Using the Box–Benkhen design (BBD) to minimize the diameter of electrospun titanium dioxide nanofibers
Introduction
Traditional methods of catalyst development and nanoparticle manufacture are based on rational approaches such as literature data and experimental design. However, variable optimization can be accomplished by full or fractional factorial designs [1], [2]. Statistical experimental design methods are routinely used to evaluate the impact of variables on chemical processes and particularly in processes where the variables influence each other [3], [4], [5]. An experimental design based method is a compilation of mathematical and statistical tools for designing experiments, constructing numerical models, evaluating the effects of variables and searching for the optimum combinations of variables [6], [7], [8]. Experimental design techniques such as response surface methodology (RSM) follow a sequential approach wherein the first step is to screen the independent variables (factors) and their levels. The second step is to build the response surface model using an appropriate experimental design method [6], [7], [9], [10]. The third step involves estimating the coefficients of the mathematical model and in the final step, the accuracy of the response is assessed using experimental data.
Among the different experimental design methods, a Full-Factorial design (FFD) is often considered impractical due to its requirement of a large number of experiments. Based upon the desirable feature of accurate prediction throughout the factor space Central-Composite design (CCD) and Box–Benkhen design (BBD) are commonly selected experimental design procedures [6], [9], [10]. However, for a quadratic response surface model with three or more factors, the BBD procedure is much more advantageous when compared to the CCD [6], [7], [11]. In the case of TiO2 nanoparticle preparation, factors controlling the particle size include surfactant, reaction temperature, calcination time and calcination temperature [12]. Establishing optimum process conditions for preparing TiO2 nanoparticles have been accomplished using factorial design [12], [13]. Hence, a similar approach can be adopted for the manufacture of TiO2 nanofibers.
In comparison to other treatment technologies, TiO2 photocatalysis is a low cost method for converting recalcitrant organic pollutants into CO2 plus water [12], [13]. The reaction is mediated by the simultaneous action of TiO2 nano-photocatalyst plus ultraviolet (UV) light. The efficiency of heterogeneous catalysts such as a photocatalyst is dependent upon the number of active sites per unit mass of catalyst interacting with the substrate molecules [14], [15]. A catalyst with a large number of catalytic active site per unit mass is highly efficient in its ability to mediate chemical reactions [14], [15], [16], [17]. In supported metal catalysts, the number of active sites per unit mass is correlated with the specific surface area (SSA) (m2 g−1) [14], [15], [16], [17]. Consequently, a major task is to manufacture high surface area TiO2 nanofiber catalysts by assessing the effect of process variables on SSA.
According to several reports, nanometer-size TiO2 formulations, primarily in form of slurry, have been manufactured and tested for their photocatalytic potential [18], [19], [20], [21], [22]. However, using nanoparticles in form of a slurry is associated with many practical constraints. These problems include solid/liquid separation for catalyst removal and recycling and implementing measures to minimize human health hazards associated with fugitive emissions during slurry preparation [22], [23], [24]. Immobilizing TiO2 nanoparticles onto a solid support can potentially eliminate many problems associated with the use of nanoparticles in the form of slurries [15], [23].
Dispersing nanometer-sized particles on a high surface area support is a popular method of producing a high surface area supported catalyst system [23]. However, a major bottleneck of this method relates to the loss of surface area due to sintering or aggregation of the nanoparticles during thermal treatment [23]. Particle sintering results in a supported catalyst with a surface area less than that of discrete nanoparticles by a few orders of magnitude [15]. Hence, another key research task is to develop an immobilized TiO2 nanocatalyst system with surface area comparable to that of discrete nanoparticles.
Fabricating ultra thin nanofibers catalysts from a broad range of polymers and polymer blends has been accomplished using the electrospinning technique [25], [26]. Recently, electrospinning coupled with the sol–gel synthesis technique has been used to fabricate TiO2 nanofibers [27], [28], [29]. According to many reports, the diameter, surface morphology and crystal structure of electrospun TiO2 nanofibers are affected by the characteristics of the spinning solution, electrospinning process variables and thermal treatment conditions [27], [30], [31], [32].
Several studies have described methods for preparing and characterizing TiO2 nanofibers and nanoparticles [33], [34]. Elecrospinning of nanofibers is a well established process which is influenced by the potential difference, the separation distance between electrodes and viscosity [27], [35], [36], [37]. According to Watthanaarun et al. [30], the formation of small diameter nanofibers can be accomplished using high potential differences (voltages). The viscosity of the electrospinning solution beyond a specific range can also affect the formation of nanofibers [35]. However, if the viscosity is maintained within the range of 130–160 centipoise (cps), fiber formation is unaffected [38]. Evidence describing the effects of separation distance between the electrodes (nozzle to collector) as well as the impact of solution flow rate on nanofiber formation has been provided by Thompson et al. [36] and Evcin and Kaya [37]. Evcin and Kaya [37] reported decreasing nanofiber diameter with decreasing flow rates while according to Thompson et al. [36], decreasing diameters were correlated with increasing separation distances between the nozzle to collector. The main driving force for producing high SSA anatase TiO2 catalyts for enhanced photocatalytic performance [31], [32], [39], [40] is related to the improved performance of these nanosize supported systems.
The objectives of this study are as follows: (1) evaluate the effects of Ti content, potential difference, infusion rate and separation distance of electrodes on nanofiber diameters diameters and (2) develop a BBD based response surface model to predict and optimize the diameter of TiO2 nanofibers.
Section snippets
Electrospinning apparatus
The main components of the electrospinning apparatus were a pumping system and a variable DC power supply (Fig. 1). The programmable syringe (PHD 22/2000, Havard Apparatus Canada, St. Laurent, QC) pumping system (configured with a 10 ml luer-lock plastic syringe (Becton Dickinson, Oakville, ON)) was fitted with a 22 gauge (0.7 mm outer diameter (OD), 0.4 mm inner diameter (ID)), 38 mm stainless steel hypodermic needle with a polypropylene hub (Becton Dickinson, Oakville, ON). The pumping system was
Effect of electrospinning process variables on nanofiber diameters
The effect of three experimental factors, potential difference, infusion rate and separation distance on the diameter of the electrospun TiO2 nanofibers were evaluated at the different experimental levels (Table 1). Ejection of the fluid jet and subsequent formation of the nanofibers involves a complex force balance [47], [48], [49]. The potential difference imposed an electrical polarization stress on the fluid drop at the needle tip and this caused the drop to become elongated. At some
Conclusions
A response surface model based on the BBD technique was developed to predict the diameter of TiO2 nanofibers produced by the sol–gel electrospinning technique. Three preliminary experimental factors considered in this study included potential difference, infusion rate and separation distance. The predicted diameters were consistent with the diameters observed experimentally except at a potential difference of 25 kV and an infusion rate of 0.6 ml h−1. Due to increasing process instability, the
Acknowledgements
Financial support for this work was provided by the Natural Sciences and Engineering Research Council of Canada (NSERC) and the University of Windsor. The authors are grateful to the Great Lake Institute of Environmental Research (GLIER) at the University of Windsor for providing access and assistance with the FESEM microscope and BET surface area analyzer.
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