Piezo-phototronic Effect in Centro-symmetric BiVO 4 Epitaxial Films

With exciting functionality, topological defects in ferroic system have attracted much attention. Under proper design, the emergence of polar domain walls in non-polar ferroelastics enables piezo-phototronic effect. In this study, we revealed ferroelastic twin texture with localized piezoelectric effect in epitaxial BiVO 4 lm by piezoresponse force microscopy. Supported by the strain eld analysis, we found the piezoresponse conned at domain wall area is attributed to the exoelectric effect induced by the presence of ferroelastic twin domains during the paraelastic to ferroelastic phase transition. The piezophototronic phenomenon was further supported the experimental evidence of dye-degradation and generation of reactive radicals. This work not only provides new insights into the introduction of piezophototronic effects in non-polar materials, but also sheds light on a new concept to use material inhomogeneity for acquiring multifunctionality.

ferroelectric/ferroelastic domain walls initially can display higher photoconductivity mainly because the photogenerated carriers are separated by a strong electric eld to accumulate at domain wall region due to the symmetry breaking. 10,11 So, the key question we want to address is: can the piezo-phototronic effect be triggered by proper design of inhomogeneity in materials with centrosymmetry? In this study, centrosymmetric BiVO 4 (BVO) is selected as a model system, because it is an attractive candidate for photoanode by virtue of its adequate bandgap (2.4 2.6 eV) and the advantages of non-toxicity and high stability. 12 Efforts on enhancing its performance were mostly focused on building up heterojunction for effective charge separation. 13,14 Less attention was paid to the control and use of material inhomogeneity. In this work, we fabricated epitaxial BVO lms on yttria-stabilized zirconia (YSZ) substrate. Due to the phase transition of BVO and metastable structure compatible with substrate, the formation of periodic domain patterns can be obtained. The temperature dependent ferroelasticity and corresponding domain structures of BVO lm were probed to explore the phase transition and understand the origin of domain formation. The importance of BVO domain patterns was highlighted by spatialresolved photoconductivity and photochemistry. We found that the exoelectricity at domain walls in centro-symmetric BVO creates a piezo-potential to enhance photoelectrochemical performance. Our results not only launch a new research direction of piezo-phototronics, but also expand potential application of utilizing material inhomogeneity especially in photoelectrochemistry.
BVO exhibits ferroelasticity at room temperature with two energy degenerate states in crystal structure.
They are two centro-symmetric space groups with different γ angle (i.e., the angle between a and b axis), 90.4° for I 2 /b and 89.6° for I 2 /a. 15, 16 One critical material inhomogeneity can be expected is the domain wall between these two structures. The properties of this inhomogeneity can be predicted by the rstprinciples density functional theory (DFT) calculations. We found that a spontaneous polarization arises at domain wall area, as depicted in Fig. 1a. In addition, Bi and V atoms at the interfaces between A/B domains are displaced appositively with respect to their surrounding oxygen cages along the domain walls, respectively. (The detailed assumptions and quanti ed polarization are described in supplementary text S1 and Figs. 2.) Such results indicate the possibility that inhomogeneity induces polarization in centrosymmetric materials. 17  indicating the two-fold symmetry. We hypothesize that the energy degenerate states of BVO as multiple domains create inhomogeneous distribution of spontaneous strain especially along in-plane direction due to the mis t between a and b axes of BVO. Thus, besides DFT calculation, the localized strain gradient at topological defects (i.e., domain walls) also suggests the possibility of inducing polarization through exoelectricity.
Based on the inherent photoactivity of BVO, the emerging polarization at the domain wall implies the existence of the piezo-phototronic effect in our system. We further explore such a piezo-phototronic behavior by conducting rhodamine B (RhB) degradation experiments since dye degradation reaction could be driven by either solar energy or mechanical vibrational energy for photoactive semiconductors with piezoelectricity (inset in Fig. 1f). 20 Surprisingly, although monoclinic space groups (i.e., I 2 /b and I 2 /a) of BVO are centro-symmetric, we observed an outstanding piezo-catalytic performance even much higher than its photo-catalytic activity in the thin lm structure. Signi cantly, the rate constant (k) of piezophototronic effect is ~10 folds higher than that of photo-catalytic effect, as shown in Figs. 9 and table S1. Considering that the piezo-phototronic effect only occurs in piezoelectric materials, this unprecedented result inspires us to further investigate the mechanism of driving piezoresponse in centrosymmetric BVO and the critical role of domain patterns.
The coupling effect between domain pattern and polarization was veri ed experimentally with spatial resolution since twin domains in the BVO/YSZ heteroepitaxy (surface height in Fig. 2a) can be visualized through piezoresponse force microscopy (PFM). In-plane PFM phase signal in Fig To further validate the exoelectric effect and understand the role of domain walls with atomicresolution, the visualization of strain eld by the geometric phase analysis (GPA) was conducted. Fig. 2e shows the plane-view STEM image containing A and B domains, andas well as (220) facets in diffraction pattern (in Fig. 2f) were selected as g1 and g2 for analysis. Maps of the local shear strain eld (ε xy~2 % in Fig. 2g) and lattice rotation (ω xy~1 .2° in Fig. 2h) are more pronounced compared with the normal strain elds (ε xx and ε yy ) as shown in Figs. 12. Although the normal strain eld is negligible, the evaluated shear strain gradient in Fig. 2i (~5×10 6 m -1 ) across the domain walls leads to the piezoresponse, which is also predicted by molecular dynamics simulations in previous study. 25 The direct observation of strain gradients is comparable with the calculated value (~4.23×10 6 m -1 in supplementary text S2), and such a strain gradient is an universal effect instead of occasional phenomenon since the in-plane strain gradient distributes throughout the lm (cross-sectional STEM and GPA in Figs. 13). Furthermore, the neighboring domains walls represent opposite shear strain gradient, which is responsible for 180 o head-to-head piezoresponse at neighboring domain walls in Fig. 2d. Combined with the domain width relationship with lm thickness (Figs. 14), the universal strain distribution is due more to the material inhomogeneity rather than the substrate clamping effect (supplementary text S3). Based on the PFM, strain eld analysis and simulation results, the origin of exoelectricity is believed to arise from the reduction of symmetry at domain wall area.
Given that the lattice mis t and the reduction of symmetry at domain walls give rise to the substantial strain gradients and result in piezo-response induced by the exoelectric effect, the direct connection between ferroelastic and piezoelectric nature for BVO could be built by in-situ modulation of temperature, especially focusing on the phase transformation. When the temperature is above the Curie temperature of BVO, 26 ferroelastic monoclinic phase would transform into paraelastic tetragonal one. Due to the reduction of energy degeneracies in crystal structure, we expect the spontaneous strain would disappear as well. In Fig. 3a, when the temperature was increased to 135℃, the splitting satellites assemble in contrast to room temperature RSM (Fig. 1c), implying not only the expansion of b-axis, but also the contraction of a-axis in BVO. After the phase transformation at 325℃ (Fig. 3b), the tetragonal diffraction spot of BVO (400) overlaps with that of YSZ (400), indicating a coherent heteroepitaxy (a BVO, tetragonal = a YSZ, cubic ). Fig. 3c shows the change of lattice parameter with temperature. The lattice mis t between A and B domains could be calculated based on the lattice parameters of monoclinic phase of BVO. The lattice mis t also reduces to 0 after the phase transition. On the other hand, when it cools down to room temperature, the satellites appear as those initially observed (Figs. 15), featuring the reversibility of the phase transition. Furthermore, high temperature in-situ TEM and PFM 27 were carried out to gain insights into domain transformation. Planeview TEM images in Figs. 16 re ect the transition temperature of ~275℃, and the strain relaxation process is mediated by the domain wall motion at 250℃. In agreement with the macroscopic phenomenon in the RSM results, the twin domain structure reappears after cooling down to room temperature. In-plane PFM signal at room temperature is shown in Fig. 3d (surface signal in Figs. 17). The inserted white curves mark the edge of bundle domain and in-plane domain distribution. The domain structure was then recorded at the temperature range from 25℃ to 275℃ (Figs. 18). Above the phase transition point of 275℃, the in-plane phase signal indeed disappears (in Fig. 3e). Moreover, in Fig. 3f, after cooling down to room temperature, the new-formed stripes of domain as well as the PFM signal appear again, featuring the generation of piezoresponse and its dependence on the phase transition. To understand the evolution of ferroelastic twin domains in BVO/YSZ heterostructure, the phase-eld simulation was conducted, and the results are shown in Figs. 19. It also con rms that the distribution of multidomain with two perpendicular orientations arises from minimizing the total energy. From these results, we suggest that the formation of ferroelastic twin domains and the exoelectricity inside twin domain wall are induced by the spontaneous strain during the tetragonal to monoclinic phase transition.
To examine the piezo-phototronic effect, we rst consider it as a combination of piezo-catalytic and photo-catalytic processes. In Fig. 4a, the photo-catalytic effect mainly occurs inside BVO lm, including absorption of light, generation of photocarriers and transport of photocarriers. 28 On the other hand, the piezo-catalytic course relies on tuning surface screen charges, as illustrated in Fig. 4b. On the surface of twin domains, the piezopotential is balanced by screen charges. Due to the head-to-head polarization distribution, screen charges, i.e., eand h + , would accumulate on A and B domain area respectively. (The calculated piezoelectrically induced open-circuit voltage across each domain wall is ~ 0.25 mV. See details in supplementary text S4.) In aqueous solution, ultrasonic vibration creates a sinusoid function of hydraulic pressure, i.e., stress. Subsequently, the localized strain gradient and piezopotential at domain wall would also uctuate with time to redistribute and release the screen charges from surface. As a consequence, the extra charges would spread into solution and serve as active charges. These free charge carriers further react with water and the dissolved oxygen, producing reactive oxygen species, e.g., •OH or •O 2 radicals, for participation in the RhB degradation reaction. 29 (Detail description is in supplementary text S5.) To identify the reactive oxygen species responsible for piezo-catalytic reaction, the production of radicals is veri ed by the electron paramagnetic resonance (EPR) technique using 5,5dimethyl-1-pyrroline N-oxide (DMPO) as spin scavenger. In Fig. 4c, the resonance peaks of DMPO-•OH were detected, showing that •OH radicals were the main reactive species. Under darkness condition, •OH radicals could be generated by mechanical vibration stimuli and the concentration increases with vibration time. Under light illumination, the concentration of •OH radicals is further enhanced (Figs. 20), demonstrating the synergy between piezo-catalytic and photo-catalytic processes, which may account for the observed piezo-phototronic effect in Fig. 1f.
In order to prove the piezo-catalytic feature on BVO, HAuCl 4 was introduced to conduct the deposition of Au nanoparticles on BVO. In the absence of reducing reagent and light illumination, HAuCl 4 can be reduced to Au nanoparticles by reacting with radicals from piezo-catalytics. 30 As evidence from Fig. 4d, under mechanical vibration and darkness condition, an appreciable amount of Au nanoparticles was produced on BVO surface. Such a feature can be further corroborated by conducting the same Au deposition experiment on pure YSZ substrate, in which Au nanoparticles were barely produced (Figs. 21).
It should be noted that the piezo-catalytic feature can enhance the photo-catalytic e ciency of BVO by intrinsic in-plane piezopotential at domain walls. As illustrated in Fig. 4e, under light illumination, radicals generated from majority of charge carriers (i.e., electrons in n-type BVO) can reduce HAuCl 4 to produced Au. Therefore, the deposition of Au nanoparticles would be prominent in the electron-rich domain. With antiparallel in-plane piezopotential at domain walls, the projected piezo-potential is from A to B domains (Fig. 2d) [31][32][33][34] , which is around three-folds larger than the twin domain width. This feature makes possible the effective separation of photo-generated electrons by the inherent electric eld and their accumulation at speci c upper domain area (B domain). Furthermore, the high density of twin domain walls also conduces to the suppression of charge carrier recombination of BVO for enhancing the photo-catalytic performance, which accounts for the observed piezo-phototronics effect as wall. It is noteworthy that the key parameter to drive piezoresponse in centrosymmetric material is the strain gradient at domain wall. Based on this concept, we expect in other photo-active semiconductors with inhomogeneities at atomic scale, piezo-phototronics effect could be triggered regardless of the material category (e.g., epitaxial thin lm or polycrystalline ceramics).

Conclusion
All in all, this study illustrates the engineering application of multifunctional topological defects and opens up opportunities for exploiting the piezo-phototronic effect based on centro-symmetric material. Ferroelastic twin domain feature of BVO was con rmed by XRD and TEM. Based on the in-plane PFM, GPA results and in-situ ferroelastic-paraelastic phase transition experiments, the strain gradient con ned in domain walls along in-plane direction generates a piezopotential due to the exoelectric effect. This piezopotential not only serves as a pathway for photogenerated carriers to accumulate at speci c domain area under light illumination, but also holds promising engineering potentials for converting mechanical vibrational energy to chemical energy for photoelectrochemical applications. Our results demonstrate the predominant role of strain gradients at domain walls by exoelectricity, and the localized piezopotential can be combined with the inherent photoactivity, lessening the gap for engineering the piezo-phototronic effect in centrosymmetric materials.   Temperature dependent characteristics of BVO/YSZ thin lm. In-plane RSM of BVO (400) with YSZ (400) reference a, At 130℃. b, At 325℃. c, Variation of lattice parameters with temperature. The high temperature analysis of PFM: The in-plane phase signal of BVO d, at room temperature, e, at 275℃. f, after the heating process and cooling down to room temperature. For high temperature PFM analysis, these results were obtained at 1.5V ac tip bias, and we used single crystal diamond conducting probe due to its superior duribility and stable resolution.