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首先声明本人不是这方面的专家，我的方向主要是光和物质相互作用方面。最近莫名其妙的接到一个邀请，为一本太阳能转化的书写其中一章，所以抓紧时间恶补了一下文献，刚刚完成草稿。其中的展望部分可以会对大家有用一些，所以贴出来分享一下。 时间很仓促，水平有限，错误以及思维局限之处在所难免，也希望大家帮我指出。 Future directions Visible catalysis of TiO2 by anodic doping and dopant-free methods Among many candidates for photocatalysts, TiO2 is the most promising material for large-scale commercial application at present due to its efficient photoactivity, good stability, environmental friendly and low cost. Since its photoactivity is more efficient in the UV region, optimization of its performance in the visible range is still a hot topic in current research. Although there is a general conclusion that metallic ion doping can extend the absorption spectrum of TiO2 to visible region, metallic ions serve as recombination centers for separated electron-hole pairs54, which decrease the efficiency of the photoactivity compared to undoped TiO2. As explained in a previous section, anodic doping of TiO2 with non-metallic elements like C, N and S, leads to the formation of new valence bands or disorder induced color centers, and is generally a better method to shift the absorption band to visible range. Unlike metallic ion doped TiO2, this method yields a photocatalyst with a much higher photoactivity of 8.35%, as reported in 2002 by Kahn et. al.65 Very recently, hydrogenation treatment was conceptually proposed to enhance the visible absorption of TiO2 by introducing disorder in the surface layers.57 We believe the performance will be further improved by delicately controlling the disorder concentration, energy level of new electronic band and distribution of disorder. There have been new strategies demonstrated recently. For example, a dopant-free, pure TiO2 phase material with a narrow band gap of ～2.1 eV was formed on the surface of rutile TiO2 (011) by oxidation of bulk titanium interstitials.66 The motivation behind this work is due to the fact that in doped TiO2, the dopant-induced defects in the crystal lattice have some negative effects on photochemical activity that are observed because doping also introduces charge carrier trapping and recombination sites. Undoped TiO2 with a narrow band gap is expected to provide much better photoactivity than its doped counterpart. Further study is necessary to understand the underlying mechanism and optimize of the structure. Novel materials Developing new photocatalytic materials is another possible method to improve the photoactivity. Of particular interests are metal organic framework (MOF) compounds, conjugated polymers, mesoporous materials (aluminosilicates) and polyoxometalates (POM). Depsite being in an early stage of research, these materials already show either comparable or higher degradation rates for the organic compounds with respect to TiO2.67 Metal organic frameworks are two dimensional or three dimensional crystalline porous materials consisting of metal ions or clusters coordinated to rigid organic ligands. The photocatalytic activity of MOF compounds is due to their semiconductor-like properties with band gaps between 1.0 and 5.5 eV.68 Upon light irradiation, they undergo a charge transfer from the ligand to metal, leaving the excited delocalized electrons with lifetimes on the microsecond time scale.69 Compared to classical photocatalysts such as TiO2, MOFs preserve a variety of pore sizes with a maximum surface area of approximately 6000 m2g-1. This is beneficial to pre-adsorption of reactants and carrier trapping on the catalyst surface. Another advantage of MOFs is their properties can be finely tuned to fit particular kinds of photocatalytic reactions or light source by choosing the constituent metals and bridging organic linkers. Polyoxometalate (POMs) represent a group of molecular clusters, which consists of three or more transition metal oxyanions (usually group 5 or group 6 transition metals) linked together by shared oxygen atoms to form a large, closed 3-dimensional framework. Some typical POM structures include: (i) Keggin [XM12O40]n−; (ii) Wells–Dawson [X2M18O62]n−; (iii) Anderson [XM6O24]n−; (iv) Lindqvist [M6O19]n− structures, where X is the heteroatom (P5+,Si4+, B3+), and M is usually Mo, V or W.67 POMs have well defined structures with a typical size of a few nanometers. They are very stable and can be easily deposited onto organic substrates for catalytic reaction by the formation of a pre-associated complex. The band gap of POMs can be adjusted for near-IR to UV absorption ranges by adjusting the size or composition of the metal oxide particulates.70 In general, decreasing the size of the particulates will increase the band gap. Their photoactivity arises due to the electron transfer between adjacent metal ions or metal–metal charge transfer (CT) bands71, which can be represented as: , where −W O represents a tungsten–oxygen bond in polyoxotungstates.72 The charge transfer from O to W leads to the formation of a hole on O, and an excited electron on W. In the absence of electron trapping reagents (such as O2, S2O82−, IO4−, BrO3−, ClO3− and H2O2), electrons accumulate on the metal oxide particulates for photocatalytic reaction. Conjugated polymers with semiconducting band gaps such as P3HT and MEH-PPV, also exhibit photocatalytic properties.73 The greatest advantage is that its light absorption and conversion capacity are easily adjusted to the solar spectrum by readily tuning the band gap. It also has the drawback of the lower durability, relative high cost and higher toxicity in contrast to inorganic TiO2. Developing cheap, stable and environmentally friendly conjugated polymers with a favorable semiconductor band gap is the ultimate goal of this field. Two good candidates filling the above requirement are graphitic carbon nitride sheets and graphene nanoribbons (Figure 18)74. They both have a two-dimensional rigid structure which is slightly different from the traditional conjugated polymers. They both exhibit good stability toward acid, strong base and high temperature and their band structures can be tuned by adjusting their size and edge structures. Although studies on both of them are still in the early stages due to the fact that they are still relatively new materials, this field is growing rapidly because of the interest generated by the favorable properties mention above. Hybrid materials As explained previously, another approach to capturing longer wavelength light includes modifying the surface of TiO2 with narrow band gap semiconductors (like CdS, PbS, CdSe, Bi2S3) and dyes as sensitizers. This approach, although it is theoretically possible to use it for water photoelectrolysis, has practical problems, principally that most photo-sensitizers would be too unstable for practical application. The interface property is essential for electron-transfer at photocatalysis process. Therefore, tuning the interfacial structure by the formation of hybrid materials can also be used to improve the photoactivity. It was already demonstrated that polyoxometalate/carbon nanotubes show remarkably improved efficiency and operational stability.75 This result opens new avenues for innovative materials designed for efficiency and cost optimization of photocatalysis. Green chemistry using photocatalysis According to the definition by United States Environmental Protection Agency, green chemistry covers such concepts as:76 I. The design of processes to maximize the amount of starting material that is incorporated into the final product; II. The use of safe, environmentally-benign substances, including solvents, whenever possible; III. The design of energy efficient processes; IV. The best form of waste disposal is to not to create any in the first place. Green chemistry aims at not producing any waste as well as maximizing the conversion efficiency of chemicals. Photocatalytic synthesis was born as a clean method and more sustainable than conventional synthesis. Following a few years’ effort, many organic photochemical reactions have been developed. According to a recent review paper by Giridhar Madras and some recent research papers, these reactions include: (i) oxidation of alkanes to alcohols, aldehydes, ketones and carboxylic acids, alcohols to aldehydes and ketones, (ii) hydroxylation of aromatics, (iii) epoxidation of alkenes, (iv) reduction of nitro-aromatics,(iv) reduction of CO2, (v) C–N and C–C coupling reactions, (vi) dehydrogenation of primary and secondary alcohols, (vii) carbonylation to produce carbamate species, (viii) oxidation of polyaromatic compounds and (viii) alkyne–azide cycloaddition.67, 77 Unlike traditional photocatalytic reactions, such as water splitting and organic material degradation, most photocatalysts for organic photochemical reactions are molecules like Ru(bpy)32+, metal porphyrins and Ru(bpy)(CO) 22+. One obstacle that has limited this application is how to separate the catalyst after homogeneous photocatalytic reaction. Developing novel hybrid catalysts or new heterogeneous catalysts should help to solve this problem. Another impediment is the developed reactions are still very few compared with the numerous organic reactions. More efforts are need to develop new photochemical reactions. [ Last edited by donkeypku on 2011-6-2 at 12:25 ] |
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小木虫(金币+0.5):给个红包，谢谢回帖
shecf(金币+5): 谢谢参与交流~ 2011-06-02 20:38:47
小木虫(金币+0.5):给个红包，谢谢回帖
shecf(金币+5): 谢谢参与交流~ 2011-06-02 20:38:47
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LZ写得挺不错的，涉及了很多光催化的最新进展。 有以下几个小问题想与LZ探讨一下： 1.LZ是侧重写均相光催化还是异相光催化呢，这两者从目前的文献看，差别应该还是比较大的，基本上属于不同的体系，我觉得LZ好像将两者放在一起了，是不是不利于读者更好的理解。除了MOFs和 Ru(bpy)32+体系，其它的基本都属于异相光催化吧。 2.我对Conjugated polymers 光催化剂的理解与LZ略有不同，我觉得它们的优点是low-cost fabrication, abundant sources and easy processibility.缺点正如LZ所说，稳定性差，另外就是光催化性能目前比较低。 3.对于graphitic carbon nitride sheets and graphene nanoribbons 这两种材料，我觉得它们与polymer 光催化剂还是很不一样，这两者应该更像无机材料，或者属于metal-free更准确一些，而不是纯粹的有机材料。 |
22楼2011-06-02 20:27:09
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wangdading
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小木虫(金币+0.5):给个红包，谢谢回帖
shecf(金币+2): 谢谢参与交流~ 2011-06-02 20:36:54
小木虫(金币+0.5):给个红包，谢谢回帖
shecf(金币+2): 谢谢参与交流~ 2011-06-02 20:36:54
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先说两个小问题: 1、我认为光活性（PHOTOACTIVITY）和光催化活性（PHOTOCATALYTIC ACTIVITY）是两个不同的概念。前者是对光响应的灵敏程度。而后者是将所吸收的光子转化为化学能（如降解污染物、分解水制氢、催化有机反应等）的能力。 2、“this method exhibit much higher photoactivity as to 8.35%.” 请问在前面如何定义“photoactivity”的？量子产率？污染物降解率？还是其他的？ |
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