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Similar to reduction, the duration of the sample oxidisation changes the thickness of the reoxidised TiO2 layer. With a sandwiched structure, the photogenerated electrons can travel through the reduced layer to the base of the electrode, while the holes stay in the outside layer for the oxidation of water. This arrangement can facilitate effective charge separation and the decreasing of electron-hole recombination. Fig. 6.10 shows the TNRs reduction, re-oxidation process and the sandwiched structure formation.

Figure 6.10 A schematic diagram shows a single TNR with the reduction and re-oxidation process.

To study the crystal structure phase changes during the oxidation treatment, XRD spectra were collected from the pristine TNRs and reduced samples at 350°C for 30 minutes and re-oxidised in air at 300°C for different time periods are shown in Fig. 6.11.

Figure 6.11 XRD patterns of the rutile TNR films on FTO substrates which were annealed in air (for adhesion improvement), reduced in hydrogen at 350°C for 30 minutes and re-oxidised in air at 300°C for different times. (a) FTO, (b) reduced rutile TiO2(c) 30 minutes oxidation (d) 60 minutes oxidation (e) 90 minutes oxidation and (f) 120 minutes oxidation.

There is no obvious phase transition observed in the diffraction patterns, since the oxidation temperature is too low. However, intensities of the rutile peaks are enhanced slightly through the oxidation stages and peaks are narrowed slightly, which is related to improvement in film crystallisation with increasing the oxidation time. The sample re-oxidised for 60 minutes showed different trend which can be referred to the bad film quality.

The corresponding morphology of the TNRs after re-oxidation at 300°C for different durations is presented in the SEM images in Fig. 6.12.

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Figure 6.12 Typical SEM images of top views of rutile-TNR films grown on FTO which were annealed in air at 550°C for 3 hours ( for adhesion improvement), reduced in hydrogen at 350°C for 30 minutes and re-oxidised in air at 300°C for different times. (a) Pristine (b) 30 minutes reduction followed by (c) 30 minutes oxidation (d) 60 minutes oxidation (e) 90 minutes oxidation and (f) 120 minutes oxidation. The scale bar is 2 µm.

Compared to the pristine sample (Fig. 6.12a), the SEM images show a slight increase in the nanorod diameter after annealing in hydrogen at 350°C for 30 minutes (Fig. 6.12b). The average nanorod diameters of the pristine and reduced samples were 187 and 197 nm with FWHM of 30 and 25 nm respectively. This can be due to the increase in the defects density in TiO2 structure. A diminishing in the nanorod diameter is occurred by oxidising the reduced sample for different annealing times in air. The average rods diameter for the re-oxidised samples at 300°C for 30, 60, 90 and 120 minutes were 170, 150, 115, 97 and 70 nm with FWHM of 30, 35, 42, 50 and 64 nm respectively.

This suggests that there is an evaporation of outer shell of disordered TNRs during the re-oxidation process. That is could be attributed to two factors; since the tip of the nanorods structure are separated while the bottom is more packed and directly attached

to the substrate, a temperature gradient from the bottom to the top will occur. In addition, the difference in the density between the top and the bottom of the nanorods allows the tips to be more exposed to the environment than the bottom, therefore the evaporation more likely to occur from the top part of the rods. The decrease in the rod diameter was also associated with decrease in the film density. Based on the SEM images, The TNRs density in this work was calculated by counting the number of TNRs using Image J software, averaged from 20 μm² areas. The densities of the pristine, reduced samples at 350°C for 30 minutes and re-oxidised at 300°C for 30, 60, 90 and 120 minutes were roughly estimated to be 17, 16, 14, 13, 12, 11 rods/μm². This observation confirms the evaporation of disordered TNRs at the tips.

The photocatalytic performance of the re-oxidised samples was also tested in a standard PEC water splitting device. The reduced film with the highest efficiency, reduced at 350°C for 30 minutes, was used for the re-oxidation procedure. Fig. 6.13a displays the I-V measurements of the untreated and reduced sample. Re-oxidation for 60 and 90 minutes increases the produced photocurrent from 2.2 mA cm^-2 to 2.84 mA cm^-2 at 0 V vs Ag/AgCl. The PE plot Fig. 6.13b shows the increase of PEC efficiency, with a maximum value of 1.89 %. Further increasing the oxidation time to 120 minutes, both the photocurrent and PEC efficiency decrease. This observation suggests that for 60 and 90 minutes’ calcination in air, the surface of the reduced TNRs was re-oxidised. There is a residual reduced layer sandwiched between the surface and core low defects TiO2. Such a structure is important for improved charge separation as the photoexcited electrons can be easily diffused to the reduced layer and effectively separated from the surface oxide, where the hole will oxidise the water. Further increase the re-oxidisation time will destroy the reduced layer and resulting reduced electron conductivity. This is responsible for the decrease of photocurrent and PEC efficiency.

Figure 6.13 The PEC performance of untreated and treated TNR films. (a) I-V curves of the reduced TNR film at 350°C for 30 minutes and oxidised TNR films at 300°C for different times. (b) Calculated photoconversion efficiency of the re-oxidized TNR films in air as a function of the oxidation time.

The EIS measurements were carried out to estimate flatband potentials, VFB and charge carrier concentrations of the re-oxidised samples in 1 M KOH electrolyte in dark at a fixed frequency of 1 kHz. Fig. 6.14 shows The Mott-Schottky plots of the reduced sample at 350°C for 30 minutes and re-oxidised at 300°C for 60, 90 and 120 minutes. The re-oxidised samples for 60, 90 and 120 minutes exhibited VFB of -0.66, -0.67 and -0.72 V. The shift of the VFB towards high negative values for the n-type semiconductor suggests an increasing of band bending and width of space charge region. The direct consequence of this would reduce the charge recombination ratio, since the space charge field strength is increased. This could result in an increase of photocurrent and photoefficiency. We believe that our unique sandwiched TNR structure is important which overcomes the reduced charge separation due to the reduced band bending.

Figure 6.14 M-S plots of reduced TNR arrays at 350°C for 30 minutes and re-oxidised for different times, measured in 1 M KOH solution (pH 13.6) in the dark. The amplitude of the sinusoidal wave was set at 10 mV at a fixed frequency of 1 kHz.

The charge carrier densities of the reduced sample at 350°C for 30 minutes before and after being oxidised at 300°C for 60, 90 and 120 minutes were 8.0×10¹⁸, 7.6×10¹⁸, 6.2×10¹⁸ and 6.0×10¹⁸ cm^-3 respectively. When the film has undergone reduction only, the resulting carrier density is at a maximum. This quantity decreases with increasing exposure time to the oxidising atmosphere. By increasing the oxidation time, the relative thickness of the reduced layer decreases which lowers the oxygen vacancy concentration.

This is the reason for the decrease in charge carrier density during oxidation. After re-oxidation for 120 min, the charge carrier density is very close to the value of the pristine sample. Thus, by then, we could expect that reduced TNRs were fully oxidised.

Therefore, it confirms that the sandwiched reduced layer disappeared after 120 min oxidation, which results in the reduction in apparent electron conductivity.

6.5 Conclusion

This chapter reports a simple and effective method for enhancing the performance of TNRs arrays as photoanode material in PEC water splitting. This study considered the surface treatment of TiO2 crystals through a reduction in hydrogen atmosphere and then

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