Interfacial charge-transfer absorption: semiclassical treatment

Optically induced charge transfer between adsorbed molecules and a metal electrode was predicted by Hush to lead to new electronic absorption features but has not been experimentally observed. However, Gerischer characterized photocurrents arising from such absorption between adsorbed metal atoms and semiconductor conduction bands. Interfacial charge-transfer absorption (IFCTA) provides information concerning the barriers to charge transfer between molecules and the metal/semiconductor and the magnitude of the electronic coupling and could thus provide a powerful tool for understanding interfacial charge-transfer kinetics. Here we provide a framework for modeling and predicting IFCTA spectra. The key feature of optical charge transfer to or from a band of electronic levels (taken to have a constant density of states and electronic coupling element) is that the absorption probability reaches half intensity at lambda + DeltaG(theta), where lambda and DeltaG(theta) are the reorganization energy and free-energy gap for the optical charge transfer, attains >90% intensity at lambda + DeltaG(theta) + 0.9 square root[4lambdak(B)T], and remains essentially constant until the top (bottom) level of the band is attained. However, when the electronic coupling and transition moment are assumed to be independent of photon energy (Mulliken-Hush model), a peaked, highly asymmetric absorption profile is predicted. We conclude that, in general, the electronic coupling between molecular adsorbates and the metal levels is so small that absorption is not detectable, whereas for semiconductors there may be intense features involving coupling to surface states.     

Interfacial charge transfer absorption: Application to metal–molecule assemblies

Optically induced charge transfer between adsorbed molecules and a metal electrode was predicted by Hush to lead to new electronic absorption features, but has been only rarely observed experimentally. Interfacial charge transfer absorption (IFCTA) provides information concerning the barriers to charge transfer between molecules and the metal/semiconductor and the magnitude of the electronic coupling and could thus provide a powerful tool for understanding interfacial charge-transfer kinetics. Here, we utilize a previously published model [C. Creutz, B.S. Brunschwig, N. Sutin, J. Phys. Chem. B 109 (2005) 10251] to predict IFCTA spectra of metal–molecule assemblies and compare the literature observations to these predictions. We conclude that, in general, the electronic coupling between molecular adsorbates and the metal levels is so small that IFCTA is not detectable. However, few experiments designed to detect IFCTA have been done. We suggest approaches to optimizing the conditions for observing the process.     

Interfacial electron transfer in metal cyanide-sensitized TiO2 nanoparticles

Electroabsorption (Stark) spectroscopy has been used to study the charge-transfer absorption from a transition-metal-cyanide complex to a TiO2 nanoparticle. Transition-metal cyanide/TiO2(particle) systems were synthesized using FeII(CN)(6)4-, RuII(CN)6(4-), MoIV(CN)(8)4-, and WIV(CN)8(4-). On formation of the M(CN)n4-/TiO2(particle) system, a new metal-to-particle charge-transfer (MPCT) absorption band is observed in the 390-480 nm region. Analysis of the absorption spectra suggests that the TiO2 level involved in the MPCT transition resides at significantly higher energy than the bottom of the conduction band and that the electronic coupling between the two metal centers is the dominant factor determining the position of the MPCT band maximum. The average charge-transfer distances determined by Stark spectra range from 4.1-4.7 A. The observation of relatively short charge-transfer distances leads to the conclusion that the MPCT absorption is from the transition-metal cyanide center to a level that is localized on the Ti atom bound to a nitrogen end of the [O2Ti-N-C-M(CN)x] system. The electronic coupling, Hab, calculated for a two state model is similar to values observed in dinuclear metal complexes.     

基于WO3?x量子点和TiO2构建高效Z型光催化产氢催化剂

TiO2具有高效、廉价、无毒及光化学稳定性好等优点,因而被广泛应用于光能转化和利用领域,如太阳能电池、光催化分解水制氢和环境污染物降解等.但是,TiO2仍然存在一些缺陷制约了其应用,其中,最关键的问题是光生电荷分离效率低.因此,人们对其进行了掺杂、异质结构建和Z型结构建等来解决这一问题,其中Z型结近年来备受关注.全固体Z型结的构建目前主要有两种方式:PSI-C-PSII和PSI-PSII.前者PSI与PSII间要插入中间导电层(如Au、rGO等)来实现界面欧姆接触;后者则无中间层,而是基于界面设计来实现欧姆接触.本文以构建PSI-PSII Z型结为目标,以TiO2和WO3为基础半导体材料,采用原位溶剂热生长的方法构建WO3量子点/TiO2结构,借助氢气还原反应在界面处引入氧缺陷.采用透射电子显微镜、X射线衍射和拉曼光谱研究了复合晶体结构,采用X射线光电子能谱、紫外可见光谱和荧光光谱等手段研究了Z型结的界面结构和能带结构.结合光催化分解水产氢活性来建立Z型结结构与光催化性能的关联关系.表征结果表明,在TiO2上进行原位溶剂热成核反应可点缀WO3量子点,并且量子点粒径随W前驱体用量的增加而变大.两种半导体材料为TiO2锐钛矿和WO3晶体结构,且WO3的XRD特征峰和Raman特征吸收峰会随W前驱体用量增加而变大.通过对WO3/TiO2进行氢气还原处理,使其表面形成大量W5+和氧缺陷,一方面提高了催化剂对可见光的吸收,另一方面在界面形成欧姆接触,实现了Z型结构的构建.Z型结构实现了光催化分解水产氢反应,其中WTH10光催化活性最好.本文为新型Z型光催化剂的设计和构建提供了新思路和策略.     

Reaction intermediates in the photoreduction of oxygen molecules at the (101) TiO2 (anatase) surface

The structural, electronic, and vibrational properties of intermediates of the O(2) photoreduction at the (101) TiO(2) (anatase) surface have been investigated by performing ab initio density functional calculations. In detail, a recently proposed approach has been used where molecules on the surface are treated like surface defects. Thus, by applying theoretical methods generally used in the physics of semiconductors, we successfully estimate the location and donor/acceptor character of the electronic levels induced by an adsorbed molecule in the TiO(2) energy gap, both crucial for the surface-molecule charge-transfer processes, and investigate the formation and the properties of charged intermediates. The present approach permits a view of the O(2) photoreduction process through several facets, which elucidates the molecule-surface charge-transfer conditions and reveals the key role played by charged intermediates. A comparison of present results with those of a highly sensitive IR (infrared) spectroscopy study of intermediates of the O(2) photoreduction leads to a deeper understanding of this process and to revised vibrational-line assignments and reaction paths.     

Charge-transfer studies of iron cyano compounds bound to nanocrystalline TiO(2) surfaces

Nanocrystalline (anatase) titanium dioxide films have been sensitized to visible light with K(4)[Fe(CN)(6)] and Na(2)[Fe(LL)(CN)(4)], where LL = bpy (2,2'-bipyridine), dmb (4,4'-dimethyl-2,2'-bipyridine), or dpb (4,4'-diphenyl-2,2'-bipyridine). Coordination of Fe(CN)(6)(4-) to the TiO(2) surface results in the appearance of a broad absorption band (fwhm approximately 8200 cm(-1)) centered at 23800 +/- 400 cm(-1) assigned to an Fe(II)-->TiO(2) metal-to-particle charge-transfer (MPCT) band. The absorption spectra of Fe(LL)(CN)(4)(2-) compounds anchored to TiO(2) are well modeled by a sum of metal-to-ligand charge-transfer (MLCT) bands and a MPCT band. Pulsed light excitation (417 or 532 nm, approximately 8 ns fwhm, approximately 2-15 mJ/pulse) results in the immediate appearance of absorption difference spectra assigned to an interfacial charge separated state [TiO(2)(e(-)), Fe(III)], k(inj) > 10(8) s(-1). Charge recombination is well described by a second-order equal concentration kinetic model and requires milliseconds for completion. A model is proposed wherein sensitization of Fe(LL)(CN)(4)(2-)/TiO(2) occurs by MPCT and MLCT pathways, the quantum yield for the latter being dependent on environment. The solvatochromism of the materials allows the reorganization energies associated with charge transfer to be quantified. The photocurrent efficiencies of the sensitized materials are also reported.     

电化学制备异质二氧化钛结构增强其在光电分解水中转化效率（英文）

本文描述了一种利用电化学方法合成第二型异质二氧化钛纳米棒结构的方法,这种纳米棒由部分还原的黑色二氧化钛和原始的白色二氧化钛组成.这种异质结结构的半还原的二氧化钛纳米棒与原始的白色二氧化钛和完全还原的黑色二氧化钛纳米棒相比,展现出更好的光电催化性能.这可归结于这种条件下合成的纳米棒具备的协同作用.一方面,这类结构增强了整个太阳光光谱的利用率.底层的黑色二氧化钛能够大量吸收可见光,而上层的白色二氧化钛能够较好的利用太阳光中紫外光的部分.另一方面,因为形成了第二型异质结结构,电荷的激发分离和传输得到了有效的控制.这种简单的制备方法可进一步拓展合成其他金属氧化物,来进一步研究光电分解水的性能.本文利用传统的三电极系统:0.05 mol/L的硫酸作为电解液,在加–0.5 V的偏压下进行.相比与已报道的还原手段,该电解液中质子的浓度相对比较低,可以保证这个还原过程逐步发生.电化学还原曲线可以明显反映二氧化钛还原的过程,尽管从扫描电子显微镜中很难观察到响应的区别,但是在该过程中,电极片的颜色发生了明显的变化.相应的,我们将这个系列的电极进行了光学和晶体学的研究,发现其在还原过程中,能带能够逐步减小.而在X射线衍测试当中,仅最终完全还原的得到的样品在20度左右出现两个额外的峰,可归结为还原状态的二氧化钛.电化学扫描进一步测试其能带的相应位置,证明了黑色被还原的二氧化钛和白色的原始二氧化钛是同时存在的.最后将这个系列光电极光电催化分解水的性能进行了比较,发现在同等的测试条件下,形成异质结结构的二氧化钛具有最高的光催化效率,其值大约是白色二氧化钛的三倍,同时也比纯黑的二氧化钛增长了20%左右.另外,异质结结构的二氧化钛与黑色的相比,在可见光区域有相同的效率的同时具备更好的紫外光转化效率.最后,我们分析了异质结形成以后的相对应的能带结构,该结构能够更好的促进光吸收和转化.     

Visible-light photocurrent response of TiO2-polyheptazine hybrids: evidence for interfacial charge-transfer absorption

We investigated photoelectrodes based on TiO(2)-polyheptazine hybrid materials. Since both TiO(2) and polyheptazine are extremely chemically stable, these materials are highly promising candidates for fabrication of photoanodes for water photooxidation. The properties of the hybrids were experimentally determined by a careful analysis of optical absorption spectra, luminescence properties and photoelectrochemical measurements, and corroborated by quantum chemical calculations. We provide for the first time clear experimental evidence for the formation of an interfacial charge-transfer complex between polyheptazine (donor) and TiO(2) (acceptor), which is responsible for a significant red shift of absorption and photocurrent response of the hybrid as compared to both of the single components. The direct optical charge transfer from the HOMO of polyheptazine to the conduction band edge of TiO(2) gives rise to an absorption band centered at 2.3 eV (540 nm). The estimated potential of photogenerated holes (+1.7 V vs. NHE, pH 7) allows for photooxidation of water (+0.82 V vs. NHE, pH 7) as evidenced by visible light-driven (λ > 420 nm) evolution of dioxygen on hybrid electrodes modified with IrO(2) nanoparticles as a co-catalyst. The quantum-chemical simulations demonstrate that the TiO(2)-polyheptazine interface is a complex and flexible system energetically favorable for proton-transfer processes required for water oxidation. Apart from water splitting, this type of hybrid materials may also find further applications in a broader research area of solar energy conversion and photo-responsive devices.     

Enhanced photocatalytic activity of zeolite-encapsulated TiO2 clusters by complexation with organic additives and N-doping.

In the literature it was found that titanium oxide clusters of a few metal atoms encapsulated inside the micropores of zeolite Y exhibit large blue shifts in the Ti-O ligand-to-metal charge-transfer band as compared to non-encapsulated bulk titanium dioxide particles. This blue shift of the Ti-O absorption band is believed to have a negative effect on the photocatalytic activity of zeolite-encapsulated TiO2. We report here on circumventing this problem and increasing visible-light absorption by means of a red shift of the absorption band caused by addition of some organic molecular modifiers containing acidic OH groups that can strongly bind with titanol groups TiOH. In the studied series of zeolite-encapsulated TiO2 samples, the red shift of the optical spectrum follows the order: catechol > 4-aminobenzoic acid > benzoic acid. Also N-doping of zeolite-encapsulated TiO2 clusters by thermal treatment with urea leads to a red shift of the TiO2 absorption band that depends on the annealing and hydration conditions. By comparison to the degradation of phenol in aqueous solution, we have demonstrated that these changes in the absorption spectrum on addition of the organic modifier are also reflected in the photocatalytic activity of the samples; a greater increase in photocatalytic activity (about 30%) was observed for the additive catechol.     

Direct observation of reactive trapped holes in TiO2 undergoing photocatalytic oxidation of adsorbed alcohols: evaluation of the reaction rates and yields

To investigate the primary process of photocatalytic oxidation of TiO2, interfacial charge-transfer reaction of trapped holes formed in nanocrystalline TiO2 films by UV irradiation was directly measured by highly sensitive femtosecond and nanosecond transient absorption spectroscopy under low intensity excitation condition to avoid fast electron-hole recombination. Accordingly, the rates and yields of photocatalytic oxidation of several alcohols adsorbed on TiO2 were evaluated successfully.     

Exciton-coupled charge-transfer dynamics in a porphyrin J-aggregate/TiO(2) complex

Exciton-coupled charge-transfer (CT) dynamics in TiO(2) nanoparticles (NP) sensitized with porphyrin J-aggregates has been studied by femtosecond time-resolved transient absorption spectroscopy. J-aggregates of 5,10,15-triphenyl-20-(3,4-dihydroxyphenyl) porphyrin (TPPcat) form CT complexes on TiO(2) NP surfaces. Catechol-mediated strong CT coupling between J-aggregate and TiO(2) NP facilitates interfacial exciton dissociation for electron injection into the conduction band of the TiO(2) nanoparticle in pulse width limited time (<80 fs). Here, the electron-transfer (<80 fs) process dominates over the intrinsic exciton-relaxation process (J-aggregates: ca. 200 fs) on account of exciton-coupled CT interaction. The parent hole on J-aggregates is delocalized through J-aggregate excitonic coherence. As a result, holes immobilized on J-aggregates are spatially less accessible to electrons injected into TiO(2) , and thus the back electron transfer (BET) process is slower than that of the monomer/TiO(2) system. The J-aggregate/porphyrin system shows exciton spectral and temporal properties for better charge separation in strongly coupled composite systems.     

Synthesis and ~(183)W NMR Characterization of α-K_7GaW_(11)(TiO_2)O_(39)·11H_2O Heteropolyanion

In recent years, the research of peroxy polyoxometalates (POMs) has attracted increasing attention because of the two reasons. First, peroxy POMs are more active as oxygenation catalysts in some oxygen transfer reactions1~5. Second, recent research from our group and others has established that peroxy POMs are more active antiviral agents and less toxic both in vitro and in vivo than the parent metal-free POMs6. In order to entirely understand the properties and activities of peroxy POMs…     

PbS/CdS-sensitized mesoscopic SnO2 solar cells for enhanced infrared light harnessing

Metal oxide semiconductors with lower lying conduction band minimum and superior electron mobility are essential for efficient charge separation and collection in PbS-sensitized solar cells. In the present study, mesoscopic SnO(2) was investigated as an alternative photoanode to the commonly used TiO(2) and examined comprehensively in PbS-sensitized liquid junction solar cells. To exploit the capability of PbS in an optimized structure, cascaded nPbS/nCdS and alternate n(PbS/CdS) layers deposited by a successive ionic layer adsorption and reaction method were systematically scrutinized. It was observed that the surface of SnO(2) has greater affinity to the growth of PbS compared with TiO(2), giving rise to much enhanced light absorption. In addition, the deposition of a CdS buffer layer and a ZnS passivation layer before and after a PbS layer was found to be beneficial for efficient charge separation. Under optimized conditions, cascaded PbS/CdS-sensitized SnO(2) exhibited an unprecedented photocurrent density of 17.38 mA cm(-2) with pronounced infrared light harvesting extending beyond 1100 nm, and a power conversion efficiency of 2.23% under AM 1.5, 1 sun illumination. In comparison, TiO(2) cells fabricated under similar conditions showed much inferior performance owing to the less efficient light harnessing of long wavelength photons. We anticipate that the systematic study of PbS-sensitized solar cells utilizing different metal oxide semiconductors as electron transporters would provide useful insights and promote the development of semiconductor-sensitized mesoscopic solar cells employing panchromatic sensitizers.     

Characteristics of high efficiency dye-sensitized solar cells

Impedance spectroscopy was applied to investigate the characteristics of dye-sensitized nanostructured TiO2 solar cells (DSC) with high efficiencies of light to electricity conversion of 11.1% and 10.2%. The different parameters, that is, chemical capacitance, steady-state transport resistance, transient diffusion coefficient, and charge-transfer (recombination) resistance, have been interpreted in a unified and consistent framework, in which an exponential distribution of the localized states in the TiO2 band gap plays a central role. The temperature variation of the chemical diffusion coefficient dependence on the Fermi-level position has been observed consistently with the standard multiple trapping model of electron transport in disordered semiconductors. A Tafel dependence of the recombination resistance dependence on bias potential has been rationalized in terms of the charge transfer from a distribution of surface states using the Marcus model of electron transfer. The current-potential curve of the solar cells has been independently constructed from the impedance parameters, allowing a separate analysis of the contribution of different resistive processes to the overall conversion efficiency.     

Donor-π-Acceptors Containing the 10-(1,3-Dithiol-2-ylidene)anthracene Unit for Dye-Sensitized Solar Cells

Two donor-acceptor molecular tweezers incorporating the 10-(1,3-dithiol-2-ylidene)anthracene unit as donor group and two cyanoacrylic units as accepting/anchoring groups are reported as metal-free sensitizers for dye-sensitized solar cells. By changing the phenyl spacer with 3,4-ethylenedioxythiophene (EDOT) units, the absorption spectrum of the sensitizer is red-shifted with a corresponding increase in the molar absorptivity. Density functional calculations confirmed the intramolecular charge-transfer nature of the lowest-energy absorption bands. The new dyes are highly distorted from planarity and are bound to the TiO(2) surface through the two anchoring groups in a unidentate binding form. A power-conversion efficiency of 3.7?% was obtained with a volatile CH(3) CN-based electrolyte, under air mass 1.5 global sunlight. Photovoltage decay transients and ATR-FTIR measurements allowed us to understand the photovoltaic performance, as well as the surface binding, of these new sensitizers.     

光促甲烷和水合成甲醇和氢催化剂的研究

用表面改性法制备TiO2/SiO2，以等体积浸渍法制备负载型复合半导体催化剂(MoO3,ZnO)-TiO2/SiO2，通过XRD、BET、TPR、IR、UV-Vis DRS和TPD等技术对材料的表面形态结构、吸光特性、化学吸附性能及光催化甲烷和水的反应性能进行了表征和分析。结果表明，MoO3-TiO2/SiO2和ZnO-TiO2/SiO2的表面物种均具有显著的量子尺寸效应，在表面分别形成Mo—O—Ti和Zn—O—Ti复合结构；MoO3和TiO2在载体表面的复合可提高对光能的利用率并可增强对甲烷的化学吸附性能， 结果使得MoO3-TiO2/SiO2的光催化反应性能明显优于ZnO-TiO2/SiO2；在固定床环隙反应器中，150℃MoO3-TiO2/SiO2光催化气相甲烷和水合成了目的产物甲醇和氢，甲醇的选择性达到87.3％。     

Synergistic enhanced photocatalytic and photothermal activity of Au@TiO2 nanopellets against human epithelial carcinoma cells

The photocatalytic and plasmonic photothermal cancer cell-killing activity of the metallic Au-capped TiO(2) (Au@TiO(2)) composite colloidal nanopellets has been investigated on HeLa cells under UV-visible (350-600 nm) light irradiation. The Au@TiO(2) composite nanopellets with the uniform Au-capped TiO(2) structure were successfully synthesized by simple reduction of HAuCl(4) on the surface of TiO(2) nanoparticles. The morphological structure and surface properties of Au@TiO(2) were characterized by using UV-visible absorption spectroscopy, TEM, SEM, XPS, EDX and XRD analyses. The formation of hydroxyl radicals (˙OH) was confirmed by photoluminescence (PL) spectra. The photocatalytic and photothermal cell-killing activity of the Au@TiO(2) nanopellets was found to vary with the molar ratio of Au to TiO(2). The direct involvement of the metal particles in mediating the electron transfer from the photoexcited TiO(2) under the band gap excitation is considered to carry out the efficient photocatalytic reaction on the cells. The plasmonic absorption spectra of Au@TiO(2) suspensions were also measured for the evaluation of photothermal cell killing. The charge separation, the interfacial charge-transfer and photothermal activity promoted the photocatalytic-photothermal cancer-cell killing more than TiO(2) alone. The cytotoxic effect of Au@TiO(2) nanopellets with low concentration of gold (TiO(2) : Au molar ratio > 1 : 1) was found to be 100%, whereas that of the commercial TiO(2) (P25) was ca. 50%. The comparative study of the cell viability using Au alone and TiO(2) alone revealed that the synergistic effect of photocatalytic hydroxyl radical formation and Au-plasmonic photothermal heat generation plays a vital role in the cancer cell killing. A plausible mechanism was also proposed for photocatalytic cancer cell killing based on the obtained results.     

Photoinduced electron injection from Ru(dcbpy)2(NCS)2 to SnO2 and TiO2 nanocrystalline films

Photoinduced electron injection from the sensitizer Ru(dcbpy)2(NCS)2 (RuN3) into SnO2 and TiO2 nanocrystalline films occurs by two distinct channels on the femto- and picosecond time scales. The faster electron injection into the conduction band of the different semiconductors originates from the initially excited singlet state of RuN3, and occurs in competition with intersystem crossing. The rate of singlet electron injection is faster to TiO2 (1/55 fs-1) than to SnO2 (1/145 fs-1), in agreement with higher density of conduction band acceptor states in the former semiconductor. As a result of competition between the ultrafast processes, for TiO2 singlet, whereas for SnO2 triplet electron injection is dominant. Electron injection from the triplet state is nonexponential and can be fitted with time constants ranging from approximately 1 ps (2.5 ps for SnO2) to approximately 50 ps for both semiconductors. The major part of triplet injection is independent of the semiconductor and is most likely controlled by intramolecular dynamics in RuN3. The overall time scale and the yield of electron injection to the two semiconductors are very similar, suggesting that processes other than electron injection are responsible for the difference in efficiencies of solar cells made of these materials.     

磁载WO3－TiO2／SiO2／Fe3O4复合光催化剂的制备及其光催化活性

 用溶胶－凝胶法在表面包覆了SiO2的磁基体Fe3O4上负载TiO2,从而得到了易于磁性固液分离的磁载WO3－TiO2／SiO2／Fe3O4复合光催化剂,并通过IR,XRD,SEM和XPS等测试手段对催化剂进行了表征．研究了磁载WO3－TiO2／SiO2／Fe3O4复合光催化剂对亚甲基蓝溶液脱色的性能,并考察了WO3掺杂量对样品催化活性的影响．结果表明,n（WO3）／n（TiO2）＝0．001时,磁载WO3－TiO2／SiO2／Fe3O4复合光催化剂的催化活性最高,循环使用3次时脱色率仍保持在98％．     

纳米TiO2光催化剂改性研究进展

TiO2光催化活性高,化学稳定性好,绿色环保及价廉等特点,在污水处理、有机污染物降解、催化制氢、抗菌抗癌及自清洁等方面有巨大的应用前景。然而TiO2半导体禁带宽,光谱范围小,光量子效率低等问题限制了其光催化性能。文章综述了TiO2光催化剂优化改性的研究进展,系统分析了TiO2半导体的结构特征与作用机理,以及TiO2光催化过程的影响因素,着重介绍了TiO2光催化剂多元优化改性方法的优势及不足,最后对未来发展新型TiO2基光催化剂提出了建议。