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The PEC water splitting performances of the undoped and doped ZnO NRs were investigated. Figure 5.9A represents the photocurrents generated from pure ZnO (red curve), Ni-doped ZnO (pink curve), Co-doped ZnO (blue curve), K-doped ZnO (green curve) and Na-doped ZnO (black curve) under simulated sunlight (100 mW cm-2 with an AM1.5 G filter) between potentials of 0.5 and 1.0 VAg/AgCl together with the dark

current measured from Na-doped ZnO NRs (orange curve) in 1.0 M KOH electrolyte (pH 13.6).

Figure 5.9 (A) Photocurrents generated from pristine and doped ZnO NRs in 1.0 M KOH electrolyte (pH 13.6) under simulated sunlight of 100 mW cm-2 _{with an AM1.5 G}

filter and (B) the corresponding output power and photoconversion efficiency under AM1.5 G full solar spectrum irradiation.

In comparison to the pure ZnO NRs photoanode, it is obvious that all the doped ZnO NRs demonstrated a significant improvement in the photocurrent density. At a typical potential of 1.0 VAg/AgCl, the photocurrents generated from pure ZnO, Ni-, Co-,

K- and Na-doped ZnO NRs photoanodes were measured to be 0.42, 0.48, 0.58, 0.62 and 0.90 mA cm-2, respectively. It is worth noting that the onset potentials of all the doped

ZnO NR photoanodes are more negative than that of pristine ZnO NR photoanode. The onset potentials of pure ZnO, Ni-, Co-, K- and Na-doped ZnO NRs were 0.35, 0.39,

0.38, 0.39 and 0.42 VAg/AgCl, respectively. The onset potential is determined by the

slow kinetics of water oxidation, where it is used to remove four electrons and four protons from two water molecules to form one molecule of oxygen. Therefore, the photogenerated electrons and holes will accumulate at the surface of the electrode and surface recombination will occur until sufficient positive potentials are applied.79 For the doped ZnO samples, the shift of the onset potentials to a more negative value can be attributed to the created defects (oxygen vacancies) of the electrodes after the introduction of dopants, causing in lowering the kinetic barrier to interfacial charge transfer.79 Therefore, a smaller overpotential will be required and thus the IV curves of

the doped ZnO samples are shifted to the left.

The photoconversion efficiency of the sample can be calculated from the ratio of electrochemical energy density to the input of photoenergy density (Equation 2.27, Section 2.6), as presented in Figure 5.9B. The maximum photoconversion efficiencies of the Ni-, Co-, K- and Na-doped ZnO NRs were calculated to be 0.14, 0.24, 0.26 and 0.29%, respectively, which were 1.3, 2.2, 2.4 and 2.6times higher than that of undoped ZnO NRs (0.11%). The higher photoconversion efficiencies suggest that the doped ZnO NRs electrodes are more efficient in light absorption and converting it to the electricity than the pure ZnO NRs electrode.

Figure 5.10 MS plots of (A) undoped, K- and Na-doped ZnO NRs and (B) Co- and Ni-doped ZnO NRs in 0.5 M of Na2SO4 (pH 6.8). The amplitude of the sinusoidal wave

was set at 10 mV at a frequency of 1 kHz.

In addition, EIS measurements were carried out to measure the capacitances of the NRs in 0.5 M of Na2SO4 (pH 6.8) in the dark with an AC voltage of 10 mV at a

fixed frequency of 1 kHz. As shown in Figure 5.10, both the undoped and doped ZnO samples show positive slopes in the MottSchottky (MS) plots, which confirm the n- type semiconductor behaviour of the electrodes. The flatband potential, 𝑉_𝐹𝐵, of the photoanodes can be obtained by MS analysis (Equation 2.23, Section 2.5). The 𝑉_𝐹𝐵 values of the photoanodes were determined by extrapolation of x-intercepts in MS plots. The values of 𝑉𝐹𝐵 of the undoped, Co-, Ni-, K- and Na-doped ZnO NRs were

measured to be 0.41, 0.52, 0.44, 0.53 and 0.50 VAg/AgCl. The measurements reveal

that the 𝑉𝐹𝐵 values of the doped ZnO photoanodes are more negative than the pristine

ZnO photoanode. This suggests the charge separation of the doped ZnO NRs is better than that of pristine ZnO NRs. This is because the doped ZnO samples have a lower potential barrier to overcome, resulting in a reduction of the charge transfer resistance and an enhancement in the charge transfer.79 _{The measurements are consistent with the}

a more negative potential than the undoped ZnO NRs. It is important to note that the obtained onset potentials of the samples are more positive than the 𝑉_𝐹𝐵 values. This can be accredited to the interfacial charge transport limitations of the samples.290

In addition, the doped ZnO electrodes show substantially smaller gradients of the MS plots as compared to the pristine ZnO electrode. This implies the donor densities of the doped ZnO samples are increased. The slopes of the MS plots were used to derive the charge carrier density of the photoanodes (Equation 2.24, Section 2.5). For ZnO, it has a relative dielectric constant value of 10 291 and the donor densities of pure ZnO, Co-, Ni-, K- and Na-doped ZnO were calculated to be 7.16 × 1018, 1.05 × 1020_{, 1.34 × 10}20_{, 2.48 × 10}19 _{and 2.20 × 10}19 _cm-3_{, respectively. In comparison}

with the dopant concentrations identified from ICPMS in Table 5.2, the large donor densities for the doped ZnO samples may be due to the oxygen vacancies, which are known as electron donor for oxide-based semiconductors.89 The increased in the electron density will improve the electron transfer at the interface between the ZnO and titanium substrate, as well as the charge transport within the ZnO. In addition, the Fermi level of the doped ZnO samples is expected to shift toward the conduction band due to the increased of the electron density. This will facilitate the charge separation at the interface of semiconductor and electrolyte by increasing the degree of band bending at the ZnO surface.265 Therefore, the PEC water splitting performances of the doped ZnO NRs are improved.

Other than that, the optical band gap of the sample also plays an important role in determining the photocatalytic water splitting. As discussed in section 5.4.5, all the doped ZnO NRs owned a smaller band gap energy than that of pristine ZnO NRs (Figure 5.7). The smaller band gap energy will extend the absorption region and increase the number of solar excited electrons and holes, thus resulting in a higher PEC

water splitting efficiency. This increased population of excitons will benefit from the increased donor densities and improved the charge mobility. In addition, Zou Z. et al. have reported that the improvement of photocatalytic of the nanostructures doped with transition metals could be due to the d-d transition induce the electron-hole separation and improved the generation of hydrogen and oxygen.292 However, the mechanism on how the d-orbitals enhanced the photoexcitation and photocatalytic activity is still remained unknown.

Another possible parameter that affect the PEC water splitting activity is the surface area to volume ratio (SAVR) of the sample, which determines geometry effects on the reaction kinetics for gas or liquid at solid surface.293 Based on the measured morphological dimensions, the SAVR values (Equation 3.10, Section 3.4.7) were calculated as 0.0181, 0.0175, 0.0184, 0.0110 and 0.0194 nm-1 for pristine ZnO, Co-, K-, Ni- and Na-doped ZnO NR arrays, respectively. Consistent with the findings, Na- and K-doped ZnO NR arrays with the highest SAVR values achieved highest photocurrent densities and photoconversion efficiencies.

In summary, the reduced band gap energy, increased carrier concentration, improved interfacial catalysis and increased SAVR are believed to be the major reasons for the improvement in photoactivity of PEC water splitting for the doped ZnO NRs.

5.6 Conclusions

Doped ZnO NRs were successfully created through the aqueous CBD method. The surface morphology of ZnO nanostructures was strongly affected by the type of charged ions formed by the dopants, which will alter the NRs morphology, either increasing or decreasing the aspect ratio. All the doped ZnO samples demonstrated a reduced in the optical band gap energy. An efficient solar hydrogen system was

developed based on the doped ZnO NRs photoanodes. When compared with the pristine ZnO NRs photoanode, all the doped ZnO NRs electrodes revealed at least ~27% enhancement in the photoconversion efficiency. Among these photoanodes, Na-doped ZnO NRs electrode exhibited the highest PEC water splitting efficiency, which was 2.6times more efficient than the pristine ZnO NRs photoanode. The improvement in photoactivity of PEC water splitting may be attributed to the increased SAVR, reduced band gaps, improved charge separation, increased donor densities and improved the interfacial charge-transfer kinetics.

Chapter 6 Enhanced Hydrogen Production through 3D ZnO Nanorods