Polarization-dependent H2/O2 evolution. Epitaxial BFO thin films were grown on (001) SrTiO3 (STO) substrates with conductive (La,Sr)MnO3 (LSMO) layer for PEC measurement. In this work, the thickness of the BFO thin films is 50 nm, which leads to a pronounced photocurrent density due to the carrier diffusion length 32. The detailed growth, piezoresponse force microscopy (PFM) and PEC measurements can be seen in Supporting Information. Fig. 1 (a, b) shows that BFO thin films with upward (downward) polarization give a cathode (anode) photocurrent. This polarization-selective photocathode and photoanode are benefited from its special energy bands (straddle the water redox levels) as illustrated in Fig. 1 (c, d). The upward-polarized BFO induces positive (negative) bond charges at the electrolyte/BFO (BFO/LSMO) interface, and downward (upward) band bending towards the electrolyte/BFO (BFO/LSMO) interface 32. This band bending promotes the photo-generated electrons to BFO surface, and thus favors the HER (Fig. 1 (c)). While, the downward polarization gives rise to the upward (downward) band bending towards the electrolyte/BFO (BFO/LSMO) interface, which promotes the holes to BFO surface, and facilitates the OER (Fig. 1 (d)). These controllable polarization and ferroelectric surface suggest that photocathode and photoanode may be assembled on a single photoelectrode.
Hydroxyl-bonded BFO surface enhanced HER and OER simultaneously. Reconstruction of surface chemical structure of the photoelectrode may increase the energy bands offset and thus facilitate charge transfer across the photoelectrode/electrolyte interface 38. Thus, we design the BFO surface structure through a controllable ferroelectric/water ionic interaction (Supporting Figure 1) 28. A dramatic enhancement of the photoemission peak (∼531.7 eV) in the X-ray photoelectron spectroscopy (XPS) spectra after the ionic interaction indicates the presence of terminal hydroxyls (metal-oxide-hydrogen, M-O-H) on the BFO surface 39, 40 as shown in Fig. 2 (a). These hydroxyls gradually desorbed from the surface when the sample was annealed at elevated temperature (Supporting Figure 2), which is similar with previous observations 41, 42, indicating that the emergent surface structure (M-O-H) is hydroxyl-bonded ferroelectric surface (BFO-OH). In water splitting process, surface hydroxyl groups may play a significant role in interfacial charge transfer 43, 44. Interestingly, the BFO-OH photoelectrodes with high photo-stability (Supporting Figure 3) not only drive hydrogen evolution with a photocurrent density of -0.06 mA·cm-2 at 0 V vs. RHE (two times larger than pristine BFO), but also catalyze oxygen production with a photocurrent density of 0.07 mA·cm-2 at 1.23 V vs. RHE as shown in Fig. 2 (b), which does not exist in the pristine BFO with the same polarization (Fig. 1 (b)).
A low charge transfer resistance in the Nyquist plot of the BFO-OH (Fig. 2 (c)) implies a new process for electrons/holes transfer across the interface, which may benefit from a reconstructed surface electronic structure in the BFO-OH (no morphology changes before and after the hydroxyl modification, Supporting Figure 4). The surface electronic structure of the BFO-OH was characterized by scanning tunneling spectroscopy (STS) 45, showing a positive shift of conduction band minimum (CBM) and valance band maximum (VBM) as seen in Fig. 2 (d). Density functional theory (DFT) calculations indicate that the surface density of states of BFO-OH, where the OH- ions bond to the Fe sites on the FeO2-ternimated energy-favorable BFO (001) surface 28, 46, 47, is consistent with the STS results, as shown in Fig. 2 (e). The up-shift of both the CBM and VBM of the BFO-OH in Fig. 2 (f) ascribes to the formation of a built-in field from hydroxylated surface to the bottom of the photoelectrode 28, leading to the decreased barriers for electrons and holes migration at the same surface, as illustrated in Supporting Figure 5. The details of DFT calculation and STS measurement are presented in Supporting Information.
Dynamic process of water splitting on the BFO-OH. To understand the dynamic reaction processes of the enhanced HER and OER on the BFO-OH, we calculated the Gibbs free energy of each step in the HER and OER and plotted the energy diagram in Fig. 3 (a) and 3 (b), respectively. In general, HER follows a two-electron step:
* + H+ + e- → *H (Eq.1)
*H + H+ + e- → H2 (Eq.2)
where the symbol “*” represents the active sites of catalysts. Due to a rational thermokinetics balance between the above two reaction steps, the Gibbs free energy for *H intermediate generation (|ΔGH*|) on the BFO-OH decreases to 0.39 eV, decreasing by a factor of 3 compared to the pristine BFO (1.46 eV), which notably enhances the HER efficiency, as illustrated in Fig. 3 (a). In contrast, OER is a four-electron process:
* + H2O → *OH + H+ + e- (Eq.3)
*OH → *O + H+ + e- (Eq.4)
*O + H2O → *OOH + H+ + e- (Eq.5)
*OOH → O2 + H+ + e- (Eq.6)
Production of oxygenated intermediates *OH in step 1 (depicted by Eq. 3) on the BFO-OH only need to overcome low energy barrier of 1.6 eV, as shown in Fig. 3(b), resulting from adsorbed water molecules dissociation followed by the Fe-OH-OH formation, which favors driving the subsequent reactions, and leading to an efficient activity for OER 38, 48, 49. However, the Gibbs free energy of production of *OH in step 1 on the pristine BFO, where the hydroxyls bound to the surface Fe atoms, is as high as 2.18 eV (Supporting Figure 6), which makes the subsequent processes rarely occur and the OER stops. The overpotential η in OER decreases to 0.53 V in the BFO-OH from 0.86 V in the pristine BFO (Supporting Information), suggesting an improved OER efficiency in the BFO-OH due to the terminal hydroxyls induced facilitated charge transfer and decreased water adsorption energy. During the OER, hydroxyls in the BFO-OH are acting as the catalytic sites other than the intermediates during the OER, which is supported by secondary ion mass spectroscopy (SIMS) in Fig. 3 (c, d). BFO-OD was designed by deuterium oxide treatment, as validated by the depth profiles (Supporting Figure 7). The intensity of deuterium on the BFO-OD almost remains identical before and after the PEC measurements in aqueous solution (Fig. 3 (d)), indicating the robust bonds (BFO-OD/OH) act as stable active sites for capturing the photo-generated electrons and holes for driving water splitting.
Printing of ferroelectric super-domains for efficient water splitting. To further enhance the water splitting efficiency, a large-area printing of checkboard down/up domains with up/down depolarization fields (Edp) on the BFO-OH are schematically demonstrated in Fig. 4 (a), where a typical 500 nm periodic super-domains structure is shown in Fig. 4 (b). A decrease of the domain size leads to an enhanced and stable photocurrent density of OER and HER, as shown in Supporting Figure 8. It is notable that the photocurrent density dramatically increases by one order of magnitude when the domain size reduces from 1 mm to 500 nm as shown in Fig. 4 (c). DFT calculations were performed for understanding the mechanism of the enhanced efficiency on the checkboard domains, where the spatial approach of the charge densities of the VBM and CBM (supported by oxygen and iron respectively) on the uniform upward-polarized BFO with downward depolarization field favors the enhancement of the electrons-phonons coupling and accelerates charge recombination. The antiparallel polarization on the BFO-OH with antiparallel depolarization field separates the electrons/holes effectively with reduced charge recombination by localizing the CBM and VBM at different domains as shown in Fig. 4 (d), and thus leads to a high current density. We expect a higher current density if the domain size reduces to the limit of electrons/holes diffusion length of the bifunctional ferroelectric photoelectrode, which may further figure out the mismatched pH for photocathode and photoanode in integrated systems (Supporting Figure 9) 37, and builds up a platform for controllable and high-efficient photocatalysis with photo- oxidation and reduction coupling reaction 50, 51.