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Polar Domains at the Surface of Centrosymmetric BiVO4

P semiconductors have recently received significant attention because their internal fields separate photogenerated electron−hole pairs and reduce recombination. Domains of opposite polarization create unbalanced charge in spatially localized areas where the domains intersect the surface. As a result, electrons are attracted to positively terminated domains where they promote reduction reactions and holes are attracted to negatively charged domains where they promote oxidation. The separation of photogenerated carriers and redox products increases reactivity by limiting recombination and back reactions; the spatial localization can also be used to create well-defined nanostructures through site specific reactions. So far, this phenomenon has been observed only in materials that have polarity deriving from their noncentrosymmetric structures. One survey identified just over 500 oxide compounds that met the symmetry requirements for polar behavior, a relatively small number considering the possible ternary, quaternary, and more complex stoichiometries that might be formulated. There are, however, polar phenomena that can occur in centrosymmetric oxides. The flexoelectric effect occurs in materials when the imposition of a strain gradient leads to a voltage. While thin film ferroelectrics have been reported to have large flexoelectric effects, the effect is thought to be small in bulk materials and especially in centrosymmetric materials. Because symmetry is broken by surfaces, they provide a mechanism for the relaxation of internal stress that arises both from microstructural discontinuities and from the discontinuous bonding at the surface. In general, any relaxation of stress at the free surface might create a strain gradient and impart flexoelectricity in an otherwise nonpolar material. Here, we focus on BiVO4, which, when cooled below 255 °C, undergoes a transition from the tetragonal scheelite structure to the monoclinic fergusonite structure (I2/b, a = 5.1935 Å, b = 5.0898 Å, c = 11.6972 Å, γ = 89.613°). This ferroelastic phase transformation leads to domains that form to partially compensate the transformation strain that occurs when the crystal changes shape. The domain microstructure is characterized by two types of straight domain walls in the zone of the c-axis. Across each wall, the a and b axes are rotated by ≈90°. Our experiments were carried out on bulk BiVO4 ceramics with the monoclinic structure (see Supporting Information Figure S1a). When the polished surface was examined microscopically, the previously reported parallel lamellar structures corresponding to the ferroelastic domains were obvious (see Supporting Information Figure S2). An example is shown in Figure 1, which shows portions of four misoriented crystals, separated by grain boundaries, each with lamellar contrast corresponding to the domain microstructure. When silver cations from solution are photochemically reduced to silver metal on the surface, the silver preferentially forms only on certain domains; this is illustrated by the image in Figure 1b. After reduction, the largest topographic features in Figure 1b appear white (see Supporting Information Figure S3). Energy dispersive spectroscopy of these features on other samples show that they contain Ag (see Supporting Information Figure S4), and X-ray diffraction patterns show the (111) peak of crystalline Ag (see Supporting Information Figure S1b), indicating that these features are reduced silver metal. In the central grain with contrast from multiple domains, marked by the star, only one parallel set of domains (the wider ones) has reduced silver, and the reduction occurs only on every other domain. This is a characteristic of all observations: when domains photochemically reduce silver, it is every other domain that is active for reduction. This is essentially the same as what has been observed in noncentrosymmetric ferroelectrics including BaTiO3, 21 BiFeO3, 22 (BaxSr1−x)TiO3, 23 and PZT, but for BiVO4, the crystal structure is centrosymmetric. Although bulk BiVO4 is centric and nonpiezoelectric, the ferroelastic domains at the surface are piezoresponsive. The piezoresponse force microscopy (PFM) image in Figure 1c shows clear contrast from the ferroelastic domains. When multiple sets of parallel domains are present, it is typically only one set that preferentially reduces silver. Piezoresponse from a polycrystalline (centric) SrTiO3 surface has previously been reported, but in that case it was a grain-by-grain response. The correlation observed here between the piezoresponse of ferroelastic domains and photochemical reactions in a centric material is unique. Similar behavior was observed on dozens of additional grains that were examined. Two additional examples (illustrated in Figure 2) show common phenomena: inhomogeneous Ag reduction, even though the piezoresponse in Figure 2c,d appears to be relatively homogeneous, and a grain that has contrast in PFM, Figure 2d, but does not reduce silver. The domain specific photochemical reduction of silver from solution on ferroelectric structures has been explained as a natural consequence of the unscreened dipolar charge in each domain. A domain with a positive polarization orientated toward the surface reduces the energy of the conduction band edge at the surface, making it easier for electrons to reach the surface and reduce cations in solution. Conversely, a