules as shown in Figure 3. The first module comprises of DC power supply unit, microcontroller16F877A unit, and stepper motor driver unit. It helps to supply necessary and controlled (Programmed pulses) to the fixed stepper motor and hence the forward and backward movements (in appropriate steps) of the piston connected to the syringe in the next module. The second module consists of syringe holder fitted with syringe and stainless steel needle connected to positive potential of high voltage power supply from module (3). The third module consists of DC power supply unit, microcontroller16F877A unit, stepper motor driver unit, flyback transformer and collector unit which is used to produce 0–30 kV variable DC power. This power is applied across stainless steel needle and collector unit.

2.3 Module I

2.3.1 DC input power source for microcontroller

The first module of the nanofibers generator consists of DC power supply unit. It provides the supply voltage of 5 V DC power to the microcontroller unit. It consists of 0–6 V step down transformer 250 mA, bridge rectifier and voltage regulator (IC-7805). The performance of the unit was checked using Digital Multi Meter (DMM) and Cathode Ray Oscilloscope (CRO). It gives a steady 5 V DC for the load resistance of 5–10,000 Ω. The waveform of generated DC output was studied using storage oscilloscope. The generated AC input waveform and bridge rectifier DC output waveforms are shown in Figure 4 [10].

Figure 4.

(a) AC input waveform and (b) DC output waveform of DC power supply.

2.3.2 Microcontroller 16F877A controlled circuit for spinneret unit

In this research PIC 16F877A is used to move the piston in desired direction and steps to inject the high viscous precursor polymer Sol–Gel for nanofibers fabrication. It is very difficult with high pressure to inject the high viscous polymer through a needle of very small diameter in the order of 0.1–0.3 mm. The unit is designed for the forward and backward movement of piston in this syringe. The microcontroller unit was fitted with three push button switches to stop and control forward and backward movement of the stepper motor. For this requirement, appropriate algorithms were written and the corresponding “C” program is written and compiled to the machine language and uploaded in the PIC 16F877A using PIC programmer [11]. Figure 5 depicts the circuit diagram of PIC 16F877A. The developed software program for this operation is attached in the Appendix-B.

Figure 5.

Circuit diagram of PIC microcontroller 16F877A.

2.3.3 Stepper motor driver unit

In order to control the motion of the stepper motor, a driving unit consists of stepper motor driver L 298 is used and the circuit diagram and cooling fan design is as shown in Figure 6. It is not possible to operate the stepper motor without the stepper driver circuit. Figure 7 describes the circuit diagram of stepper motor driver unit.

Figure 6.

Circuit diagram of stepper driver photograph.

Figure 7.

Stepper driver circuit diagram.

Necessary voltage for the rpm of stepper motor with high starting pulse can be obtained only using stepper driver unit. For each step movement of the signal is obtained from the microcontroller through this driver unit. L 298 plays a vital role in generating high voltage and high current output for the stepper motor operation. A micro controller or stepper motor controller can be used to activate the drive transistors in the right order, and this will ease of operation making unipolar motors popular with hobbyists; this is the cheapest way to get precise angular movements.

The L 298 driver circuit can control concurrently up to two stepping motors. Only one stepper motor was used towards this motor driver circuits. It can handle maximum of 4 A, i.e., two amps per motor, however to get the maximum current make sure to add a heat sink on the top of the body [12]. The L 298 has a large cooling brim with a hole in it, making it easy to attach a metal heat sink to it. The motor driver unit system primarily comprises of three significant components namely stepper driver, indexer, and user interface as shown in stepper driver circuit diagram (Figure 7). In this research work 23LM C355-20 stepper motor and L 298 stepper motor driver is produced by automation process. The L 298 is a 15—pin multi watt and high power monolithic integrated circuit. This dual full-bridge driver is specifically designed to accommodate TTL logic levels and effectively drive inductive loads like relays and stepping motors, offering both high voltage and high current capabilities. The device includes two enable inputs, which allow for independent enabling or disabling of the driver, regardless of the input signals.

The stepper motor leads were connected in series for a higher inductance and therefore attained higher performance with lower speed. The motor driver will be efficiently able to control and drive a four phase unipolar stepper motor with continuous output current ratings to 1.5 A per phase and 1.5 Ω [13]. The schematic diagram shows a basic connection diagram for controlling motors using the L 298 motor bridge IC. There are three input pins for each motor: Input1, Input2, and Enable1 controls Motor1. Input3, Input4, and Enable2 controls Motor2. Table 1 describes the input and basic functions of designed stepper driver circuit. In order to activate the motor, the enable1 line must be high. To control the motor and its direction by applying a LOW or HIGH signal to the Input1 and Input2 lines, as shown in Table 1.

Enable	Input-I	Input-II	Function
Low	Low	Low	Off
Low	High	Low	Off
Low	Low	High	Off
Low	High	High	Off
High	Low	Low	Motor break
High	High	Low	Motor run forward
High	Low	High	Motor run backward
High	High	High	Motor break

Table 1.

Stepper driver input and function.

2.3.4 Stepper motor

Stepper motors are a type of brushless synchronous motors that complete a full rotation cycle in a predetermined number of steps. These motors are specifically designed to be driven by applying excitation pulses to the phase windings, as they cannot be operated simply by connecting the positive and negative leads of a power supply. Instead, they require a stepping sequence generated by a microcontroller to drive their movement. The motor advances in discrete steps according to this sequence. Stepper motors can be classified based on their construction type.

1. Variable Reluctance (VR) stepper motor.

2. Permanent Magnet (PM) stepper motor.

3. Hybrid stepper motor

Variable reluctance stepper motors are characterized by having a rotor composed of ferromagnetic materials. When the stator is energized, it becomes an electromagnet, exerting a pull on the rotor in that direction. The ferromagnetic material naturally seeks to align itself along the path of minimum reluctance. By energizing the coils, a magnetic field is generated, and the airgap reluctance is adjusted, hence the name “variable reluctance stepper motor.” In this type of motor, the motor’s direction is independent of the direction of the current flowing in the windings [14].

On the other hand, the rotor of a Permanent Magnet (PM) stepper motor is permanently magnetized. Therefore, the motor’s movement is determined by the attraction and repulsion forces between the magnetic poles of the stator and rotor. In this motor, the direction of the motor depends on the direction of the current flowing in the windings since the magnetic poles are reversed by changing the current flow through the rotor. The hybrid stepper motor, as the name suggests, combines the advantages of both the permanent magnet stepper motor and the variable reluctance stepper motor to provide improved efficiency [15]. These two types of stepper motors are the most commonly used. The main distinction lies in the fact that the rotor of the variable reluctance stepper motor is made of a ferromagnetic material, whereas the rotor of the permanent magnet stepper motor is permanently magnetized. The full view and cut view of the original stepper motor are depicted in Figure 8a and b.

Figure 8.

(a) Stepper motor full view and (b) cut view.

2.4 Module II

2.4.1 Syringe holder unit or spinneret unit

Syringe pumps, also known as spinnerets, are small pumps precisely operated by stepper motors. Microcontroller 16F877A drives the stepper motor using stepper driver to generate necessary mechanical force [16]. The generated mechanical force is used to move the syringe tap on either direction. During forward motion, the mechanical force moves the plunger of the syringe in the forward direction resulting in pushing the Sol–Gel to the syringe needle tip, where as in the reverse direction the mechanical force generated by the stepper motor brings the plunger of the syringe to its initial position for refilling. The microcontroller16F877A is programmed using “C” language to generate a controlled forward and reverse mechanical force. This software enabled microcontroller accurately generates controlled mechanical forces in forward duration and reverse duration. By varying software code in the microcontroller the mechanical force duration can be varied. A screw rod is connected in between the stepper motor and syringe needle, due to this set up, the system requires a high power DC supply. The 5 Amps 12 Volt DC source is used to energize the stepper motor via stepper driver circuit. Cooling fans are installed to dissipate the enormous amount of heat generated during the generation of mechanical force to move the plunger of the syringe and to keep the driver unit at optimal temperature. Figure 9 describes the block diagram of spinneret unit.

Figure 9.

Block diagram of a syringe holder unit/spinneret unit.

3. Conclusion

• Hardware and software design of DC power supply unit, microcontroller unit, MOSFET switching circuit, high voltage power supply unit and spinneret unit design has been completed.

• Electronic components and circuit boards needed in the proposed research have been systematically identified and incorporated in this research.

• Microcontroller based high voltage power supply and syringe holder unit has been designed by using PIC16F877A. A variable frequency from 250 Hz to 25 kHz can be generated for the high voltage. Inbuilt PWM generator has been used to generate pulse of high voltage with a variable frequency.

• The “C” language software works as expected. High DC voltage output can be configured from PIC16F877A interface software via serial communication. The observed output voltages were varied from 0–30 kV with the help of variable potentiometer and its accurate results were displayed on liquid crystal display screen (LCD).

• Two different metal collectors were initially designed and studied, both collectors yield best results. It provides uniform fibers on the metal collector plate with good morphological characteristics. It is also observed that there is no bead like structure found in the fiber obtained with both type of collectors.

• Polymer flow rate can be configured from microcontroller interface software via control circuits in the main board.

• The overall results show that voltage, frequency and duty cycle are controlled by the resistance of the switching circuit, and the output voltage proportionally changes by changing the resistance.

• The electrospinning generator design has been optimized and formulated.

• This improvised microcontroller controlled nanofibers generator has a number of advantages over conventional techniques: Simple, cost effective, portable and able to fabricate continuous nanofibers with small scale production.