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Concept: Photocatalytic water splitting


Flower-like AgCl microstructures with enhanced visible light-driven photocatalysis are synthesized by a facile one-pot hydrothermal process for the first time. The evolution process of AgCl from dendritic structures to flower-like octagonal microstructures is investigated quantitatively. Furthermore, the flower-like AgCl microstructures exhibit enhanced ability of visible light-assisted photocatalytic degradation of methyl orange. The enhanced photocatalytic activity of the flower-like AgCl microstructure is attributed to its three-dimensional hierarchical structure exposing with [100] facets. This work provides a fresh view into the insight of electrochemical process and the application area of visible light photocatalysts.

Concepts: Light, Structure, Hierarchy, Chemical synthesis, Photocatalysis, Photocatalytic water splitting, Visible spectrum, Microstructure


The generation of hydrogen from water using sunlight could potentially form the basis of a clean and renewable source of energy. Various water-splitting methods have been investigated previously, but the use of photocatalysts to split water into stoichiometric amounts of H2 and O2 (overall water splitting) without the use of external bias or sacrificial reagents is of particular interest because of its simplicity and potential low cost of operation. However, despite progress in the past decade, semiconductor water-splitting photocatalysts (such as (Ga1-xZnx)(N1-xOx)) do not exhibit good activity beyond 440 nm (refs 1,2,9) and water-splitting devices that can harvest visible light typically have a low solar-to-hydrogen efficiency of around 0.1%. Here we show that cobalt(II) oxide (CoO) nanoparticles can carry out overall water splitting with a solar-to-hydrogen efficiency of around 5%. The photocatalysts were synthesized from non-active CoO micropowders using two distinct methods (femtosecond laser ablation and mechanical ball milling), and the CoO nanoparticles that result can decompose pure water under visible-light irradiation without any co-catalysts or sacrificial reagents. Using electrochemical impedance spectroscopy, we show that the high photocatalytic activity of the nanoparticles arises from a significant shift in the position of the band edge of the material.

Concepts: Spectroscopy, Energy, Light, Hydrogen, Photoelectrochemical cell, Photocatalysis, Photocatalytic water splitting, Fuel cell


The increasing human need for clean and renewable energy has stimulated research in artificial photosynthesis, and in particular water photoelectrolysis as a pathway to hydrogen fuel. Nanostructured devices are widely regarded as an opportunity to improve efficiency and lower costs, but as a detailed analysis shows, they also have considerably disadvantages. This article reviews the current state of research on nanoscale-enhanced photoelectrodes and photocatalysts for the water splitting reaction. The focus is on transition metal oxides with special emphasis of Fe(2)O(3), but nitrides and chalcogenides, and main group element compounds, including carbon nitride and silicon, are also covered. The effects of nanostructuring on carrier generation and collection, multiple exciton generation, and quantum confinement are also discussed, as well as implications of particle size on surface recombination, on the size of space charge layers and on the possibility of controlling nanostructure energetics via potential determining ions. After a summary of electrocatalytic and plasmonic nanostructures, the review concludes with an outlook on the challenges in solar fuel generation with nanoscale inorganic materials.

Concepts: Oxygen, Energy, Hydrogen, Atom, Photoelectrochemical cell, Transition metal, Photocatalytic water splitting, Artificial photosynthesis


The combination of photocatalysis under visible light irradiation and sonolysis in the continuous system has been used to degrade an aqueous solution of phenol. ZnFe(2)O(4)/TiO(2)-GAC was employed as the photocatalysts which were obtained by sol-gel process and characterized by spectroscopic X-ray diffraction (XRD), scanning electron microscopy with energy dispersive X-ray microanalyses (SEM-EDX) and Brunauer-Emmett-Teller sorptometer (BET). It was observed that the rates of phenol degradation were affected by the initial pH value of phenol solution, salt addition, gas supplying and the recycling times of the recovered photocatalyst. The kinetic law for the phenol degradation can be apparently expressed as the first-order with respect to the concentration of phenol. Degradation of phenol solution in the continuous system, i.e., photocatalysis and sonolysis has synergistic effect in comparison with the photocatalytic reaction and sonolysis, respectively.

Concepts: Electron, X-ray, Light, Electromagnetic radiation, Concentration, Chemistry, Photocatalysis, Photocatalytic water splitting


A new 1D core-shell strategy is demonstrated for a hydrogen-generation photo-electrochemical cell (PEC). This Si/iodine-doped poly(3,4-ethylenedioxythiophene) (PEDOT) 1D nanocable array shows an encouraging solar-to-chemical energy-conversion efficiency. Coating with iodine-doped PEDOT can effectively enhance the photocatalytic efficiency and stability of SiNW arrays. The PEC model proposed shows a potentially promising structure for H(2) production using solar energy.

Concepts: Sun, Mass, Model theory, Photoelectrochemical cell, Titanium dioxide, Photocatalysis, Photocatalytic water splitting, Hydrogen production


There is increasing interest in developing artificial systems that can mimic natural photosynthesis to directly harvest and convert solar energy into usable or storable energy resources. Photocatalysis, in which solar photons are used to drive redox reactions to produce chemical fuel, is the central process to achieve this goal. Despite significant efforts to date, a practically viable photocatalyst with sufficient efficiency, stability and low cost is yet to be demonstrated. It is often difficult to simultaneously achieve these different performance metrics with a single material component. The heterogeneous photocatalysts with multiple integrated functional components could combine the advantages of different components to overcome the drawbacks of single component photocatalysts. A wide range of heterostructures, including metal/semiconductor, semiconductor/semiconductor, molecule/semiconductor and multi-heteronanostructures, have been explored for improved photocatalysts by increasing the light absorption, promoting the charge separation and transportation, enhancing the redox catalytic activity and prolonging the functional life-time. The present review gives a concise overview of heterogeneous photocatalysts with a focus on the relationship between the structural architecture and the photocatalytic activity and stability.

Concepts: Photon, Chemical reaction, Light, Catalysis, Absorption, Titanium dioxide, Photocatalysis, Photocatalytic water splitting


Ohmic contact formation at the interface of the Au nanoparticle (NP)-ZnO nanosheet (NS), which facilitates photoelectron transfer from ZnO NSs to Au NPs, is determined by scanning Kelvin microscopy for the first time. Reduction of charge recombination in the ZnO NSs confirmed by the quench of green band emission results in the enhancement of photocatalytic activity of the Au NP-ZnO NS composite.

Concepts: Photon, Light, Solar cell, Photocatalysis, Photocatalytic water splitting, Ohmic contact


Recently, it has been proven that directional flow of photogenerated charge carriers occurs on specific facets of TiO(2) nanocrystals. Herein, we demonstrate that the photocatalytic activity of anatase TiO(2) nanocrystals in both photoreduction and photooxidation processes can be enhanced by selectively depositing Pt nanoparticles on the {101} facets, which strengthens spontaneously surface-induced separation between photogenerated electrons and holes in the photocatalysis process. An optimal ratio of the oxidative {001} facets to the reductive {101} facets exists with regard to the photocatalysis of the faceted TiO(2) nanocrystals, and this is crucial for balancing the recombination and redox reaction rates of photogenerated electrons and holes. The present work might help us gain deeper insight into the relation between the specific surface of semiconductor photocatalysts and their photocatalytic activities and provides us with a new route to design photocatalysts with high photocatalytic activity.

Concepts: Photosynthesis, Hydrogen, Redox, Titanium dioxide, Photocatalysis, Photocatalytic water splitting


Formaldehyde (HCHO) is a major indoor pollutant and long-term exposure to HCHO may cause health problems such as nasal tumors and skin irritation. Photocatalytic oxidation is considered as the most promising strategy for the decomposition of HCHO. Herein, for the first time, a direct g-C3N4-TiO2 Z-scheme photocatalyst without an electron mediator was prepared by a facile calcination route utilizing affordable P25 and urea as the feedstocks. Photocatalytic activities of the as-prepared samples were evaluated by the photocatalytic oxidation decomposition of HCHO in air. It was shown that the photocatalytic activity of the prepared Z-scheme photocatalysts was highly dependent on the g-C3N4 content. At the optimal g-C3N4 content (sample U100 in this study), the apparent reaction rate constant was 7.36 × 10(-2) min(-1) for HCHO decomposition, which exceeded that of pure P25 (3.53 × 10(-2) min(-1)) by a factor of 2.1. The enhanced photocatalytic activity could be ascribed to the formation of a g-C3N4-TiO2 Z-scheme photocatalyst, which results in the efficient space separation of photo-induced charge carriers. Considering the ease of the preparation method, this work will provide new insights into the design of high-performance Z-scheme photocatalysts for indoor air purification.

Concepts: Nitrogen, Reaction rate constant, Photocatalysis, Chemical kinetics, Rate equation, Irritation, Photocatalytic water splitting, Reaction rate


How to extend ultraviolet photocatalysts to the visible-light region is a key challenge for solar-driven photocatalysis. Herein, we show that ultraviolet ZnO photocatalysts can present high visible-light photocatalytic activity when combined with CuO quantum dots (QDs; <3 nm). Theoretical analysis demonstrates that the quantum size effect plays a key role in the photoactivity of the CuO/ZnO composite. For CuO QDs smaller than 3 nm, the separated charges could transfer from CuO QDs to the conduction bands of ZnO due to quantum splitting of the CuO energy level and phonon compensation for the difference in the conduction band minimum of CuO and ZnO; however, this process would not occur with the disappearance of the quantum size effect. Further structural analysis demonstrates that interfacial charge separation and transfer between ZnO and CuO dominate the photocatalytic processes instead of a single CuO or ZnO surface. Compared with ZnOnoble metal structures (e.g., ZnOAg or ZnOAu), these ZnOCuO QD composites present wider absorption bands, higher visible photocatalytic efficiencies, and lower costs.

Concepts: Photon, Energy, Condensed matter physics, Solar cell, Titanium dioxide, Photocatalysis, Photocatalytic water splitting, Band gap