3.1. Photoelectrochemical Performance
The PEC performance of these as-prepared samples was further investigated. Under chop** irradiation, a linear sweep voltammetry (LSV) measurement was carried out under AM 1.5 G light, as presented in
Figure 5a. Ce/TiO
2 shows a photocurrent density of 10.9 μA/cm
2 at 1.0 V vs. Ag/AgCl, which is 2.5 and 2.4 times higher than that of pure TiO
2 (4.3 μA/cm
2) and CeO
2-TiO
2 (4.5 μA/cm
2), respectively. The highly increased photocurrent of Ce/TiO
2 mainly originates from the efficient charge separation. Ce do** in TiO
2 enhances the efficiency of photogeneration and separation of electron-hole pairs, resulting in a high photocurrent response of Ce/TiO
2.
Figure S2 presents the chopped LSV curves of the as-prepared TiO
2 samples, which were modified with different Ce loadings or were annealed at different temperatures.
Chronoamperometry measurement was used to investigate the chemical stability of the samples at 0.8 V vs. Ag/AgCl, as presented in
Figure 5b. The CeO
2-TiO
2 showed a higher photocurrent density than the pure TiO
2, which resulted from the heterojunction between TiO
2 and CeO
2. The maximum photocurrent densities of the pure TiO
2, CeO
2-TiO
2, and Ce/TiO
2 were 3.2, 4.5, and 8.0 μA/cm
2 at 0.8 V vs. Ag/AgCl, respectively. These results indicate that Ce do** into the crystal lattice of TiO
2 is beneficial to the separation of photogenerated electrons and holes. Ce ions in TiO
2 form an intermediate energy level, which results in the formation of oxygen vacancies. Under light radiation, the absorbed oxygen on the surface of the photocatalyst generates hydroxyl radicals and superoxide radicals, improving the photocatalyst’s activity. The Ce/TiO
2 sample exhibited steady photocurrent densities during the long cycling, implying that its photostability significantly improved.
The onset potential of the photocurrents indicates the photocatalytic activity and the flat band potential of the photoanodes. A high onset potential implies a low utilization efficiency of solar energy [
30]. As shown in
Figure 6, the onset potential of the Ce/TiO
2 (0.0 V) was more negative compared to that of the pure TiO
2 (0.159 V) and CeO
2-TiO
2 (0.103 V). The low onset potential of the Ce/TiO
2 indicates an efficient carrier transfer process in the Ce-doped TiO
2 [
31].
The photocurrent densities versus the monochromatic light of the samples were measured in electrochemical noise (ECN) mode, which is a nondestructive and in situ monitoring technique to investigate the spontaneous electrochemical reactions of photoanodes [
16,
32], as shown in
Figure 7a. The photocurrent density of the Ce/TiO
2 was slightly lower than that of the pure TiO
2 under the UV light region. It is worth noting that the Ce/TiO
2 showed visible light absorption up to 500 nm, while the pure TiO
2 and CeO
2-TiO
2 exhibited no obvious response under the visible light region. The successful Ce do** highly improves the visible light utilization efficiency of TiO
2. The incident photon-to-current efficiencies (IPCEs) spectra were obtained from the photocurrent–wavelength curves, as shown in
Figure 7b. The Ce/TiO
2 had the highest IPCE value of 0.10% in the region of 350 nm and an obvious visible light response. The Ce/TiO
2 still exhibited an IPCE value of 0.02% at 400 nm. The photocurrents and IPCE versus monochromatic light of Ce/TiO
2 under applied voltages are shown in
Figure 7c. The photocurrent response of the Ce/TiO
2 increased with bias, which is consistent with the results of the LSV curves.
The band gap energy
Eg of these samples can be calculated from the IPCE spectra by a Tauc plot of (IPCE % × hv)
1/2 versus photon energy (hv) [
16,
33,
34,
35], as illustrated in
Figure 8a. The band gaps of the pure TiO
2, CeO
2-TiO
2, and Ce/TiO
2 were 3.23, 3.24, and 2.73 eV, respectively. The band gap of the pure TiO
2 is consistent with the reported data. The Ce/TiO
2 had a lower Eg than pure TiO
2 owing to the formation of oxygen defect levels above the valence band of TiO
2. Thus, Ce do** in TiO
2 narrows the band gap and enhances the visible light absorption of TiO
2.
The charge transport in the pure TiO
2, CeO
2-TiO
2, and Ce/TiO
2 was also studied by electrochemical impedance spectroscopy (EIS), as shown in
Figure 8c. The Ce/TiO
2 sample shows a smaller arc in the Nyquist plot, indicating a lower charge transfer resistance at the electrode interface [
36]. Ce do** introduces a new electronic state within the band gap of TiO
2 near the conduction band, facilitating the charge separation and reducing the recombination rate of photogenerated electron/hole pairs.
Furthermore, the Mott-Schottky (M-S) measurement was used to determine the flat band potential (V
fb) of these samples, as illustrated in
Figure 8b. The curve slopes of the three samples are all positive, indicating their n-type semiconductor properties. The flat band potential can be regarded as the conduction band (CB) potential of n-type semiconductors, which is calculated by intercepting the slopes with the potential axis according to the M-S equation [
25,
34]. The values of V
fb of pure TiO
2, CeO
2-TiO
2, and Ce/TiO
2 are about 0.0, −0.13, and −0.16 V vs. RHE, respectively. The negative shift in the flat band potential of the Ce/TiO
2 compared to the pure TiO
2 implies a decrease in the transfer energy barrier of the interfacial electrons and the charge transfer resistance after Ce do** in TiO
2 [
35]. In addition, the slope of the M-S plot of the Ce/TiO
2 is significantly smaller than that of the pure TiO
2, indicating an increase in the charge carrier density according to the M-S equation.
3.2. PC H2 Evolution Performance and Mechanism for Water Splitting under Solar Light
The photocatalytic H
2 evolution of the pure TiO
2, CeO
2-TiO
2, and Ce/TiO
2 was evaluated under simulated solar light, as shown in
Figure 9a. The H
2 evolution rate of Ce/TiO
2 was approximately 0.33 μmol/h/g, which is more than twice that of the pure TiO
2 (0.12 μmol/h/g) and CeO
2-TiO
2 (0.16 μmol/h/g). Moreover, Ce/TiO
2 also showed superior photostability for the photocatalytic H
2 evolution over 10 h. The good performance in the water splitting of the Ce/TiO
2 resulted from the narrow band gap and the effective separation of the photogenerated carriers. Ce do** in the lattice of TiO
2 highly improves the photocatalytic performance of TiO
2. The unique energy level structure of the Ce element provides do** levels in TiO
2, facilitating electron transfer and charge separation.
Based on the above results, the mechanism of Ce do** in TiO
2 for an improved photocatalytic performance was proposed. The energy band diagram of the pure TiO
2, CeO
2-TiO
2, and Ce/TiO
2 is illustrated in
Figure 9b. Ce do** in the lattice of TiO
2 narrows the band gap of TiO
2, resulting in a visible-light response. The negatively shifted conduction band in Ce/TiO
2 improves the carrier transfer process and reduces the recombination of photogenerated electron/hole pairs. Therefore, Ce/TiO
2 exhibits a high photocatalytic performance for H
2 evolution from water splitting.