The Journal of Physical Chemistry Letters October 6, 2016 Volume 7(Issue 19) pp:3908-3912
Publication Date(Web):September 20, 2016
DOI:10.1021/acs.jpclett.6b01903
Among various nonradiative photophysical processes related to energy migration in singlet–triplet coupled molecular systems, unlike such processes as internal conversion, intersystem crossing (ISC), and intramolecular vibrational relaxation that have been extensively addressed, the reverse ISC (rISC) is a unique one that apparently lacks sufficient interrogation probably owing to its intrinsically elusive nature. In particular, it still remains a nontrivial task to quantitatively describe the rISC pathway. Here we introduce a new, simple route to this end, just through explicit modeling and simulations on routinely available, experimental data from ultrafast transient absorption spectroscopy. We demonstrate on a proof-of-concept, rare-earth chelate molecular system that our approach, featuring spectral profile analysis together with wavelength-dependent global kinetics fitting, enables facile retrieval of the rISC rate from experimental data.
Co-reporter:Xingchen Jiao, Zongwei Chen, Xiaodong Li, Yongfu Sun, Shan Gao, Wensheng Yan, Chengming Wang, Qun Zhang, Yue Lin, Yi Luo, and Yi Xie
Journal of the American Chemical Society June 7, 2017 Volume 139(Issue 22) pp:7586-7586
Publication Date(Web):May 17, 2017
DOI:10.1021/jacs.7b02290
The effect of defects on electron–hole separation is not always clear and is sometimes contradictory. Herein, we initially built clear models of two-dimensional atomic layers with tunable defect concentrations, and hence directly disclose the defect type and distribution at atomic level. As a prototype, defective one-unit-cell ZnIn2S4 atomic layers are successfully synthesized for the first time. Aberration-corrected scanning transmission electron microscopy directly manifests their distinct zinc vacancy concentrations, confirmed by positron annihilation spectrometry and electron spin resonance analysis. Density-functional calculations reveal that the presence of zinc vacancies ensures higher charge density and efficient carrier transport, verified by ultrafast photogenerated electron transfer time of ∼15 ps from the conduction band of ZnIn2S4 to the trap states. Ultrafast transient absorption spectroscopy manifests the higher zinc vacancy concentration that allows for ∼1.7-fold increase in average recovery lifetime, confirmed by surface photovoltage spectroscopy and PL spectroscopy analysis, which ensures promoted carrier separation rates. As a result, the one-unit-cell ZnIn2S4 layers with rich zinc vacancies exhibit a carbon monoxide formation rate of 33.2 μmol g–1 h–1, roughly 3.6 times higher than that of the one-unit-cell ZnIn2S4 layers with poor zinc vacancies, while the former’s photocatalytic activity shows negligible loss after 24 h photocatalysis. This present work uncovers the role of defects in affecting electron–hole separation at atomic level, opening new opportunities for achieving highly efficient solar CO2 reduction performances.
Chemical Science (2010-Present) 2017 vol. 8(Issue 5) pp:4087-4092
Publication Date(Web):2017/05/03
DOI:10.1039/C7SC00307B
Understanding the photoexcitation processes in semiconductors is critical for the design of advanced photocatalytic materials. Nevertheless, traditional viewpoints focus on photogenerated free charge carriers, which are somehow invalid once the many-body effects are taken into account, especially for polymeric photocatalysts. Here we systematically investigate the photoexcitation processes involved in the polymer matrix of graphitic carbon nitride (g-C3N4) by combining photoluminescence spectroscopy and ultrafast transient absorption spectroscopy, validating the strong excitonic effects in the well-known photocatalyst for the first time. The identification of the robust triplet–triplet annihilation process, in which two triplet excitons collide to produce a singlet exciton, highlights an important nonradiative depopulation pathway of excited species and thereby offers potential strategies to regulate the photocatalytic activities of polymeric g-C3N4. The work establishes a new understanding of the photocatalytic mechanism in the polymeric g-C3N4 matrix, and thus paves the way for designing effective polymeric photocatalysts through excitonic engineering.
Co-reporter:Hao Huang; Lei Zhang; Zhiheng Lv; Ran Long; Chao Zhang; Yue Lin; Kecheng Wei; Chengming Wang; Lu Chen; Zhi-Yuan Li; Qun Zhang; Yi Luo;Yujie Xiong
Journal of the American Chemical Society 2016 Volume 138(Issue 21) pp:6822-6828
Publication Date(Web):May 13, 2016
DOI:10.1021/jacs.6b02532
Harnessing surface plasmon of metal nanostructures to promote catalytic organic synthesis holds great promise in solar-to-chemical energy conversion. High conversion efficiency relies not only on broadening the absorption spectrum but on coupling the harvested energy into chemical reactions. Such coupling undergoes hot-electron transfer and photothermal conversion during the decay of surface plasmon; however, the two plasmonic effects are unfortunately entangled, making their individual roles still under debate. Here, we report that in a model system of bimetallic Au–Pd core–shell nanostructures the two effects can be disentangled through tailoring the shell thickness at atomic-level precision. As demonstrated by our ultrafast absorption spectroscopy characterizations, the achieved tunability of the two effects in a model reaction of Pd-catalyzed organic hydrogenation offers a knob for enhancing energy coupling. In addition, the two intrinsic plasmonic modes at 400–700 and 700–1000 nm in the bar-shaped nanostructures allow for utilizing photons to a large extent in full solar spectrum. This work establishes a paradigmatic guidance toward designing plasmonic–catalytic nanomaterials for enhanced solar-to-chemical energy conversion.
Co-reporter:Wentuan Bi, Lei Zhang, Zhongti Sun, Xiaogang Li, Tao Jin, Xiaojun Wu, Qun Zhang, Yi Luo, Changzheng Wu, and Yi Xie
ACS Catalysis 2016 Volume 6(Issue 7) pp:4253
Publication Date(Web):June 1, 2016
DOI:10.1021/acscatal.6b00913
Recently, the implantation of non-noble-metal electrocatalysts into photocatalysts has brought dramatically improved hydrogen evolution activities; yet, the mechanistic details are still under debate, because of the poor understanding of interfacial charge carrier dynamics. Here, for the first time, we unravel that it is the electrocatalytic process that plays the critical role in these heterostructured systems. Spectroscopic characterizations, combined with theoretical calculations, give a clear physical picture that the photoexcited electrons transfer from photocatalysts to phosphides electrocatalysts, then driving H2 evolution reaction similar to electrocatalysis; and also reveal the Fermi level of electrocatalysts as a feasible descriptor for the photocatalytic activity.Keywords: co-catalysts; electrocatalysts; electron transfer; hydrogen evolution reaction; photocatalysis
Co-reporter:Qin Liu;Qichao Shang;Adnan Khalil;Qi Fang;Shuangming Chen;Qun He;Ting Xiang;Daobin Liu; Qun Zhang; Yi Luo; Li Song
ChemCatChem 2016 Volume 8( Issue 16) pp:2614-2619
Publication Date(Web):
DOI:10.1002/cctc.201600504
Abstract
The replacement of expensive noble-metals cocatalysts with inexpensive, earth-abundant, metallic nonmetal materials in most semiconductor-based photocatalytic systems is highly desirable. Herein, we report the fabrication of stable 1T-MoS2 slabs in situ grown on CdS nanorods (namely, 1T-MoS2@CdS) by using a solvothermal method. As demonstrated by ultrafast transient absorption spectroscopy, in combination with steady-state and time-resolved photoluminescence, the synergistic effects resulting from formation of the intimate nanojunction between the interfaces and effective electron transport in the metallic phase of 1T-MoS2 largely contribute to boosting the photocatalytic activity of CdS. Notably, the heterostructure with an optimum loading of 0.2 wt % 1T-MoS2 exhibits an almost 39-fold enhancement in the photocatalytic activity relative to that exhibited by bare CdS. This work represents a step towards the in situ realization of a 1T-phase MoS2-based heterostructure as a promising cocatalyst with high performance and low cost.
Journal of the American Chemical Society 2015 Volume 137(Issue 42) pp:13440-13443
Publication Date(Web):October 4, 2015
DOI:10.1021/jacs.5b08773
It is highly desirable to convert CO2 to valuable fuels or chemicals by means of solar energy, which requires CO2 enrichment around photocatalysts from the atmosphere. Here we demonstrate that a porphyrin-involved metal–organic framework (MOF), PCN-222, can selectively capture and further photoreduce CO2 with high efficiency under visible-light irradiation. Mechanistic information gleaned from ultrafast transient absorption spectroscopy (combined with time-resolved photoluminescence spectroscopy) has elucidated the relationship between the photocatalytic activity and the electron–hole separation efficiency. The presence of a deep electron trap state in PCN-222 effectively inhibits the detrimental, radiative electron–hole recombination. As a direct result, PCN-222 significantly enhances photocatalytic conversion of CO2 into formate anion compared to the corresponding porphyrin ligand itself. This work provides important insights into the design of MOF-based materials for CO2 capture and photoreduction.
Co-reporter:Bo Wu; Jiahua Hu; Peng Cui; Li Jiang; Zongwei Chen; Qun Zhang; Chunru Wang;Yi Luo
Journal of the American Chemical Society 2015 Volume 137(Issue 27) pp:8769-8774
Publication Date(Web):June 22, 2015
DOI:10.1021/jacs.5b03612
Endohedral metallofullerenes (EMFs) have become an important class of molecular materials for optoelectronic applications. The performance of EMFs is known to be dependent on their symmetries and characters of the substituents, but the underlying electron dynamics remain unclear. Here we report a systematic study on several scandium EMFs and representative derivatives to examine the cage symmetry and substituent effects on their photoexcited electron dynamics using ultrafast transient absorption spectroscopy. Our attention is focused on the visible-light (530 nm as a demonstration) photoexcited electron dynamics, which is of broad interest to visible-light solar energy harvesting but is considered to be quite complicated as the visible-light photons would promote the system to a high-lying energy region where dense manifolds of electronic states locate. Our ultrafast spectroscopy study enables a full mapping of the photoinduced deactivation channels involved and reveals that the long-lived triplet exciton plays a decisive role in controlling the photoexcited electron dynamics under certain conditions. More importantly, it is found that the opening of the triplet channels is highly correlated to the fullerene cage symmetry as well as the electronic character of the substituents.
Co-reporter:Jing Ge, Qun Zhang, Jun Jiang, Zhigang Geng, Shenlong Jiang, Kaili Fan, Zhenkun Guo, Jiahua Hu, Zongwei Chen, Yang Chen, Xiaoping Wang and Yi Luo
Physical Chemistry Chemical Physics 2015 vol. 17(Issue 19) pp:13129-13136
Publication Date(Web):20 Apr 2015
DOI:10.1039/C5CP00323G
A molecule or a molecular system always consists of excited states of different spin multiplicities. With conventional optical excitations, only the (bright) states with the same spin multiplicity of the ground state could be directly reached. How to reveal the dynamics of excited (dark) states remains the grand challenge in the topical fields of photochemistry, photophysics, and photobiology. For a singlet–triplet coupled molecular system, the (bright) singlet dynamics can be routinely examined by conventional femtosecond pump–probe spectroscopy. However, owing to the involvement of intrinsically fast decay channels such as intramolecular vibrational redistribution and internal conversion, it is very difficult, if not impossible, to single out the (dark) triplet dynamics. Herein, we develop a novel strategy that uses an ultrafast broadband white-light continuum as a excitation light source to enhance the probability of intersystem crossing, thus facilitating the population flow from the singlet space to the triplet space. With a set of femtosecond time-reversed pump–probe experiments, we report on a proof-of-concept molecular system (i.e., the malachite green molecule) that the pure triplet dynamics can be mapped out in real time through monitoring the modulated emission that occurs solely in the triplet space. Significant differences in excited-state dynamics between the singlet and triplet spaces have been observed. This newly developed approach may provide a useful tool for examining the elusive dark-state dynamics of molecular systems and also for exploring the mechanisms underlying molecular luminescence/photonics and solar light harvesting.
Journal of Molecular Spectroscopy 2015 Volume 313() pp:49-53
Publication Date(Web):July 2015
DOI:10.1016/j.jms.2015.05.002
•We have recorded the LIF excitation spectra of YS in the 17 860–20 700 cm−1 region.•We also measured the DF spectra of YS in the 17 860–20 700 cm−1 region.•A new 2Σ+2Σ+ state, preserving charge-transfer nature, has been observed.•The FC factors calculation for the new bands has been performed.•The spin-rotation parameter γ(new2Σ+)γ(new2Σ+) has been determined to be 0.0206 cm−1.We have investigated the laser-induced fluorescence (LIF) excitation spectra and dispersed fluorescence (DF) spectra of yttrium monosulfide (YS) in the energy range of 17 860–20 700 cm−1. Rotational analyses indicated that almost all of the intense vibronic bands can be attributed to the new [19.38]2Σ+(υ′)-X2Σ+(υ″)[19.38]2Σ+(υ′)-X2Σ+(υ″) transition system. The missing (1, 2), (2, 1) and (3, 3) bands are caused by their very small Franck–Condon factors, as confirmed by our calculations. The new 2Σ+2Σ+ state has been suggested to arise from the electronic configuration of (core)1σ22σ21π33σ2π, featuring a charge-transfer nature. Moreover, the spin-rotation parameter γ for the newly observed 2Σ+2Σ+ state has been determined to be 0.0206 cm−1, the magnitude of which is larger than the known γ(X2Σ+)γ(X2Σ+) (0.001427 cm−1) but smaller than γ(B2Σ+)γ(B2Σ+) (−0.1515 cm−1).
Co-reporter:Lili Wang;Jing Ge;Ailun Wang;Mingsen Deng;Xijun Wang;Song Bai;Rui Li; Jun Jiang; Qun Zhang; Yi Luo ; Yujie Xiong
Angewandte Chemie International Edition 2014 Volume 53( Issue 20) pp:5107-5111
Publication Date(Web):
DOI:10.1002/anie.201310635
Abstract
A practical strategy is proposed to facilitate the migration of holes in semiconductor (the low rate of which limits photocatalytic efficiency) by taking advantage of the Schottky barrier between p-type semiconductor and metal. A high work function is found to serve as an important selection rule for building such desirable Schottky junction between semiconductor surface facets and metal. The intrinsic charge spatial distribution has to be taken into account when selecting the facets, as it results in accumulation of photoexcited electrons and holes on certain semiconductor facets. Importantly, the facets have a high work function, the same characteristic required for the formation of Schottky junction in a p-type semiconductor–metal hybrid structure. As a result, the semiconductor crystals in the hybrid design may be better enclosed by single facets with high work function, so as to synergize the two effects: Schottky barrier versus charge spatial separation.
The charge state of the Pd surface is a critical parameter in terms of the ability of Pd nanocrystals to activate O2 to generate a species that behaves like singlet O2 both chemically and physically. Motivated by this finding, we designed a metal–semiconductor hybrid system in which Pd nanocrystals enclosed by {100} facets are deposited on TiO2 supports. Driven by the Schottky junction, the TiO2 supports can provide electrons for metal catalysts under illumination by appropriate light. Further examination by ultrafast spectroscopy revealed that the plasmonics of Pd may force a large number of electrons to undergo reverse migration from Pd to the conduction band of TiO2 under strong illumination, thus lowering the electron density of the Pd surface as a side effect. We were therefore able to rationally tailor the charge state of the metal surface and thus modulate the function of Pd nanocrystals in O2 activation and organic oxidation reactions by simply altering the intensity of light shed on Pd–TiO2 hybrid structures.
Co-reporter:Yue Yuan, Shenlong Jiang, Qingqing Miao, Jia Zhang, Mengjing Wang, Linna An, Qinjingwen Cao, Yafeng Guan, Qun Zhang, Gaolin Liang
Talanta 2014 Volume 125() pp:204-209
Publication Date(Web):1 July 2014
DOI:10.1016/j.talanta.2014.02.063
•A water-soluble, biocompatible, fluorescent chemosensor for label-free, simple, and fast detection of Hg2+ in aqueous solution and in cells with high selectivity.•Mechanisms responsible for its fluorescence turn-on effect was gained from ultrafast transient absorption spectroscopy.•A simple device for fast and effective removal of Hg2+ from contaminated water has been constructed.A water-soluble, biocompatible, and fluorescent chemosensor (1) for label-free, simple, and fast detection of mercury ions (Hg2+) in aqueous solutions and in HepG2 cells with high selectivity is reported herein. Chelation of 1 with Hg2+ results in the disappearance of its fluorescence emission at 350 nm and the appearance of a new emission at 405 nm. Selectivity and interference studies indicated that 1 could be selectively chelated by Hg2+ without interference from other metal ions. Insight into the mechanisms responsible for its fluorescence effect was gained from ultrafast transient absorption spectroscopy. With these properties, 1 was successfully applied for imaging Hg2+ in living cells and for removing Hg2+ from river water. Moreover, we also constructed a simple device for fast and effective removal of Hg2+ from contaminated liquid samples.A water-soluble, biocompatible, fluorescent chemosensor for label-free, simple, and fast detection of mercury ions (Hg2+) in aqueous solution and in HepG2 cells with high selectivity was reported. Moreover, a facile device for fast and effective removal of Hg2+ from contaminated liquid samples was also constructed.
Angewandte Chemie International Edition 2014 Volume 53( Issue 12) pp:3205-3209
Publication Date(Web):
DOI:10.1002/anie.201309660
Abstract
The charge state of the Pd surface is a critical parameter in terms of the ability of Pd nanocrystals to activate O2 to generate a species that behaves like singlet O2 both chemically and physically. Motivated by this finding, we designed a metal–semiconductor hybrid system in which Pd nanocrystals enclosed by {100} facets are deposited on TiO2 supports. Driven by the Schottky junction, the TiO2 supports can provide electrons for metal catalysts under illumination by appropriate light. Further examination by ultrafast spectroscopy revealed that the plasmonics of Pd may force a large number of electrons to undergo reverse migration from Pd to the conduction band of TiO2 under strong illumination, thus lowering the electron density of the Pd surface as a side effect. We were therefore able to rationally tailor the charge state of the metal surface and thus modulate the function of Pd nanocrystals in O2 activation and organic oxidation reactions by simply altering the intensity of light shed on Pd–TiO2 hybrid structures.
Co-reporter:Qun Zhang ; Hongjun Zheng ; Zhigang Geng ; Shenlong Jiang ; Jing Ge ; Kaili Fan ; Sai Duan ; Yang Chen ; Xiaoping Wang ;Yi Luo
Journal of the American Chemical Society 2013 Volume 135(Issue 33) pp:12468-12474
Publication Date(Web):July 29, 2013
DOI:10.1021/ja407110r
Graphene oxide (GO) is an attractive alternative for large-scale production of graphene, but its general structure is still under debate due to its complicated nonstoichiometric nature. Here we perform a set of femtosecond pump–probe experiments on as-synthesized GO to extrapolate structural information in situ. Remarkably, it is observed that, in these highly oxidized GO samples, the ultrafast graphene-like dynamics intrinsic to pristine graphene is completely dominant over a wide energy region and can be modified by the localized impurity states and the electron–phonon coupling under certain conditions. These observations, combined with the X-ray photoelectron spectroscopy analysis and control experiments, lead to an important conclusion that GO consists of two types of domain, namely the carbon-rich graphene-like domain and the oxygen-rich domain. This study creates a new understanding of the realistic domain structure and properties of as-synthesized GO, offering useful guidance for future applications based on chemically modified/functionalized graphenes.
Co-reporter:Li Wang, Junfeng Zhen, Jianqiang Gao, Qun Zhang, Yang Chen
Chemical Physics Letters 2010 Volume 493(4–6) pp:245-250
Publication Date(Web):25 June 2010
DOI:10.1016/j.cplett.2010.05.073
Abstract
Laser-induced fluorescence excitation spectrum of NiS in the wavelength range 450–560 nm has been recorded and analyzed. Thirty-five vibronic transition bands have been observed, 29 of which are reported for the first time. Rotational analyses indicated that the observed bands can be attributable to the transitions of 58NiS (and 60NiS). Twenty-five bands have been suggested to be grouped into four vibrational progressions. Furthermore, through dispersed fluorescence measurements we directly obtained the energies for the ground-state vibrational levels up to υ″=6 as well as the vibrational frequency and the anharmonicity constant for the ground state of 58NiS.
Co-reporter:Hui Wang, Shenlong Jiang, Shichuan Chen, Xiaodong Zhang, Wei Shao, Xianshun Sun, Zhi Zhao, Qun Zhang, Yi Luo and Yi Xie
Chemical Science (2010-Present) 2017 - vol. 8(Issue 5) pp:NaN4092-4092
Publication Date(Web):2017/03/24
DOI:10.1039/C7SC00307B
Understanding the photoexcitation processes in semiconductors is critical for the design of advanced photocatalytic materials. Nevertheless, traditional viewpoints focus on photogenerated free charge carriers, which are somehow invalid once the many-body effects are taken into account, especially for polymeric photocatalysts. Here we systematically investigate the photoexcitation processes involved in the polymer matrix of graphitic carbon nitride (g-C3N4) by combining photoluminescence spectroscopy and ultrafast transient absorption spectroscopy, validating the strong excitonic effects in the well-known photocatalyst for the first time. The identification of the robust triplet–triplet annihilation process, in which two triplet excitons collide to produce a singlet exciton, highlights an important nonradiative depopulation pathway of excited species and thereby offers potential strategies to regulate the photocatalytic activities of polymeric g-C3N4. The work establishes a new understanding of the photocatalytic mechanism in the polymeric g-C3N4 matrix, and thus paves the way for designing effective polymeric photocatalysts through excitonic engineering.
Co-reporter:Jing Ge, Qun Zhang, Jun Jiang, Zhigang Geng, Shenlong Jiang, Kaili Fan, Zhenkun Guo, Jiahua Hu, Zongwei Chen, Yang Chen, Xiaoping Wang and Yi Luo
Physical Chemistry Chemical Physics 2015 - vol. 17(Issue 19) pp:NaN13136-13136
Publication Date(Web):2015/04/20
DOI:10.1039/C5CP00323G
A molecule or a molecular system always consists of excited states of different spin multiplicities. With conventional optical excitations, only the (bright) states with the same spin multiplicity of the ground state could be directly reached. How to reveal the dynamics of excited (dark) states remains the grand challenge in the topical fields of photochemistry, photophysics, and photobiology. For a singlet–triplet coupled molecular system, the (bright) singlet dynamics can be routinely examined by conventional femtosecond pump–probe spectroscopy. However, owing to the involvement of intrinsically fast decay channels such as intramolecular vibrational redistribution and internal conversion, it is very difficult, if not impossible, to single out the (dark) triplet dynamics. Herein, we develop a novel strategy that uses an ultrafast broadband white-light continuum as a excitation light source to enhance the probability of intersystem crossing, thus facilitating the population flow from the singlet space to the triplet space. With a set of femtosecond time-reversed pump–probe experiments, we report on a proof-of-concept molecular system (i.e., the malachite green molecule) that the pure triplet dynamics can be mapped out in real time through monitoring the modulated emission that occurs solely in the triplet space. Significant differences in excited-state dynamics between the singlet and triplet spaces have been observed. This newly developed approach may provide a useful tool for examining the elusive dark-state dynamics of molecular systems and also for exploring the mechanisms underlying molecular luminescence/photonics and solar light harvesting.
