Electrohydrodynamic casting or electrospinning is a simple, versatile, economic, and efficient strategy in preparing fibers in nanometer scales for various polymers and oxidized metals [1
]. To incorporate oxide nanoparticles into electrospun fibers, metal- organic compounds are typically used. Titanium oxide nanoparticles can be prepared through the hydrolysis reaction of titanium isopropoxide in humid air with highly concentrated acetic acid as a stabilizer. Titanium oxide is manufactured in the industry as a main functional component due to its high photocatalytic activity [2
]. Examples of its applications include in solar cell technology and environmental science [3
]. The combination of the excellent properties of nanostructured titanium dioxide and its high surface area gives the titanium-oxide-containing nanofibers vast applicability in cosmetics, scaffolds for tissue engineering, catalytic devices, sensors, solar cells, and optoelectronic devices. During electrospinning, the electrostatic charging of the droplet results in the formulation of the well-known Taylor cone. This allows for nanofiber formation from a mixture of solutions with multiple components. To examine the photovoltaic responses of the resultant fiber, the fiber itself must be conductive. Therefore, partially carbonizing the precursor through a series of heat treatments has been performed as shown in earlier studies [4
]. The high photoactive phase of the TiO2
nanofiber is anatase, and it makes the nanofiber viable for application in sensors, solar cells, etc. [9
]. In general, a higher degree of molecular orientation in the original PAN precursor fiber results in carbon fibers with better mechanical properties—particularly the tensile modulus. Based on this fact, the polyacrylonitrile (PAN) and dimethylformamide (DMF) solution was considered the base polymer for forming the nanofiber [10
]. However, the PAN and DMF solution itself is an excellent material to form porous nanocomposites through the electrospinning process. By incorporating the titanium dioxide into the fibers, enhancement on the photosensitive characteristic should be possible.
It has been reported that graphene-based materials can be used as the counter electrode or photoanode for making dye-sensitized solar cells (DSSCs) [11
]. Specifically, the photovoltaic performance of the DSSCs consisting of graphene composites with carbon nanotubes (CNTs), titanium dioxide (TiO2
), and other substances, such as organic semiconductors and ionic liquids, were compared with standard reference solar cells. One of the interesting findings was that the graphene TiO2
nanocomposite anode had better photovoltaic properties than the commonly used pure TiO2
photoanode. Uddin et al. [12
] made solid-state dye-sensitized photovoltaic micro-wires (DSPMs) with carbon nanotubes yarns (CNYs) as the counter electrode. Through optimizing the numbers of CNYs and CNYs–TiO2
interface, the open circuit voltage and current density of the micro-wire solar cells. The advantages of the cells include: high strength, high flexibility, and excellent electrical conductivity. But the energy conversion efficiency remains to be increased.
This work presents the usage of a new composite fiber: TiO2 coated carbon fiber for making a flexible photoelectric energy converter. The TiO2 nanoparticles were sporadically coated at the fiber surface to form a composite fiber. This composite fiber showed fast response to visible light which allowed us to measure the open circuit voltage. The microstructure of the composite fiber was examined and its composition was analyzed by the scanning electron microscopy (SEM).
2. Materials and Methods
Polyacrylonitrile (PAN) with an approximate molecular weight of 150,000 was supplied by Scientific Polymer Products, Inc., Ontario, NY, USA. Dimethylformamide (DMF) was purchased from Alfa Aesar, Ward Hills, MA, USA. Titanium (IV) isopropoxide (95%) was also purchased from Alfa Aesar. A high voltage direct current power source was supplied by Spellman, Inc., Hauppauge, NY, USA. A Fusion 200 precision syringe pump was purchased from Chemyx, Inc., Stafford, TX, USA. An OTF-1200X compact split tube furnace made by MTI Corporation, Richmond, CA, USA was used for heat treating the prepared materials.
Electrohydrodynamic casting or electrospinning was the process used to fabricate the fibers in the experiment. This method has the advantage of easy deposition and is versatile for the manufacturing of polymeric materials, composites, and ceramics [13
]. This process provides sufficient electrostatic force by applying a very large DC voltage difference between the solution and collector to overcome and surface tension of the PAN and titanium isopropoxide solutions, which made the titanium oxide particle-forming solution and the PAN solution co-spun or co-cast onto the collector, and formed a composite microfiber in the reaction spinning process. Figure 1
a shows the layout of the equipment and Figure 1
b illustrates the spinneret for the fiber processing.
The following forms of materials were used to make the solutions for this process. Polyacrylonitrile polymer (PAN) was supplied in powder form. Dimethylformamide (DMF) was used as the solvent for PAN. Titanium oxide (TiO2) (Cal Poly Pomona, Pomona, CA, USA) was made in nanoscale powder from the hydrolysis of titanium metal-organic compound. Acetic acid (CH3 COOH) (Alfa Aesar, Ward Hill, MA, USA) was used as the stable agent. The first step was to prepare the titanium oxide powder mixed into acetic acid with no critical percentage ratio because the solution would absorb water from ambient air. Roughly 10 mL of ethanol was used to dissolve about 1.0 g of titanium isopropoxide. Then, several drops of acetic acid were added into the solution as the stabilizer. The dissolving process took 4 h in a test tube mixer to ensure the solution quality. The next step was to prepare the PAN solution, with a 1 to 9 ratio; 1.0 g of polyacrylonitrile polymer per 9 mL of dimethylformamide solvent to obtain the concentration of 10% PAN solution. This mixing process was done in the test tube mixer.
With the completion of preparing two solutions, the PAN and titanium isopropoxide solutions were filled into two separated syringes. These two 15 mL syringes were connected with a coaxial spinneret. The PAN-DMF core fluid was pumped by a Chemxy precision syringe pump and the syringe was connected to the 20-gauge inner needle. The syringe that contained titanium isopropoxide solution was attached to a flexible PVC tube and a 16-gauge spinneret. Both needles were bonded together to form a coaxial spinneret setup, which is shown in Figure 1
b. Since the titanium isopropoxide solution formed the outer-layer flow, it reacted with water in the air to produce titanium hydroxide. This setup provided a structure where the titanium isopropoxide would generate a titanium hydroxide-containing shell layer on the PAN-DMF jet.
When the fiber was finally produced, it was necessary to go to the second step—i.e., the heat treatment step of the experiment. Before putting these two syringes into the programmable precision syringe pump, the air was purged from both of the syringes to ensure the consistency of flow rate of solution. A distance of 15 cm was kept between the tip of the needles and the fiber collector plate. A DC power supply was linked in between the tip of the coaxial spinneret and the final collector plate. This power supply was connected to a high voltage AC-DC converter, which had its positive lead on the spinneret, and negative lead on the collector plate. This setup facilitated the electrospinning process under room temperature and generic atmospheric conditions. The injection speeds of both syringes were adjusted between 0.001 to 0.01 mL/min depending on the collecting condition. A DC voltage of 15 kV was applied to electrify the solutions, and this potential difference led the charged jet to cast fibers on the collector plate. The resultant microfiber was sprayed and collected on a tissue paper which was placed on the stainless-steel collector plate, which gave the advantage of separating the fabricated fiber easier than itself. The entire electrospinning and collecting process took approximately 2 h so that the fiber mat could have the desired thickness and strength for the later heat treatment process. It was measured that the collected fiber mat samples after electrospinning reached a thickness of 400 µm. The fiber has an average diameter in the micron meter range. The yield of the electrospinning process is about 90%.
Heat treatment of the fabricated microfiber caused oxidation of PAN and complete dehydration of titanium hydroxide by setting the specimen into a quartz tube furnace at 250 °C for 2 h. The air in the quartz furnace was replaced with hydrogen after the heating temperature was raised to 500 °C and the heating process continued for an additional 2 h before cool down. At this high temperature in hydrogen, the PAN fiber was converted into a partially carbonized state. Also, the titanium hydroxide was completely converted into dioxide. During the cooling process, the specimen remained in its position inside the furnace tube; this process could take different amounts of time due to different ambient temperature. Figure 2
represents the complete time-temperature profile associated with the heat treatment process. After the heat treatment, the average diameter of the fiber was reduced significantly.
The photovoltaic tests were performed on the titanium dioxide coated carbon fiber composite. A fluorescent light tube was used to generate visible light. A CHI440C Electrochemical Workstation, made by CH Instrument, Austin, TX, USA was used to record the voltage response data. The sample fiber was set on transparent glass slides and aluminum foil strips were used as the electrical conducting path. The light source was controlled as 25 s ON followed by 25 s OFF. The test results were obtained by plotting the open circuit potential versus time.