Congjie Zhang

Charge Separation between Polar {111} Surfaces of CoO Octahedrons and Their Enhanced Visible-Light Photocatalytic Activity

Crystal facet engineering of semiconductors has been proven to be an effective strategy to increase photocatalytic performances. However, the mechanism involved in the photocatalysis is not yet known. Herein, we report our success in that photocatalytic performances of the Cl(-) ion capped CoO octahedrons with exposed {111} facets were activated by a treatment using AgNO3 and NH3·H2O solutions. The clean CoO {111} facets were found to be highly reactivity faces. On the basis of the polar structure of the exposed {111} surfaces, a charge separation model between polar {111} surfaces is proposed. There is an internal electric field between polar {111} surfaces due to the spontaneous polarization. The internal electric field provides a driving force for charge separation. The reduction and oxidation reactions selectively take place on the positive and negative polar {111} surfaces. The charge separation model provides a clear insight into charge transfer in the semiconductor nanocrystals with high photocatalytic activities and offer guidance to design more effective photocatalysts, solar cells, photoelectrodes, and other photoelectronic devices.

Visible-light photocatalysis in Cu2Se nanowires with exposed {111} facets and charge separation between (111) and () polar surfaces

The search for active narrow band gap semiconductor photocatalysts that directly split water or degrade organic pollutants under solar irradiation remains an open issue. We synthesized Cu2Se nanowires with exposed {111} facets using ethanol and glycerol as morphology controlling agents. The {111} facets were found to be the active facets for decomposing organic contaminants in the entire solar spectrum. Based on the polar structure of the Cu2Se {111} facets, a charge separation model between polar (111) and () surfaces is proposed. The internal electric field between polar (111) and () surfaces created by spontaneous polarization drives charge separation. The reduction and oxidation reactions occur on the positive (111) and negative () polar surfaces, respectively. This suggests the surface-engineering of narrow band gap semiconductors as a strategy to fabricate photocatalysts with high reactivity in the entire solar spectrum. The charge separation model can deepen the understanding of charge transfer in other semiconductor nanocrystals with high photocatalytic activities and offer guidance to design more effective photocatalysts as well as new types of solar cells, photoelectrodes and photoelectric devices.

Flowerlike Cu2Te architectures constructed from ultrathin nanoflakes as superior dye adsorbents for wastewater treatment

Flowerlike architectures assembled from Cu2Te nanoflakes have been synthesized using a hydrothermal reaction of Te, Cu foils, and KBH4 with H2O in the presence of ethanol amine (EA) at 200 °C for 12 h. The Cu2Te flowerlike architectures with diameters of 4.0–6.0 μm are assembled from the nanoflakes with a thickness of 4.0–6.0 nm. When the as-synthesized Cu2Te flowerlike nanoarchitectures serve as adsorbents for acid fuschin (AF), malachite green (MG), methylene blue (MB) and rhodamine B (RhB) in water, the adsorption capacities are 3798.1, 1122.8, 77 and 14.4 mg g−1, respectively. This suggests that the Cu2Te flowerlike architectures are excellent adsorbents for AF and MG and are promising materials for the removal of AF and MG pollutants from wastewater. The excellent adsorption performance is attributed to the renewed assembly of the constituent ultrathin Cu2Te nanoflakes with thickness of 4.0–6.0 nm.

Enhanced Visible-Light Photocatalytic H 2 Evolution inCu 2 O/Cu 2 Se Multilayer HeterostructureNanowires Having {111} Facets and Physical Mechanism

It is rather challenging to develop photocatalysts based on narrow-band-gap semiconductors for water splitting under solar irradiation. Herein, we synthesized the Cu2O/Cu2Se multilayer heterostructure nanowires exposing {111} crystal facets by a hydrothermal reaction of Se with Cu and KBH4 in ethanol amine aqueous solution and subsequent annealing in air. The photocatalytic H2 production activity of Cu2O/Cu2Se multilayer heterostructure nanowires is dramatically improved, with an increase on the texture coefficient of Cu2O(111) and Cu2Se(111) planes, and thus the exposed {111} facets may be the active surfaces for photocatalytic H2 production. On the basis of the polar structure of Cu2O {111} and Cu2Se {111} surfaces, we presented a model of charge separation between the Cu-Cu2Se(111) and O-Cu2O(1̅ 1̅ 1̅) polar surfaces. An internal electric field is created between Cu-Cu2Se(111) and O-Cu2O(1̅ 1̅ 1̅) polar surfaces, because of spontaneous polarization. As a result, this internal electric field drives the photocreated charge separation. The oxidation and reduction reactions selectively occur at the negative O-Cu2O(1̅ 1̅ 1̅) and the positive Cu-Cu2Se(111) surfaces. The polar surface-engineering may be a general strategy for enhancing the photocatalytic H2-production activity of semiconductor photocatalysts. The charge separation mechanism not only can deepen the understanding of photocatalytic H2 production mechanism but also provides a novel insight into the design of advanced photocatalysts, other photoelectric devices, and solar cells.

The photovoltaic effect in a [001] orientated ZnO thin film and its physical mechanism

We report a new type of photovoltaic effect. The photovoltaic device was constructed using a [001] orientated wurtzite ZnO thin film synthesized by heating Zn(NO3)2 solution. The open-circuit voltage (Voc) and short-circuit current (Isc) of the ZnO photovoltaic device are 0.16 mV and 0.25 μA, respectively, under 365 nm ultraviolet lamp (3 W) illumination. Current rectification across the top and bottom planes of the ZnO thin film was observed. The photovoltaic and rectifying properties of the ZnO thin film are related to the magnitude of the TC(002). An internal electric field is produced in the ZnO film by spontaneous polarization in the [001] direction. The presence of the internal electric field is the fundamental physical basis of the photovoltaic effect, and a new physical mechanism of photon-to-electron conversion is proposed. The electrostatic potential provides a driving force for flow of the photogenerated electrons and holes in the semiconductor and in an external load, and thus the photo-to-electron conversion is achieved. Our result suggests thin film texturing as a strategy to develop photovoltaic devices beyond p–n junction. In addition, the photo-to-electron conversion model provides new insights into the understanding of the photovoltaic effect in ferroelectric and pyroelectric materials as well as the design and fabrication of advanced solar cells and other electronic and optoelectronic devices.

Visible-light photocatalysis in Cu2Se nanowires with exposed {111} facets and charge separation between (111) and (1[combining macron]1[combining macron]1[combining macron]) polar surfaces.

The search for active narrow band gap semiconductor photocatalysts that directly split water or degrade organic pollutants under solar irradiation remains an open issue. We synthesized Cu2Se nanowires with exposed {111} facets using ethanol and glycerol as morphology controlling agents. The {111} facets were found to be the active facets for decomposing organic contaminants in the entire solar spectrum. Based on the polar structure of the Cu2Se {111} facets, a charge separation model between polar (111) and (1[combining macron]1[combining macron]1[combining macron]) surfaces is proposed. The internal electric field between polar (111) and (1[combining macron]1[combining macron]1[combining macron]) surfaces created by spontaneous polarization drives charge separation. The reduction and oxidation reactions occur on the positive (111) and negative (1[combining macron]1[combining macron]1[combining macron]) polar surfaces, respectively. This suggests the surface-engineering of narrow band gap semiconductors as a strategy to fabricate photocatalysts with high reactivity in the entire solar spectrum. The charge separation model can deepen the understanding of charge transfer in other semiconductor nanocrystals with high photocatalytic activities and offer guidance to design more effective photocatalysts as well as new types of solar cells, photoelectrodes and photoelectric devices.