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首先聲明本人不是這方面的專家，我的方向主要是光和物質(zhì)相互作用方面。最近莫名其妙的接到一個(gè)邀請，為一本太陽能轉(zhuǎn)化的書寫其中一章，所以抓緊時(shí)間惡補(bǔ)了一下文獻(xiàn)，剛剛完成草稿。其中的展望部分可以會對大家有用一些，所以貼出來分享一下。 時(shí)間很倉促，水平有限，錯(cuò)誤以及思維局限之處在所難免，也希望大家?guī)臀抑赋觥?br /> 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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