Plasmonic antenna effects on photochemical reactions

Efficient solar energy conversion has been vigorously pursued since the 1970s, but its large-scale implementation hinges on the availability of high-efficiency modules. For maximum efficiency, it is important to absorb most of the incoming radiation, which necessitates both efficient photoexcitation and minimal electron-hole recombination. To date, researchers have primarily focused on the latter difficulty: finding a strategy to effectively separate photoinduced electrons and holes. Very few reports have been devoted to broadband sunlight absorption and photoexcitation. However, the currently available photovoltaic cells, such as amorphous silicon, and even single-crystal silicon and sensitized solar cells, cannot respond to the wide range of the solar spectrum. The photoelectric conversion characteristics of solar cells generally decrease in the infrared wavelength range. Thus, the fraction of the solar spectrum absorbed is relatively poor. In addition, the large mismatch between the diffraction limit of light and the absorption cross-section makes the probability of interactions between photons and cell materials quite low, which greatly limits photoexcitation efficiency. Therefore, there is a pressing need for research aimed at finding conditions that lead to highly efficient photoexcitation over a wide spectrum of sunlight, particularly in the visible to near-infrared wavelengths. As characterized in the emerging field of plasmonics, metallic nanostructures are endowed with optical antenna effects. These plasmonic antenna effects provide a promising platform for artificially sidestepping the diffraction limit of light and strongly enhancing absorption cross-sections. Moreover, they can efficiently excite photochemical reactions between photons and molecules close to an optical antenna through the local field enhancement. This technology has the potential to induce highly efficient photoexcitation between photons and molecules over a wide spectrum of sunlight, from visible to near-infrared wavelengths. In this Account, we describe our recent work in using metallic nanostructures to assist photochemical reactions for augmenting photoexcitation efficiency. These studies investigate the optical antenna effects of coupled plasmonic gold nanoblocks, which were fabricated with electron-beam lithography and a lift-off technique to afford high resolution and nanometric accuracy. The two-photon photoluminescence of gold and the resulting nonlinear photopolymerization on gold nanoblocks substantiate the existence of enhanced optical field domains. Local two-photon photochemical reactions due to weak incoherent light sources were identified. The optical antenna effects support the unprecedented realization of (i) direct photocarrier injection from the gold nanorods into TiO(2) and (ii) efficient and stable photocurrent generation in the absence of electron donors from visible (450 nm) to near-infrared (1300 nm) wavelengths.     

Influence of orientation and ferroelectric domains on the photochemical reactivity of La2Ti2O7

The effects of crystal orientation and ferroelectric domain structure on the photochemical reactivity of La₂Ti₂O₇ have been measured. Electron backscattered diffraction was used to determine the orientations of the surfaces of crystals in a ceramic specimen, piezo-force microscopy was used to determine the domain structure, and the photocathodic reduction of silver from an aqueous silver nitrate solution was used to evaluate the photochemical reactivity. The reactivity is greatest on (001) surfaces (this is the orientation of the layers in this (110)_p layered perovskite structure) while surfaces perpendicular to this orientation have the least reactivity. Complex domain structures were observed within the grains, but they appeared to have no effect on the photocathodic reduction of silver, in contrast to previous observations on other ferroelectrics. La₂Ti₂O₇ is an example of a ferroelectric oxide in which the crystal orientation has a greater influence on the photochemical reactivity than polarization from the internal domain structure.     

Photoresponsive polymers having pendant chlorocinnamoyl moieties: synthesis, reactivity ratios and photochemical properties

New photoreactive homo- and copolymers containing pendant chlorocinnamoyl moieties were respectively synthesized by homopolymerization of corresponding acrylic monomer and copolymerization with glycidyl methacrylate. The diverse structures were characterized by different spectroscopic techniques. The thermal properties of the photosensitive polymers were investigated by thermogravimetric analysis in air and differential scanning calorimetry under nitrogen atmosphere. The compositions of polymers were determined by ¹H-NMR analysis. The reactivity ratios of both comonomers were calculated using the conventional linearization methods such as Fineman-Ross, Kelen-Tudos, extended Kelen-Tudos and a non-linear error-in-variables model (EVM) method using a computer program, RREVM, in order to optimize the reaction conditions for industrial applications. The photoreactivity of newly synthesized homo- and copolymers containing pendant chlorocinnamoyl moieties was investigated in solution as well as in thin films. The effects of solvent nature, concentration, temperature, copolymer compositions and photosensitizers on the rate of photocrosslinking of these new photoresponsive polymers were evaluated. Suitable conditions were discussed for using as negative photoresists in industries.     

腐殖酸的光化学过程模拟及其对五氯苯酚结合系数的影响

研究了2种有代表性的腐殖酸美国Swuannee河提取的腐殖酸(国际腐殖酸协会标准)和德国森林土壤腐殖酸,在模拟太阳光照射过程中不同照射时间、溶解氧浓度和入射光波长范围对五氯苯酚结合性质的影响.结果表明腐殖酸经模拟太阳光照射后,其与五氯苯酚的结合系数Koc下降.光化过程中腐殖酸的分子量或SUVA270与lg(Koc)之间有良好的线性关系.溶解氧浓度的升高能加速腐殖酸的光氧化,使结合系数进一步下降.氮气饱和条件下照射则使腐殖酸与五氯苯酚的结合系数升高.UV-B、UV-A和可见光部分对Swuannee河腐殖酸与五氯苯酚的结合系数Koc的下降的贡献分别为18%、47%和35%.     

Substituent effects on the reactivity of the silicon-carbon double bond

Laser flash photolysis of various organosilicon compounds such as aryl-, vinyl-, and alkynyldisilanes, silacyclobutanes and silacyclobutenes, and alpha-silylketenes and -diazomethanes leads to the formation of reactive silenes which can be detected directly in solution, allowing detailed studies of the kinetics and mechanisms of their reactions with nucleophiles. Over 30 transient silenes have now been studied by these methods, providing the opportunity to systematically assess the effects of substituents at silicon and carbon on the reactivity of the Si=C bond.     

煤的结构对直接液化反应性影响的分析

系统地分析了煤的组成结构及液化前的预处理对直接液化反应性的影响,包括煤阶、岩相组成、各种矿物质及酸洗脱除、水分及烘干、孔隙性、物理结构及预溶胀、H／C比等,指出不同的工艺条件下它们对直接液化的影响是不同的。     

Some kinetic aspects of radical copolymerization: influence of the reaction medium on the reactivity ratios

The reactivity ratios for the free radical copolymerization of styrene and methyl methacrylate at 50°C have been evaluated in dioxane, acetone and dimethylformamide solutions. In all these systems there is a marked solvent effect on both r₁ and r₂, which can be correlated to the variation in the dielectric constant of the solvent. The role of solvents in enhancing the polarization of growing chains and the alternation tendency is discussed.     

Some physical properties of Canadian coals and their effects on coal reactivity

The pore volume, surface area and compressibility of eleven Canadian coals, varying in rank from lignite to semianthracite, have been determined by mercury porosimetry, gas adsorption method and relations derived from helium and mercury densities. The total pore volume was measured in the diameter range of 0.2 nm–2.98 μm, which was subdivided into two groups, namely the micropore region (< 0.0036 μm) and the combined meso- and macropore region (0.0036–2.98 μm). It has been determined that the porosity of the eleven coals studied varies from 2 to 39%. It has been found that the total pore volume, micropore volume, surface area and the apparent compressibility of these coals decrease with increase in the carbon content, or the rank of the coals. The effect of the total pore volume, micropore volume and surface area on chemical reactivity of the coal is discussed separately. A good correlation was obtained between the carbon content and helium density of the coal after correction is made for the mineral content.     

Photocatalytic reactivity and diffusing OH radicals in the reaction medium containing TiO2 particles

The generation of OH radicals on UV-illuminated TiO₂ surface is mainly responsible for the photocatalytic oxidation of pollutants in various contaminated environmental media. Although the reactivity of OH radicals is largely limited within the surface region, the possibility of OH desorption and diffusion into the reaction medium has been often raised. This study provides several examples for the presence of diffusing OH radicals in aqueous solution and polymer matrix containing TiO₂ particles. The photocatalytic degradation rates of (CH₃)₄N⁺ in TiO₂ suspension were comparable between acidic and alkaline conditions, which could not be explained by a simple electrostatic surface charge model. From the present mechanistic study, it is suggested that the photocatalytic oxidation of (CH₃)₄N⁺ at acidic pH mainly proceeds through free OH radicals in the solution bulk, not on the surface of TiO₂. The diffusing OH radicals also played the role of main oxidants in the solid phase. The photolysis of TiO₂-embedded PVC composite films generated cavities around the imbedded TiO₂ particles and the development of cavity diameter continued even after the direct contact between the PVC and TiO₂ was prohibited. This implied that active oxygen species that were photogenerated on TiO₂ surface desorbed and diffused across a few micrometers to react with the polymer matrix.     

Xanthene dyes/amine as photoinitiators of radical polymerization: A comparative and photochemical study in aqueous medium

The efficiency of several xanthene dyes as photoinitiators of the free radical polymerization in aqueous medium was evaluated. These results show that dyes with triplet quantum yield higher than 0.1 present similar efficiencies, independently of their different chemical structure. A detailed study of the photophysics of the dyes under the polymerization conditions was carried out using laser time-resolved spectroscopies. These studies show that the active radicals are those which formed in the interaction of excited triplet state of the dye with the amine through an electron transfer process. In spite of this, the photoinitiation efficiency is not correlated with the triplet quantum yield. Also, the photophysics studies show that the quantum yield of the different pathways of the decomposition of the charge transfer intermediate is an important parameter to predict the efficiency of these photoinitiator systems. The experimentally measured active radical formation is well correlated with that calculated from the polymerization rate. The presence of heavy atoms in the xanthene ring increases the triplet quantum yield, but decreases the active radical yield, and then the polymerization rate.     

Ligand effects on dioxygen activation by copper and nickel complexes: reactivity and intermediates

Copper and nickel complexes having various active-oxygen species M n -O 2 ( n = 1 or 2), such as trans-(micro-1,2-peroxo)Cu (II) 2, bis(micro-oxo)M (III) 2, bis(micro-superoxo)Ni (II) 2, and ligand-based alkylperoxo-M (II) n , can be produced by a series of tetradentate tripodal ligands (TMPA analogues) containing sterically demanding 6-methyl substituent(s) on the pyridyl group(s), where TMPA = tris(2-pyridylmethyl)amine. Roles of the methyl substituent(s) for the formation of the active-oxygen species and their oxidation reactivities are reported.     

Combined effects of inorganic constituents and pyrolysis conditions on the gasification reactivity of coal chars

A detailed phenomenological study of the gasification behavior of a North Dakota lignite was undertaken and the fundamental parameters that determine char reactivity were investigated. Differences in reactivity of up to three orders of magnitude were obtained by varying the conditions of coal pretreatment and pyrolysis. Pretreatment included demineralization with HCl and HF, ion exchange with ammonium acetate and back exchange with calcium acetate. Pyrolysis temperature, residence time and heating rate were varied in the range 975–1475 K, 0.3 s-1 h and 10 K/min-10⁴ K/s, respectively. The observed reactivity differences were rationalized in terms of variations in the concentration of carbon and catalyst active sites.     

介质粒度对Falcon离心分选效果的影响

为提高Falcon重介离心分选的脱硫降灰效果,在分析介质性质的基础上,研究了介质粒度对Falcon重介离心分选产品粒度组成、灰分、硫分、可燃体回收率及脱硫效率的影响,同时考察了各产品可燃体回收率与脱硫效率的关系。结果表明:Falcon重介离心分选对+0.500 mm和-0.045 mm煤样脱硫降灰效果较差,这是由于分选过程中高密度小颗粒和低密度大颗粒容易发生错配,影响分选效果;而0.125~0.074 mm脱硫效果最好,说明0.125~0.074 mm中黄铁矿硫的解离程度相对其他粒级要高,硫分容易脱除。1号介质粒度分布比较均匀,2号介质粒度较细,形成的悬浮液也较稳定,因此可燃体回收率相同时,2号介质Falcon离心分选的脱硫效率要高于1号介质,分选效果较好。     

Double diffusive effects on radial fingering in a porous medium or a Hele-Shaw cell

Double diffusive effects on the growth of radial viscous fingering in a porous medium or a Hele-Shaw cell were analyzed theoretically. Under linear stability theory, the stability equations were derived in a self-similar domain. The stability equations were transformed using the normal mode analysis and solved analytically and numerically. Regardless of the diffusivity ratio, Le, the Péclet number, Pe, makes the system unstable, i.e., accelerates the growth of instabilities. The double diffusive effects on the growth of the instabilities were strongly dependent on the viscosity distribution. If the displaced phase has stable viscosity distribution, as Le increases, the system becomes unstable regardless of the magnitude of Pe. However, as Le increases, the growth of the instabilities is suppressed if the displaced phase has unstable viscosity distribution.     

Ultraviolet sources for photochemical application

This article reviews recent developments in laser and UV source technologies that are potentially useful for photochemical processes and photopolymerization processes in particular. Methods for measurement and calibration of light energy are described. A few publications on newer photochemical processes are also briefly reviewed.     

Gas absorption with photochemical reaction

Gas-liquid reactions can be activated by high energy radiations (photons). When gas absorption is accompanied by pseudo-first order reaction further enhancement in the specific rate of absorption can be realised for situations where either the reactive species in the liquid phase is activated or the dissolved solute gas is activated.     

PVC compounds for photochemical crosslinking

The best of all photoinitiator systems for crosslinking PVC compounds, according to tests made with a Tesla RVIM discharge tube, is the combination of chlorthioxanthone and triethanolamine. Pentaerithroltriacrylate is the most effective crosslinking agent. Lead stabilizers give the best heat stability and a higher rate of photochemical crosslinking than tin and antimony stabilizers.     

Diffusion in porous silicon: effects on the reactivity of alkenes and electrochemistry of alkylated porous silicon

Alkenes are known to react with hydrogen-terminated silicon surfaces to produce robust organic monolayers that are attached to the surface via covalent SiC bonds. In this report we investigate the dependence of the rate of alkylation of porous silicon samples on the reaction time using photochemical initiation. The kinetics of the photochemical alkylation of hydrogen-terminated porous silicon by undec-1-ene in toluene were observed to be pseudo first order, however the apparent rate constant decreased as the concentration of undec-1-ene increased. This behaviour is opposite to what would be expected if the rate-limiting process was an elementary chemical reaction step involving the alkene. Instead, it suggests that transport of the alkene to reactive sites and in the correct orientation is the rate-limiting step. Comparison of the rates of alkylation of porous silicon by undec-1-ene and dimethoxytrityl (DMT)-undecenol is consistent with such an interpretation as the bulky DMT headgroup gives a lower rate of alkylation. The diffusion of some simple redox-active probe molecules in porous silicon was investigated using a scanning electrochemical microscope (SECM). The probe molecules are converted at diffusion-controlled rate at an inlaid disk ultramicroelectrode (UME) consisting of the cross-section of a microwire sealed in glass. If the microelectrode is placed a short distance above the porous silicon, the microelectrode current depends on kinetics of the electrochemical reactions at the porous silicon and the mass transport properties within the open thin layer cell formed by the microelectrode and the alkylated porous silicon. In order to differentiate the effects of finite heterogeneous kinetics at silicon from diffusion limitations, current-distance curves were fitted over a wide range of applied potentials (on the Si) and it was observed that the diffusion coefficient in the porous layer was strongly anisotropic. The measured diffusion rates are comparable to those in bulk water along the pores, but with negligible diffusion between pores. This indicates that few pore-pore interconnections exist in the porous silicon.     

氧对光固化胶粘剂固化影响的研究

本文论述了自然环境中的O2对辐射固化高分子材料辐射固化过程中的O2抑制原理、O2抑制作用对辐射固化高分子材料性能的影响、解决O2抑制的各种途径和方法及辐射固化高分子材料今后的发展趋势。