Photogenerated Charge Separation between Polar Crystal Facets Under a Spontaneous Electric Field

Abstract

As we know, photocatalytic reactions occur at the surfaces of catalysts, thus the photocatalytic performance of a catalyst is strongly related to its exposed crystal faces, which determines the surface structure at the atomic scale. [11,12] Recently, the separation of photoinduced charges on the exposed faces of semiconductor nanocrystals has been studied using surface photovoltage techniques [13] and synchronous illumination X-ray photoelectron spectroscopy. [14] However, these studies only focus on one crystal facet, and the structures at atomic scale of the reactive facets of the semiconductor photocatalysts have never been taken into consideration. Based on the atomic arrangement of ZnO photocatalytic reactive polar {001} planes, we present a model of photo-produced charge separation promoted by an interior electric field between Zn-ZnO (001) and O-ZnO (001) crystal planes for the first time. [15] Here, we provide evidence that the charge separation between polar facets is a general photocatalytic reaction mechanism by demonstrating the polar structures of facets with photocatalytic activity, photo-induced selective redox reactions on polar planes, and photovoltaic and rectification effects in polar-direction-orientated semiconductor thin film. Polar Crystal Facets and Their Photochemical Reactions Polar Crystal Facets with Enhanced Photocatalytic ActivityChoy et al. [16] have first observed that ZnO nanoplates with exposed {001} crystallographic planes exhibit higher photocatalytic activity for H 2 O 2 generation compared with ZnO microrods and dumbbell-shaped microrods. Subsequently, hexagonal ZnO quantum dots [17] and nanodisks [18] and porous nanodisks [19] with dominant {001} crystal faces were found to display enhanced photocatalytic activity in the decomposition of rhodamine B [17,18] and methyl orange. [15,19] Thus, the exposed ZnO {001} crystal planes are regarded as the active surfaces for photocatalytic decomposition of organic dyes. According to the literature, [15,20] the exposed {001} surfaces of wurtzite ZnO are polar, which are comprised of a positive Zn-ZnO(001) and a negative O-ZnO (001) surface. The [001] of ZnO is a polar direction. In the [001] direction, Zn and O are not located at the same atomic layer. The Zn and O atoms are positively and negatively charged, respectively. The positive Zn-and negative O-atomic layers stack alternately along the [001] direction (Figure 1a,b).Until now, the photogenerated charge separation mechanism is not clear in photocatalysis. Based on the structures at the atomic scale of the polar crystal facets with photocatalytic activities, a general separation model of photogenerated charge between polar facets under a spontaneous electric field is presented. This charge separation model is proven by photo-induced selective redox reactions on polar planes, photovoltaic and rectification effects in the polar direction orientated thin films, and electric field assisted assembly and alignment of semiconductor nanowires grown along the polar direction. This polar s...

Polarity-Driven Directional [0001] Electron Transfer on Nonpolar ZnO (101̅0) Crystal Plane

“…According to refs and for the nonlayered compounds, the [001] in hexagonal and tetragonal crystals and the [111] in cubic crystals are usually the polar direction. In these polar directions, the directional electron migration driven by E s /V s should be exhibited.…”

“…The internal electric field from the spontaneous polarization in the [0001] polar direction is named spontaneous electric field (E s ). , The existence of E s in polar structure has been found to play a crucial role in the photocatalysis and photovoltaic, rectification, and piezoelectric effects. A general model of photogenerated charge separation between polar crystal facets and a novel piezoelectric mechanism which can change the current understanding of piezoelectric phenomena are presented. As a result, an unified polar structure theory has been preliminarily established.…”

“…Subsequently, there appeared a flurry of reports on polar crystal facets discovered as active crystalline surfaces for photocatalysis. − In addition, the enhanced rectification, photovoltaic, and piezoelectric generation performances are observed in the [0001] polar orientated wurtzite ZnO thin films , and nanorods . The internal electric field from the spontaneous polarization in the [0001] polar direction is named spontaneous electric field (E s ). , The existence of E s in polar structure has been found to play a crucial role in the photocatalysis and photovoltaic, rectification, and piezoelectric effects. A general model of photogenerated charge separation between polar crystal facets and a novel piezoelectric mechanism which can change the current understanding of piezoelectric phenomena are presented.…”

“…To clarify the mechanism of ZnO {001} crystal plane enhancing photocatalysis, we first proposed the concept of internal electric field between the Zn-terminated (0001) and O-terminated (000) polar surfaces, and presented a model of charge separation promoted by the interior electric field . Subsequently, there appeared a flurry of reports on polar crystal facets discovered as active crystalline surfaces for photocatalysis. − In addition, the enhanced rectification, photovoltaic, and piezoelectric generation performances are observed in the [0001] polar orientated wurtzite ZnO thin films , and nanorods . The internal electric field from the spontaneous polarization in the [0001] polar direction is named spontaneous electric field (E s ). , The existence of E s in polar structure has been found to play a crucial role in the photocatalysis and photovoltaic, rectification, and piezoelectric effects.…”

“…30,32 The existence of E s in polar structure has been found to play a crucial role in the photocatalysis and photovoltaic, rectification, and piezoelectric effects. A general model of photogenerated charge separation between polar crystal facets 30 and a novel piezoelectric mechanism which can change the current understanding of piezoelectric phenomena 32 are presented. As a result, an unified polar structure theory has been preliminarily established.…”