oa Editorial [Hot Topic: Light-Driven Reactions and Materials in the Environmental Technology (Guest Editor: Adriana Zaleska)]

This special issue highlights a number of new directions in light-driven reactions and materials relevant to environmental technology. Light - either solar or artificial - could be used to drive heterogeneous electrochemical reactions at the surface of an optically active photocatalyst (e.g. TiO2 or modified TiO2), resulting in the solar-to chemical energy conversion via water splitting (H2 generation) and CO2 reduction into light hydrocarbons. Light-mediated photooxidation and reduction offer potentially an environmental friendly and cheap method for removing pollutants from gas and liquid streams. Methods as UV/hydrogen peroxide, heterogeneous photocatalysis (HP), photo-Fenton (PF), UV plus ozone and combination of these technologies are belonging to photochemical-based processes (PAOPs). All PAOPs are based on the production of very reactive species (especially hydroxyl radicals, HO˙) able to degrade or transform chemical pollutants, provoking ultimately total mineralization. However, one of the major drawbacks of PAOPs is that the operational costs are relatively high compared to less expensive technologies such as biological treatments, use of activated carbon, etc. However, these technologies can be used combined with themselves or with conventional technologies. According to the Malato et al. [1] the most important features of PAOPs making it applicable to the treatment of contaminated aqueos effluents are: (a) the process take place at ambient temperature and without ovepressure; (b) oxidation of the substances into CO2 and other inorganic species is complete; (c) the oxygen necessary for the reaction can be directly obtained from atmosphere, (d) the semiconductor photocatalyst is cheap, innocuous and can be reused; (e) the photocatalyst could be immobilized to the different types of inert matrices; and (f) the energy for photo-excitation of the photocatalyst can be obtained from Sun. Furthermore, irradiation can be not only employed to pollutants degradation but also to synthesize of organic compounds or nanoparticles of controlled size, morphology and structure, which allow to study their properties and to optimize their applications. Gold-based bimetallic nanoparticles of controlled composition and structure (Au-Ag, Au-Pt, Au-Pd…) and composite materials could be obtained by photochemical reaction, which induce homogeneous nucleation and reduction. One of the most studied light-driven material is titanium(IV) oxide. Strong oxidation and reduction power of photoexcited titanium(IV) dioxide was realized from the discovery of Honda-Fujishima effect. In 1972 Fujishima et al. [2] reported photoinduced decomposition of water on TiO2 electrodes. Since Frank and Bard [3] first examined the possibilities of using TiO2 to decompose cyanide in water, there has been an increasing interest in environmental applications. Based on these works, TiO2 photocatalysts are widely used for air purification, deodorization, sterilization, anti-fouling, and mist removal. TiO2 is the sole potential photocatalyst usable in many practical applications, such as indoor deodorization or outdoor self-cleaning, since it has high stability under photoirradiation without self-decomposition, adequate reduction and oxidation power of photoexcited electrons and positive holes, respectively, to drive a variety of redox reactions, non-toxicity and availability. Nevertheless, due to large band gap of pure TiO2 (from 3.0 to 3.2 eV depending on the crystal structure) it could be activated only by UV irradiation, limiting the utilization of sunlight as an irradiation source in photocatalytic reactions. To improve the efficiency of solar-driven photocatalysis a few approaches were proposed such as metal-ion implantation, TiO2 reducing, non-metal doping, preparation of composites of TiO2 with semiconductor having lower band gap energy (e.g. Cd-S particles, sensitizing of TiO2 with dyes and TiO2 doping by upconversion luminescence agent. Current research in the field of light-driven reactions and materials are mainly focused on the improvement of photoreaction efficiency. Two main approaches are proposed: (a) preparation of new materials, such as TiO2 modification to increase the spectral sensitivity of TiO2-based photocatalysts to visible light and to compete with electron-hole recombination, and (2) enhancement of interface surface area by photoreactor design. Selection of a light source together with photoreactor geometry play an important role in the reaction efficiency. Sufficient UV penetration into the radiated liquid/gas is of crucial importance. High mass transfer rates for efficient interaction between the pollutant and the photocatalyst and for high oxygen uptake at the gas-liquid interface is another requirement for practical applications. In this regard, reactor design for efficient liquid or gas phase treatment has been a challenging problem. Coming back to last year, a snapshot of current areas of research and pointing toward new directions for potential future breakthroughs were provide during some conferences in the field of light-driven reactions and materials. 18th International Conference on Photochemical Conversion and Storage of Solar Energy (July 25-30, 2010, Korea University, Seoul, Korea) attracted more than 300 participants from all over the world and gave some new ideas especially in the topic of new materials used in photocatalysis. The principle and design of functional novel materials, including precisely dimension-controlled nanostructures, were detailed presented and discussed. Presentations were focused on the area of environmental applications of heterogeneous photocatalysis. The 15th International Conference on TiO2 Photocatalysis: Fundamentals and Applications (November 15-18 2010, San Diego, USA) dealt mainly with methods of photocatalyst modification together with photocatalytic treatment and disinfection of water, wastewater and gas phase. Examples of solar-driven installation used for aqueous phase treatment/ purification was also presented. However, my personal impression is that solar-to-chemical energy conversion, such as artificial photosynthesis will be the leading topic in coming future. This special issue of Recent Patents and Engineering on Light-driven reactions and materials in the environmental technology aims to provide a survey of the current topics and the major development lines in this rapidly growing research area. The scope includes any aspect of condensed and gas phase photochemical processes employed in the environmental technologies. A broad range of processes and techniques in photochemistry are covered such as: heterogeneous and homogenous photocatalysis, others light-induced processes for water/wastewater and gas treatment, photoreactor design, light sources, photochemical materials, photochemical synthesis of nanoparticles, and hydrogen production via water splitting (fuel from solar). Light-driven reactions and materials in the environmental technology presented review manuscripts showing stateof- the-art combined with recent patent coverage so that the readers gain access to a broader perspective of the field.