Optimization of electrospinning process parameters to develop the smallest ZnO + PVP nanofibres using Taguchi experimental design and ANOVA

Electrospinning is a well-known and straightforward technique for creating nanofibres from various materials, such as metals, ceramics, and polymers. The process takes place in a strong electric field, causing the polymer solution to stretch, resulting in micro/nanoscale fibres. The process parameters of electrospinning influence the morphology of nanofibres. In the present study, zinc oxide (ZnO) nanofibres were created in a polyvinylpyrrolidone (PVP) polymer combining electrospinning and sol–gel methods. To measure the effects of electrospinning and sol–gel on the diameter of ZnO + PVP nanofibres, a Taguchi design of experiment (DoE) approach was adopted, which comprised the “PVP concentrations, flow rate, needle tip-to-collector distances, and applied voltage”. S/N ratio, orthogonal L9 arrays with Taguchi design, and variance analysis. Several trials and investigations are planned using ANOVA to observe the best circumstances for synthesising ZnO + PVP. In DoE studies, it was analysed that the PVP concentration is the most crucial determinant of the nanofibre diameter, followed by flow rate. For electro-spun ZnO + PVP nanofibres, an optimal combination was also identified to produce the lowest diameters with the least variance. Interaction plot values were also recommended for experimentation with good interaction and a further selection of parameter values.


Introduction
Electrospinning is a well-known and straightforward technique for creating nanofibres from various materials, such as metals, ceramics, and polymers. The process takes place in a strong electric field, causing the polymer solution to stretch, resulting in micro/nanoscale fibres [1]. Due to its exceptional qualities, including a large surface area/volume, superior aspect ratio, elasticity, and high absorbency [2], this process creates electro-spun nanofibres suited for various industrial applications [2]. Tissue engineering, filtering, medicine, personal care, sensing devices, sound absorbers, energy production, and storage are a few application areas [3][4][5]. By applying an electric charge to a polymer solution, electrospinning stretches the jet of the solution in a powerful electric field to produce nanofibres [3,4]. The Taylor cone is formed when the polymer solution lengthens and widens as it exits the needle, which would be attached to a DC power supply with high voltage [2]. The charged polymer jet extrudes as fibres as a result of the electrostatic force created when the conical droplet overcomes surface tension with an enhanced electric field. Due to the negative charge, thin, stretched fibres move more quickly to the collector's end and gather into webs [3]. The process parameters of electrospinning influence the morphology of nanofibres. There are three groups of parameters: "(a) solution parameters (concentration, fluidity, surface tension, molar mass, and solvent type) and (b) processing variables (applied voltage, flow rate, as well as needle-collector distance), collector shape, and speed, and (c) environmental factors (temperature, humidity, and ambient pressure)" [2, 5]. As a result, optimising and modelling the electrospinning parameters can lead to the desired fibre morphology. The concentration of a polymer solution may be changed to alter the viscosity, which is the most crucial factor determining fibre form, diameter, and beading [2]. According to some studies, higher voltages promote the production of smaller fibres by increasing electrostatic repulsion forces. The flow speed affects the jet's speed and the fibre's diameter. An inferior flow rate is preferable to eliminate beaded threads because the solution has more drying time, and the jet is stretched more [6]. The separation between the needle and the collecting screen influences the electric field's strength, which governs fibre form. Smaller distances allow the solvent to evaporate, causing fibres to stick together and produce larger-diameter fibres. High voltage is necessary for fibre creation over extended distances, however. The spacing must be changed since the solvent drying process alters the geometry of the fibre [7].

1-dimensional zinc oxide nanofibres
Zinc oxide (ZnO) has recently received much attention in the scientific community as a future material. Similar to GaN, ZnO's broad band gap of 3.4 eV and robust excited state binding energy of around 60 meV at room temperature make it essential for blue and ultraviolet optical systems [8]. However, ZnO offers several benefits over GaN in this application, the two most notable of which are its greater exciton binding energy and its capacity to create single-crystal substrates [9][10][11]. The remarkable characteristics of zinc oxide (ZnO) nanostructures, such as their large aspect ratio, large electron mobility, and electrical and optical anisotropy, have generated a lot of attention during the last ten years [12][13][14]. There are numerous ways to produce ZnO, including the solgel process, electrospinning, precipitation in aqueous solution, hydrothermal synthesis, and vapour deposition that enable the production of products with particles of different shapes, sizes, and spatial structures. Electrospinning is one of the most basic, versatile, inexpensive, and successful methods to synthesise 1-dimensional ZnO nanofibres [15].
PVP is a synthetic, biocompatible, water-soluble polymer that shows improved adhesion, lesser toxicity, and higher solubility in a range of organic solvents [2]. By combining PVP with several fillers, such as titanium dioxide (TiO 2 ), tin(IV) oxide, as well as zinc oxide (ZnO) and (SnO 2 ), nanofibres have been created. Several solvents, including ethanol, deionised H 2 O, N, N-dimethylformamide (DMF), methanol, and dichloromethane (DCM), were used to explore how PVP nanofibres were made utilising the electrospinning process [2, 16].

Taguchi design
R.A. Fisher created the design of experiments (DOE), a complex statistical method, in England in the 1920s to investigate the impact of several variables that concurrently provide the greatest variety in the result. Genechi Taguchi worked as a researcher at Japan's Electronically Controlled Laboratory in the late 1940s and did vital DOE research. He put in a lot of time and effort to make this experimental approach more user-friendly (simple to use) before using it to increase the quality of the items produced. Dr. Taguchi's standardised OE, called the Taguchi Technique as Taguchi Approach, was introduced in the USA in the early 1980s [17]. The Taguchi Robust Experiments Technique is a valuable engineering technique for determining the ideal values of processing parameters least susceptible to different sources of variation. Furthermore, this method may highlight the individual and interaction impacts of a huge and complex collection of components. In general, two tools are required: an orthogonal array (OA) to concurrently accommodate various experimental design components and a signal-to-noise ratio (S/N) to determine the most trustworthy set of operating circumstances from changes in the finding [18][19][20][21].
This study aims to pinpoint the electrospinning process parameters that can be altered to reduce the diameter of ZnO?PVP nanofibres which gives a high surface-to-volume ratio, which is well suited for use in dye-sensitised solar cells. High-surface area nanofibres scatter light. Nanofibres scatter DSSC light, prolonging the optical path. Dye molecule photon absorption boosts photocurrent and PCE. Mie scattering theory explains NF light scattering. Nanofibres speed electron transport in DSSCs. Due to the interconnected nanofibre network, dye molecules swiftly transfer electrons to TCO layers. Electron transport maximises photocurrent and DSSC efficiency. Thus, nanofibre-based DSSCs have better PCE due to Mie scattering theory-enhanced light scattering and faster electron transport [22,23]. Utilising Taguchi's L9 orthogonal designs (OA), statistical analysis of variance (ANOVA), with optimisation, the design of experiments (DOE) approach was utilised to determine the electrospinning parameters for ZnO?PVP nanofibres [2]. Voltage, concentration, needle-collector distance, and flow rate are amongst the electrospinning control parameters examined in this study. In addition, Taguchi's Design of Experiments was used to determine the minimum fibre diameter and its variance. As a result, this research can provide precise analysis and understanding of ZnO?PVP fibre diameter affected by electrospinning process parameters [20,24,25].

Materials used
No components are further purified before usage; they are all acquired from Sigma-Aldrich. Polyvinylpyrrolidone (PVP) (C 6 H 9 NO) x utilising a 99% pure LS material as a binder with a relative molecular mass of 1,300,000, N, N-dimethylformamide (HCON(CH 3 ) 2 as a solvent and zinc acetate (CH 3 CO 2 ) 2 Zn with a molecular weight of 183.48 and 99.99% purity as a precursor.

Process parameters
The four elements (A, B, C, and D), or the four primary electrospinning process parameters, were studied using the Taguchi technique: A: PVP in solution, in percent weight/volume, B: Flow rate, in ml/h, C: Distance between needle and collector in cm, and D: Applied voltage in kV.
An approximate value for these critical electrospinning parameters is required before selecting the various values for each parameter. To this end, some early investigations were carried out, approximating data from the literature [15,[26][27][28][29]. The sol-gel technique was used to create the viscous solution in two steps. The first 1.37 g of PVP was added to 5 ml of DMF and stirred at room temperature using a magnetic stirrer for 4 h. Next, PVP was dissolved in DMF and then 0.45 g of zinc acetate was added and stirred for 2 h to get the final solution. To create a spine, this prepared solution is electro-spun. Figure 1 shows the used laboratory electrospinning setup. Electrospinning tests were performed in this study using a commercial structure with a single nozzle and a bottom-up vertical configuration (E-Spin-NANO). The polymer solution pump via a 5-ml syringe has a capacity of 0.010 ml/h and a highvoltage supply range of 0 to 40 kV. A fixed, grounded, circular aluminium collector wrapped with aluminium foil is used to collect electro-spun fibres. During the experiment, the system's ventilation was activated to maintain a temperature of roughly 30°C and humidity levels of about 40% in the cabin. During the experiment, thirty minutes of continuous electrospinning time was employed to gather electrospun fibres like a web.

Electrospinning laboratory setup
The prepared solution is placed in a 5-ml plastic syringe for synthesis. The sample was created with high voltages of 28 kV, a flow rate of 0.5 ml/h, and a distance of 25 cm between both the needle and the collector. To completely remove the organic polymer components, a piece of the synthesised sample was calcined at 450°C in the presence of air for 2 h. Figure 2 shows that the zinc acetate and PVP solution directed good spinning properties and produced smooth and continuous nanofibres. However, the degradation of polymer components causes fibre diameter to shrink during calcination, converting asspun nanofibres into porous nanofibres (minimum diameter 31.58 nm for a pilot experiment).

Taguchi technique parameters and levels
Three different values were chosen for each parameter after preliminary testing. Table 1  The Taguchi L9 Orthogonal L9 (34) was then constructed by determining the number of trials and parameter value combinations to be used in each experiment using the Minitab 21 tool, as shown in Tables 2 and 3. Instead of 81 trials, it was discovered that just nine experiments (E1-E9) would be necessary to acquire optimum process parameters, leading to a considerable decrease in time, labour, and material consumption (Table 3).

ANOVA
The predominant effect of the different controllable parameters was found to use an analysis of variance. Many variables were measured and categorised in the form of a standard table in an analysis of variance, comprising the degrees of freedom, a sum of the squares, means of squares (variance), variance ratio, a pure sum of the squares, as well as the percentage contribution of each parameter [30][31][32].
Total variance (VT): The following Eq. (1) expresses the amount of variance (VT), which represents the sum of the squares of all experimental results [18]: where Ei is the average fibre diameter and N indicates the number of experiments included in the research (Taguchi DoE) [18,20] The total variance of each factor (Vi): At three distinct levels, the sum squares V A , V B , V C , and V D were employed to indicate four factors: concentrations, flow rate, needle-to-collector distances, and voltage. For all elements, C.F. remains constant, and this is the correction factor. The correction factor is used to calculate all sums of squares (C.F) [19].
Percentage contribution (%): The ratio of the overall fluctuation of each P A , P B , P C , or P D component determines its percentage contribution. (V A , V B , V C , as well as V D ) to the total variance (VT) [18]: "where i is the number of factors (in this case, i=4)" [18]. "Signal to noise (S/N)" of electro-spun ZnO?PVP nanofibre diameter: As shown in Eq. (7), the ideal combination factors for ZnO?PVP nanofibres were identified using a smaller is the better characteristic formula to minimise fibre diameter and variation.
where N is the measurements that are taken, S/N is the signal-to-noise ratio, and d is the diameter of electro-spun ZnO?PVP nanofibres [18]. The smaller the variance of ZnO?PVP electro-spun Nanofibres, the bigger the value of S/N [21,32].

Field emission scanning electron microscopy (SEM)
Electro-spun ZnO?PVP nanofibres are made electrically conductive by coating them in layers of platinum using a sputter-coating technique, thereby resolving the surface charge problem. In addition, FESEM was used to evaluate the materials' surface morphologies (FEI Nova NanoSEM 450).

Imaging analysis
ImageJ software was used to evaluate electro-spun nanofibre diameters from FESEM images. First, each fibre's diameter was measured manually perpendicular to the fibre axis. Then, in each FESEM image, allfibre measurements were made, and the mean and standard deviations of the nanofibres were calculated.
4 Results and discussion

Morphology and diameter of nanofibres
Nine different ZnO?PVP electrospinning experiments (E1-E9) were conducted after the Taguchi L9 orthogonal experiment matrix was created. FESEM was then used to determine the average fibre diameter distributions of electro-spun fibres collected for each condition. Figures 3, 4, and 5 show a relatively smooth and almost bead-free electro-spun fibre shape was obtained following each testing condition. Table 4 consists of the average fibre diameters obtained from FESEM pictures (using ImageJ software), the procedure parameters, and their values in each experiment. In addition, the S/N ratios (signalto-noise ratios) were also summarised in Table 4. These values were determined using Eq. 7 connection and the "smaller is better" approach [18,33].
The DoE investigation's frequency contribution graphs in the diameter range between 0 and 350 nm are displayed in Figs. 3, 4, and 5. E7 had the largest average diameter (196.15 nm) and the widest diameter range (0-500 nm) in the DoE study. The big nanofibre diameter was caused by the greatest viscosity levels of the sol-gel solution with a PVP Fig. 2 FESEM image of as-spun PVP/Zinc acetate nanofibres before calcination and after calcination content of 20%, the lowest applied voltage (28 kV), as well as a greater flow rate (0.4 ml/h). The dispersed diameter fibres for E8 were observed because stretching the fibres becomes more challenging and unstable as the viscosity of the polymer solution rises. However, the average diameters for E8 and E9 nanofibres were 138.7 nm and 159.5 nm, respectively.
The sol-gel concentrations in E8 and E9 are the same as in E7, but the electrospinning parameters differ. The needle tip-to-collector distance (28 cm) and flow rate (0.2 ml/h) for E8 contributed to the decrease in fibre diameter. E6 had the smallest mean diameter (35.34 nm) in the DoE study. The primary causes of the decreased nanofibre diameter were the lowest solution feed rate (0.2 ml/h) and the greatest applied voltage (32 kV).
However, fibre breaks down when the flow rate is low, and the applied voltage is high. For E1, E2, and E3, the 14-wt% PVP concentration was the primary factor in the decrease in diameter size, despite their tip-to-collector spacing, the applied voltage, and the flow rate. Figure 6 displays the influence of

Results of nine Taguchi experiments
and analysis of variance (ANOVA) Figure 7 gives each factor's influence which is shown as a percentage. The most crucial component in fibre diameter was PVP content, which accounted for approximately 68.80% of the variation. Solution flow rate is the second most effective parameter at 18.87%; however, compared to PVP concentration, the applied high voltage and needle-to-collector distance had minimal influence on fibre diameter. Applied voltage and needle-to-collector distance proved negligible for fibre diameter, with 10.87% and 1.439% percentage contributions, respectively. The final two variables have minimal effect on electro-spun nanofibres; nonetheless, this circumstance may be exploited to adjust the amounts of these two variables as required. Figure 8 shows an interaction plot with one axis for one variable's levels and a distinct line for each level of the other variable's means. The dependent variable is represented on the Y-axis. The connection between concentration, applied voltage, flow rate, and the separation between the needle tip and the collector is depicted in the interaction diagram. The interaction of the factors is used for future prediction or selection of the element.
It was revealed that raising the PVP concentration, extending the needle tip-to-collector distance, and employing a voltage setting whilst reducing the flow rate all resulted in a drop in fibre diameter, which is consistent with earlier research [6,18,[34][35][36].
The delta number in Table 5 reflects the gap between the highest and lowest S/N ratio. The rank data illustrate how important each processing parameter is based on how large the delta values are ranked from most significant to the most minor. Thus, it can be shown that the concentration of the PVP solution appears to be the most critical processing parameter for the shortest fibre diameter, with the flow rate coming second.
The same software was used to assess the level of contribution of each processing parameter to a variance in the average diameter for electro-spun fibres using the analysis of variation (ANOVA) approach, using the S/N ratio method of the Taguchi method. As a result, it is feasible to compare the rank data from the S/N response table to the % contribution data acquired using the ANOVA technique.   Table 6 summarises "the ANOVA calculations in terms of degrees of freedom (DOF), the adjusted sum of squares (Adj SS), adjusted mean squares (Adj MS), and percentage contribution". The DOF in this method equals "the number of levels minus one" for each parameter. [21]. "Adj SS is the square of the deviation from the overall mean for each parameter, whilst Adj MS is calculated by dividing Adj SS by the corresponding DOF value" [21]. Lastly, the percentage influence of each parameter can be calculated as the ratio of the Adj SS to the total Adj SS multiplied by a factor of one hundred. Table 6 demonstrates that 68.80 percent of the variation is due to Parameter A (Polymer Solution Concentration) of the total variance in electro-spun fibre average diameter, and parameter D (Distance from Needle and Collector) accounts for 1.439% of the total variation in electro-spun fibre average diameter. Consequently, the percentage contribution statistics from the ANOVA methodology agree with the critical ranking data from the Taguchi S/N response method.

Interaction plots
The lines in this interaction diagram are not parallel. This interaction effect demonstrates that the applied high voltage, flow velocity, and distance between the needle tip and the collector influence the relationship between concentration and fibre diameter. Furthermore, the flow rate and the distance between the needle tip and the collector affect the relationship     between the applied high voltage and the fibre diameter [21]. The distance between the needle tip and the collector also influences the connection between flow rate and fibre diameter [21].

An optimum combination of factors
The S/N ratios for electrospinning ZnO?PVP nanofibres in Taguchi's nine DoE investigations were computed using Eq. (7), Table 4. Because the purpose of this study is to locate the shortest nanofibre with the lowest variation, the greatest S/N ratio from the Taguchi DoE work provides the ideal condition. As shown in Table 4 To find the optimal level factor for its contribution towards reducing the mean diameter and standard deviation of electro-spun ZnO?PVP nanofibres, the average S/N ratio was computed, as shown in Fig. 9. According to the results, a minor variation in tiny electro-spun ZnO?PVP nanofibre can be achieved using the following parameters: PVP concentration of 17 wt%, an applied voltage of 32 kV, a solution flow rate of 0.2 ml/h, and needle tip-to-collector distance of 25 cm (A 17 B 32 C 0.2 D 25 ).
The results also show that the concentration of PVP and the flow velocity of the solution are the two most crucial parameters. In contrast, the applied voltage and the distance from the needle tip to the collector seem insignificant [21].

Confirmation test
After choosing the optimal design parameters, the final step is to validate the result by comparing the theoretical values calculated using the regression equation and the experimental values. The regression equation for the average fibre diameter achieved is given as follows: Achieved average fibre diameter=67?13.84* concentration-9.09 * Applied high voltage?231* Flow rate?0.49* Distance between needle tip to collector. Table 7 shows a comparison of theoretical and experimental fibre diameter values. Again, with a confirmation accuracy of 98.85%, the agreement between the expected and actual reaction is high.

Conclusion
The impacts of PVP content, flow rate, needle tip-tocollector distance, and applied voltage were investigated across three levels at the nanofibre diameter of electro-spun ZnO?PVP using Taguchi DoE analysis. Two key parameters, namely the PVP content and the flow rate, were shown to control the tiny nanofibre diameter with a minor change. The regression equation developed had good significance with the experimental values. Good interaction is predicted by the parametric values selected and the experiment performed. Specifically, we found that the optimal conditions were a PVP concentration of 17 wt%, an applied voltage of 32 kV, a solution flow rate of 0.2 mL/h, and a needle tip-to-collector distance of 25 cm (A 17 B 32 C 0.2 D 25 ).

Author contributions
All authors contributed to the study's conception and design. HVM contributed to the conceptualization, methodology, and writing of the original draft. HVM and KYK contributed to data curation and formal analysis. HVM and NN contributed to the methodology and editing of the original draft. KYK and NN contributed to data curation and revision of the original draft. HVM and NN contributed to the formal analysis and revision of the original draft. KYK and NN contributed to the writing, reviewing, and editing of the original draft.

Funding
Open access funding provided by Manipal Academy of Higher Education, Manipal. This research has not received external funding.

Data availability
All data and material collected are presented in the manuscript. Clarification on any matter can be made through the corresponding author.

Declarations
Conflict of interest The authors declare no competing financial interest.

Open Access
This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://c reativecommons.org/licenses/by/4.0/.