Tianquan Lian

Homogeneous light-driven water oxidation catalyzed by a tetraruthenium complex with all inorganic ligands.

A totally homogeneous, molecular, visible-light-driven water oxidation system is reported. The three system components are (i) a water oxidation catalyst, 1 (a Ru(IV)(4)O(4) cluster stabilized by oxidatively resistant [SiW(10)O(32)](8-) ligands); (ii) a photosensitizer, [Ru(bpy)(3)](2+); and (iii) a sacrificial electron acceptor, S(2)O(8)(2-). Dioxygen is formed rapidly with an initial turnover frequency of approximately 8 x 10(-2) s(-1) and an estimated quantum yield (defined as the number of O(2) molecules formed per two photons absorbed) of approximately 9%.

Efficient Light-Driven Carbon-Free Cobalt-Based Molecular Catalyst for Water Oxidation

The abundant-metal-based polyoxometalate complex [Co(4)(H(2)O)(2)(PW(9)O(34))(2)](10-) is a hydrolytically and oxidatively stable, homogeneous, and efficient molecular catalyst for the visible-light-driven catalytic oxidation of water. Using a sacrificial electron acceptor and photosensitizer, it exhibits a high (30%) photon-to-O(2) yield and a large turnover number (>220, limited solely by depletion of the sacrificial electron acceptor) at pH 8. The photocatalytic performance of this catalyst is superior to that of the previously reported precious-metal-based polyoxometalate water oxidation catalyst [{Ru(4)O(4)(OH)(2)(H(2)O)(4)}(γ-SiW(10)O(36))(2)](10-).

Efficient hot-electron transfer by a plasmon-induced interfacial charge-transfer transition

Improving electron harvesting Small metal nanostructures generate electrons from light by creating surface plasmons, which can transfer “hot electrons” to a semiconductor. The efficiency of this process, however, is often low because of electron-electron scattering. Wu et al. demonstrate a pathway that allows the plasmon to directly excite an electron in a strongly coupled semiconductor acceptor (see the Perspective by Kale). Cadmiun selenide nanorods bearing gold nanoparticles on their ends strongly damped plasmons via interfacial electron transfer with a quantum efficiency exceeding 24%. Science, this issue p. 632; see also p. 587 The plasmon of a gold nanoparticle on a cadmium selenide nanorod is strongly damped by interfacial electron transfer. [Also see Perspective by Kale and Christopher] Plasmon-induced hot-electron transfer from metal nanostructures is a potential new paradigm for solar energy conversion; however, the reported efficiencies of devices based on this concept are often low because of the loss of hot electrons via ultrafast electron-electron scattering. We propose a pathway, called the plasmon-induced interfacial charge-transfer transition (PICTT), that enables the decay of a plasmon by directly exciting an electron from the metal to a strongly coupled acceptor. We demonstrated this concept in cadmium selenide nanorods with gold tips, in which the gold plasmon was strongly damped by cadmium selenide through interfacial electron transfer. The quantum efficiency of the PICTT process was high (>24%), independent of excitation photon energy over a ~1–electron volt range, and dependent on the excitation polarization.

Controlling charge separation and recombination rates in CdSe/ZnS type I core-shell quantum dots by shell thicknesses.

Type I core/shell quantum dots (QDs) have been shown to improve the stability and conversion efficiency of QD-sensitized solar cells compared to core only QDs. To understand how the shell thickness affects the solar cell performance, its effects on interfacial charge separation and recombination kinetics are investigated. These kinetics are measured in CdSe/ZnS type I core/shell QDs adsorbed with anthroquinone molecules (as electron acceptor) by time-resolved transient absorption spectroscopy. We show that the charge separation and recombination rates decrease exponentially with the shell thickness (d), k(d) = k(0)e(-βd), with exponential decay factors β of 0.35 ± 0.03 per Å and 0.91 ± 0.14 per Å, respectively. Model calculations show that these trends can be attributed to the exponential decrease of the 1S electron and hole densities at the QD surface with the shell thickness. The much steeper decrease in charge recombination rate results from a larger hole effective mass (than electron) in the ZnS shell. This finding suggests possible ways of optimizing the charge separation yield and lifetime by controlling the thickness and nature of the shell materials.

Ultrafast Charge Separation and Long-Lived Charge Separated State in Photocatalytic CdS–Pt Nanorod Heterostructures

Colloidal semiconductor-metal nanoheterostructures that combine the light-harvesting ability of semiconductor nanocrystals with the catalytic activity of small metal nanoparticles show promising applications for photocatalysis, including light-driven H(2) production. The exciton in the semiconductor domain can be quenched by electron-, hole-, and energy transfer to the metal particle, and the competition between these processes determines the photocatalytic efficiency of these materials. Using ultrafast transient absorption spectroscopy, we show that, in CdS-Pt heterostructures consisting of a CdS nanorod with a Pt nanoparticle at one end, the excitons in the CdS domain dissociate by ultrafast electron transfer (with a half-life of ∼3.4 ps) to the Pt. The charge separated state is surprisingly long-lived (with a half-life of ∼1.2 ± 0.6 μs) due to the trapping of holes in CdS. The asymmetry in the charge separation and recombination times is believed to be the key feature that enables the accumulation of the transferred electrons in the Pt tip and photocatalysis in the presence of sacrificial hole acceptors.

Hole removal rate limits photodriven H2 generation efficiency in CdS-Pt and CdSe/CdS-Pt semiconductor nanorod-metal tip heterostructures.

Semiconductor-metal nanoheterostructures, such as CdSe/CdS dot-in-rod nanorods with a Pt tip at one end (or CdSe/CdS-Pt), are promising materials for solar-to-fuel conversion because they allow rational integration of a light absorber, hole acceptor, and electron acceptor or catalyst in an all-inorganic triadic heterostructure as well as systematic control of relative energetics and spatial arrangement of the functional components. To provide design principles of such triadic nanorods, we examined the photocatalytic H2 generation quantum efficiency and the rates of elementary charge separation and recombination steps of CdSe/CdS-Pt and CdS-Pt nanorods. We showed that the steady-state H2 generation quantum efficiencies (QEs) depended sensitively on the electron donors and the nanorods. Using ultrafast transient absorption spectroscopy, we determined that the electron transfer efficiencies to the Pt tip were near unity for both CdS and CdSe/CdS nanorods. Hole transfer rates to the electron donor, measured by time-resolved fluorescence decay, were positively correlated with the steady-state H2 generation QEs. These results suggest that hole transfer is a key efficiency-limiting step. These insights provide possible ways for optimizing the hole transfer step to achieve efficient solar-to-fuel conversion in semiconductor-metal nanostructures.

Photoinduced Ultrafast Electron Transfer from CdSe Quantum Dots to Re-bipyridyl Complexes

Ultrafast dissociation of excitons in CdSe quantum dots via electron transfer to adsorbed Re-bipyridyl complexes was demonstrated. The dissociation pathway was determined by the observation of reduced adsorbate using femtosecond IR spectroscopy. The rate of electron transfer was shown to increase at smaller QD sizes. Electron transfer time as fast as 2.3 ps was observed, faster than the exciton annihilation time in CdSe. The ultrafast charge separation in this quantum dot-adsorbate donor-acceptor complex provides a potential approach for separating multiple excitons in quantum dots.

Evidences of hot excited state electron injection from sensitizer molecules to TiO 2 nanocrystalline thin films

Electron injection dynamics in dye sensitized TiO2 nanocrystalline thin films are studied with femtosecond mid-infrared spectroscopy. Three classes of sensitizer molecules, Ru(dcbpy)2(X)2 (X2 = 2SCN, 2CN, and dcbpy), Fe(dcbpy)2(CN)2, and ReCl(CO)3(dcbpy), are used to examine the dependence of injection rate and yield on the excited state redox potentials. We observed that electron injection occurred on the <100 fs time-scale and injection quantum yield depended on the redox potential for the series of Ru dyes. These results suggest that electron injection to TiO2 competes with electronic and vibrational relaxation within the sensitizer excited states and the branching ratio between these two processes determines the injection quantum yield for sensitizer molecules with an excited state redox potential below the conduction band-edge.

Near Unity Quantum Yield of Light-Driven Redox Mediator Reduction and Efficient H2 Generation Using Colloidal Nanorod Heterostructures

The advancement of direct solar-to-fuel conversion technologies requires the development of efficient catalysts as well as efficient materials and novel approaches for light harvesting and charge separation. We report a novel system for unprecedentedly efficient (with near-unity quantum yield) light-driven reduction of methylviologen (MV(2+)), a common redox mediator, using colloidal quasi-type II CdSe/CdS dot-in-rod nanorods as a light absorber and charge separator and mercaptopropionic acid as a sacrificial electron donor. In the presence of Pt nanoparticles, this system can efficiently convert sunlight into H(2), providing a versatile redox mediator-based approach for solar-to-fuel conversion. Compared to related CdSe seed and CdSe/CdS core/shell quantum dots and CdS nanorods, the quantum yields are significantly higher in the CdSe/CdS dot-in-rod structures. Comparison of charge separation, recombination and hole filling rates in these complexes showed that the dot-in-rod structure enables ultrafast electron transfer to methylviologen, fast hole removal by sacrificial electron donor and slow charge recombination, leading to the high quantum yield for MV(2+) photoreduction. Our finding demonstrates that by controlling the composition, size and shape of quantum-confined nanoheterostructures, the electron and hole wave functions can be tailored to produce efficient light harvesting and charge separation materials.

Plasmon-Induced Hot Electron Transfer from the Au Tip to CdS Rod in CdS-Au Nanoheterostructures

The plasmon-exciton interaction mechanisms in CdS-Au colloidal quantum-confined plexcitonic nanorod heterostructures have been studied by transient absorption spectroscopy. Optical excitation of plasmons in the Au tip leads to hot electron injection into the CdS rod with a quantum yield of ~2.75%. This finding suggests the possibility of further optimization of plasmon-induced hot electron injection efficiency through controlling the size and shape of the plasmonic and excitonic domains for potential light harvesting applications.