Fabrication of TiO 2 nanofibre photoelectrode for photoelectrochemical cells

. The TiO 2 nanofibres (NFs), prepared with the electrospinning method, acted as the photoanode in a photoelectrochemical cell (PEC) for hydrogen generation. The fabrication parameters of Ti/PVP (polyvinylpyrrolidone) fibres were determined with the field-emission scanning electron microscopy (FE-SEM) method. The structure and morphology of the TiO 2 fibres were characterized by using X-ray diffraction (XRD), FE-SEM, transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HR-TEM). The average diameter of the TiO 2 fibre is 132 ± 16 nm. A three-electrode potentiostat was used to study the photoelectrochemical properties of the photoanode. The density photocurrent reached the saturation value of 80  A·cm – 2 at 0.2 V under the irradiation of a Xenon lamp.


Introduction
Many scientists believe that photochemical water splitting to generate hydrogen is one of the most promising technologies because it is based on converting water with sunlight, an available endless energy source [1]. A photoelectrochemical cell (PEC) is a device integrating light absorption on semiconductor materials and electrochemical processes in the cell. After absorbing the appropriate light photon, oxidation and reduction reactions occur on electrodes to separate water into hydrogen and oxygen [1]. Fujishima and Honda were the first to report the water separation with a PEC by using a titanium dioxide (TiO2) electrode [1]. In PEC technology, the semiconductor material used as a photoelectrode must satisfy the following two basic conditions.
First, the bandgap (Eg) must be greater than 1.23 eV, which is the energy needed for splitting water molecules [1]. Second, for the reaction to occur without applying an external voltage, the conductor band bottom must be higher than the water oxidation level, and the valence band peak must be higher than the hydrogen generation potential. Despite having a large Eg, titanium is still one of the most attractive materials for PEC technology because of its stability to corrosion and photo corrosion, low cost, high availability, and low toxicity [2]. It has the conduction band bottom and valence band top position within the oxidation-reduction potentials of H2O and thus satisfies the requirements for the water splitting process.
The optical conversion efficiency of the PEC depends primarily on the photoelectrode material.
An ideal photoelectrode with high efficiency and stability must satisfy the following requirements:

Preparation of ITO substrate
Cut the ITO (indium tin oxide, Shenzen, China) sheet into small pieces of about 2 cm 2 (2 × 1 cm).
They were cleaned ultrasonically in acetone for 30 min, rinsed with distilled water for 15 min, and dried at 50 °C. We found the optimal conditions as follows: TTip/PVP volume ratio 2:1, spray rate 0.4 mL·h -1 , and electric field 0.6 kV·cm -1 .

Sample manufacturing process
To investigate the photoelectrochemical properties of the material, we sprayed the TTip/PVP fibres on the ITO substrate for 20 min.
After spraying, the electrodes were calcined at 500 °C in the air for 2 h with a heating rate of 2 °C·min -1 . We obtained TiO2/ITO nanostructured fibre electrodes.

Characterization
The

Photoelectrochemical measurement
The PEC properties were investigated by using a three-electrode electrochemical analyzer All the measurements were performed with frontside illumination of the photoanodes. The potential was swept linearly at a scan rate of 10 mV·s -1 with the potential range from -0.6 to 0.6 V (vs Ag/AgCl). The electrode illuminated area exposed to the electrolyte was fixed at 1 cm 2 by using nonconductive epoxy resin. The conversion efficiency was calculated following the equation where Jp is the photocurrent density (mA·cm -2 ); I0 is the irradiance intensity of the incident light The images show that the 3:1 ratio (Fig. 1a) does not form fibres. The fibres are stuck together because of the small polymer portion in the spray solution. At the 2.5:1 ratio (Fig. 1b), titanium salt begins to form several short fibres. The fibres form more clearly at the ratio 2:1 (Fig. 1c) and 1.5:1 (Fig. 1d). However, at the ratio of 1.5:1, the fibres are uneven because of the liquid's high viscosity that hinders the spraying process. We also reduced the ratio to 1:1, but the viscosity   (Table 1) and displayed the diameter distribution (Fig. 3).

Effect of electric fields
It can be seen that the diameter of Ti 2+ /PVP fibre decreases as the electric field increases. This decreasing trend is consistent with that of previous research [10,11]. We found that at the 0.4 kV·cm -1 electric field, the fibre does not form.
Thus, the electric field of 0.5 kV·cm -1 is neccessary to create inductive charges in the spray solution and causes the electrostatic force to form fibres.   Although the fibre diameter is relatively large (273 nm) at the electric field of 0.6 kV·cm -1 , it is highly uniform (relative error 6.6%), and spinning does not break. Low et al. [11] reported that it was possible to fabricate fibres with an average diameter of 70 nm at an electric field of 2.2 kV·cm -1 , but the diameters were uneven (about 50% relative error). Therefore, we choose the electric field of 0.6 kV·cm -1 for the fabrication of photoelectrodes.

Effect of spray rates
The SEM images of Ti 2+ /PVP materials at different spray rates are shown in Fig. 4. At the spray rate of 0.3 mL·h -1 (Fig. 4a) and 0.4 mL·h -1 (Fig. 4b), the fibre diameter is relatively uniform, and the spray is continuous. When the spray rate increases to 0.5 and 0.6 mL·h -1 , the fibre diameter becomes smaller and uneven. The higher the spray rate, the more uniformity of the fibre diameter decreases, and the spraying process is interrupted. This  These results are consistent with some previously published results [9,11]. According to Kim, when the heating temperature increases to 600 °C, there is a transition from the anatase to rutile phases [12]. Because the ITO substrate used in the experiment breaks at 550 °C, we did not investigate the phase transition of TiO2 fibres according to the calcining temperature.  particle has a size of 12 nm (Fig. 8a). The lattice planes were determined from the HR-TEM image (Fig. 8b). The lattice planes corresponding to the TiO2 anatase crystal are (101) (d101 = 0.35 nm) and (004) (d004 = 0.24 nm). This result is completely consistent with those of the X-ray diffraction above.  (Fig. 9b). The film thickness corresponding to 20-min spraying is 3 m (Fig.   9c). The highest optical current density of the sample in this study is roughly equivalent to that of some published TiO2 structures [9,13].

Conclusion
Our study shows that TiO2 nanofibre structure