Novel bipolar charge collecting structure enabling overall water splitting on ferroelectric photocatalysts

Due to the unidirectional charge separation and above-gap photovoltage, ferroelectrics have been considered as excellent photocatalytic candidates for solar fuel production. However, the performance of ferroelectric photocatalysts is often moderate. Few reports demonstrated that these kinds of photocatalyst could achieve overall water splitting. Here we propose a novel approach to fabricate interfacial charge collecting nano-structures on ferroelectric’s positive and negative domains, enabling overall water splitting in ferroelectric photocatalysts. We observed efficient accumulations of photogenerated electrons and holes within their thermalization length (about 50 nm) around the Au nanoparticles located in the positive and negative domains of BaTiO 3 single crystal. Photocatalytic overall water splitting was firstly observed on ferroelectric BaTiO 3 single crystal after assembly oxidation and reduction cocatalysts on the positive and negative charged Au nanoparticles. The idea of fabricating bipolar charge collecting structure on the ferroelectrics to achieve overall water splitting paves the new way for utilizing the energetic photogenerated charges in solar energy conversion.


Introduction
Ferroelectrics with switchable spontaneous polarization have shown tantalizing potential in memory storage and integrated microelectronics [1][2][3][4][5] . Meanwhile, power conversion efficiency exceeds unity, and large photovoltage above the bandgap have been reported in some ferroelectrics [6][7][8][9][10] . The photoelectric characters of ferroelectrics have drawn much interest in solar fuel production [11][12][13][14] . Compared to the traditional charge separation driving force via drift or diffusion mechanisms in common semiconductor photocatalyst, ferroelectric semiconductors possess charge separation driving force due to spontaneous polarization [15][16][17][18][19][20] . The unique characters in asymmetric crystals endows ferroelectric semiconductors the bulk photovoltaic effect (BPVE), favoring efficient photogenerated charge separation within the nonthermalizaion length 21,22 . With these photogenerated charges, the Shockley-Queisser limit for the power conversion efficiency in the ferroelectric devices have been exceeded under one sun illumination (AM 1.5 G) 6 . Despite the enormous potential applications, the application of ferroelectrics in photovoltaic devices remains scarce. In particular, ferroelectric semiconductors are not yet reported for photocatalytic overall water splitting. Although they possess both spontaneous ferroelectric polarization induced internal field for massive charge separation and thermodynamically suitable energy band structure for overall water splitting.
The charge separation mechanisms in ferroelectric, known as BPVE, are often explained by two mechanisms: shift and ballistic 23 . The shift mechanism originates from a quantum phenomenon in the noncentrosymmetric crystal. It is the result of the coherent evolution of a quantum wave packet and the photoexcitation-induced shift of real space. The ballistic mechanism is related to the photogenerated, nonthermalized charges with asymmetric momentum distribution in the noncentrosymmetric crystal (FIG. 1a). The nonthermalized charges descend to the band bottom via a length L0, also called thermalization length. L0 depends on materials and incident photons in tens to hundreds of nanometers. Within L0, all the photogenerated charges contribute to BPVE and yield the highest solar energy conversion efficiency. Hexagonal close-packed metallic electrode arrays with accurate distance were predicted to have the highest collection and utilization of photogenerated charges (FIG. S1). Based on this principle, Spanier et al. prepare a device with a single-tip electrode contact and an array with 24 tips. The device generated a current density of 17 mA cm -2 under the illumination of AM 1.5 G 6 . Photogenerated charges are concentrated around every individual tip and then collected via the ITO electrode. However, the fully-covered ITO electrode hinders the transmissivity in the ultraviolet range, where BTO has the most significant absorption coefficient. As a result, the performance of this device is mediocre and performs well below expectations. The utilization of photogenerated charges in ferroelectrics for high-efficiency solar energy conversion remains a longstanding challenge, despite the theoretical basis seeming quite clear. Thus, well-designed micro/nanostructures in ferroelectric-based semiconductors are of substantial importance in solar energy conversion. There left plenty of room for the explore of charge separation mechanisms at micro/nanoscale to achieve photocatalytic overall water splitting.
In this work, we proposed a novel approach to fabricate nano charge collecting structures at metal/ferroelectric interface to enable overall water splitting ability in ferroelectric photocatalysts, Au array patterned BaTiO3 single crystal. We find the anomalous concentration of photogenerated electrons and holes in Au particles, located at +P and -P domains in BaTiO3 single crystal, respectively. It is proved that the photogenerated charges are concentrated around Au particle within a hemisphere of radius L0, the thermalization length, about 50 nm. Due to the energetic photogenerated charges, fabricated Au array/BTO photocatalysts show substantial photocatalytic overall water splitting performance. The measured thermalization length L0 is also the key experimental prescription in designing high efficiency ferroelectrics in solar energy conversion at nanoscales.  31,32 . The enhanced built-in voltage at the two types of Au/BTO interface proves that the Schottky-like depleting layer at -P domain is further depleted and the quasi-Ohmic-like accumulation layer at +P domain is also further accumulated. The above results provide strong evidences that photogenerated charges are concentrated around Au particles in the SCRs, agreeing with Spanier's speculation 6 . In the surface SCRs of bare BTO and common semiconductors, the built-in voltage decrease under illumination (FIG. S4, S5, S6) [31][32][33][34] . This phenomenon at Au/BTO is quite anomalous, entirely different from common metal/semiconductor junction.
To obtain further information, a detailed quantitative analysis is carried out. As Ferroelectric photocatalyst is then designed based on the above experiment phenomenon and measured essential experimental prescription. Except for the thermalization length of BTO, several other factors should also be concerned, such as array density, Au particle size. The work function, metal-ferroelectric interface, and surface plasmon resonance (SPR) of Au particles are pronouncedly size-dependent 41,42 .
Despite the Au array's density decrease with Au particle size, large Au particles with higher charge capacity, better metal/ ferroelectric interface, and red-shift SPR are preferred. Besides, the electric field around the charged Au particle arrays should also be well considered. Thus, the distance between the margin of two adjacent Au particles should be more than twice L0 due to the electrostatic repulsion between them (FIG. S18 and S19). Based on these aspects, appropriately designed ferroelectric photocatalysts are shown in FIG. 4a. Periodic hexagonal close-packed (hcp) Au particles on BTO are prepared with self-assemble polystyrene microsphere template (Details in Supporting   Information, FIG. S12-16). The Au particles are about 200-230 nm in diameter with 500 nm periodicity. The distance between the margin of two adjacent Au particles is about 250-300 nm. The electric field simulation indicates that the electric field surrounding Au array is massively enhanced and radially expands, but different from an individual one. The enhanced field around the center Au particle is almost a hemisphere and contracted horizontally compared with individual one due to the electrostatic repulsion between the neighbor Au particles. The enhanced field extends about 80 nm from the margin of Au particle. A nonenhanced area is also found between the two Au particles due to the electrostatic repulsion. When the distance between the margin of two adjacent Au particles is 100 nm, i.e., twice of L0, the periodicity decreases to 300 nm. The strong electrostatic repulsion between the neighbor Au particles enables a shrunken and reduced electric field (FIG. S19). The electric field extends less than the L0 and cannot conform to the demand of charge collection within L0. Based on the simulation results, we demonstrate that the designed Au array on BTO is rational. Because the BE difference between the Au 0 and Au 1+ is usually about 1.5 eV. And it is the same with Au 1+ and Au 3+ . Thus, the Au element is in the chemical state of Au 0 but different charge density due to the constructed heterojunction with BTO. In detail, the higher BE of Au 0 4f can be assigned to the Au particle at +P with quasi-Ohmic contact.
Analogously, the lower BE of Au 0 4f can be assigned to the Au particle at -P with After selectively photodeposition (Rh/Cr2O3 and CoOOH) 43,44 , the overall water splitting is achieved in pure water (FIG. 4d). This could be the first case in the literature that ferroelectric structures can split pure water via photocatalysis. Even though perovskite BTO possesses both thermodynamically suitable energy band and massive charge separation driving force for water splitting, the overall water splitting of BTO is still not reported yet. After constructing nanostructures to collect and utilize the photogenerated charges, we successfully demonstrate that the overall water splitting in pure water can be achieved. These results emphasize the significance of utilizing the photogenerated charges in ferroelectrics within the thermalization length.

Conclusions
In summary, we have shown that the overall photocatalytic water splitting can be achieved in ferroelectric photocatalysts via collecting and utilizing the photogenerated charges within the thermalization length in a prototype of Au/BTO photocatalysis.
Using KPFM, we have observed the concentration of photogenerated charges within the thermalization length of BTO at the Au/BTO interface. Measured thermalization length is an essential experimental prescription for fabricating high-efficiency photocatalytic and photovoltaic devices at the nanoscale. With this novel structure design, constructed ferroelectric photocatalysts can perform photocatalytic overall water splitting. The experimental design definitely opens a paradigm of designing the ferroelectric photocatalysts for efficient solar energy conversion. Overall water splitting reactions of Au array/BTO with cocatalysts in pure water.

Supporting Information
Supporting Information is available from the xxx or from the author.