Efficient Photoinduced Charge Transfer Research Articles

Manipulating Hot-Electron Injection in Metal Oxide Heterojunction Array for Ultrasensitive Surface-Enhanced Raman Scattering.

Efficient photoinduced charge transfer (PICT) resonance is crucial to the surface-enhanced Raman scattering (SERS) performance of metal oxide substrates. Herein, we venture into the hot-electron injection strategy to achieve unprecedented enhanced PICT efficiency between substrates and molecules. A heterojunction array composed of plasmonic MoO2 and semiconducting WO3-x is designed to prove the concept. The plasmonic MoO2 generates intense localized surface plasmon resonance under illumination, which can generate near-field Raman enhancement as well as accompanied plasmon-induced hot-electrons. The hot-electron injection in direct interfacial charge transfer and plasmon-induced charge transfer process can effectively promote the PICT efficiency between substrates and molecules, achieving a record Raman enhancement factor among metal oxide substrates (2.12 × 108) and the ultrasensitive detection of target molecule down to 10-11 M. This work demonstrates the possibility of hot-electron manipulation to realize unprecedented Raman enhancement in metal oxides, offering a cutting-edge strategy to design high-performance SERS substrates.

Fabrication of Ag nanoparticles decorated hieratical Ni0.25Co0.75(OH)2 microflowers photocatalyst toward efficient environmental remediation

The bimetallic hydroxide NixCo1-x(OH)2 was first prepared through hydrothermal method. The prepared NixCo1-x(OH)2 architecture showed good photocatalytic degradation performance toward oxytetracycline (OTC), especially the Ni0.25Co0.75(OH)2 hierarchical microflowers. After deposition of Ag nanoparticles, Ag/Ni0.25Co0.75(OH)2 nanocomposites with further improved photocatalytic activity were obtained especially on 0.2% Ag/Ni0.25Co0.75(OH)2. The modified band structure with optimal loading amount of Ag nanoparticles was convinced to have positive effect on the efficient photo-induced charge separation and transfer during photocatalytic procedure. This study expanded the application of hydroxides in the field of photocatalysis, and provided ideas for the construction of new photocatalytic system.

Rational Modification of Two-Dimensional Donor-Acceptor Covalent Organic Frameworks for Enhanced Visible Light Photocatalytic Activity.

Covalent organic frameworks (COFs) are promising crystalline materials for photocatalytic solar- to hydrogen-energy conversion due to the tunable chemical structures and energy band gaps. Herein, we report a chemical modification strategy for improving the photocatalytic activity of COFs. A benzene-1,3,5-tricarbaldehyde (BT)- and benzothiadiazole derivative-based two-dimensional donor-acceptor (D-A) COF, denoted as BT-COF, were fabricated and further modified by using an alternative electron-donating unit, 2-hydroxybenzene-1,3,5-tricarbaldehyde (HBT), to the polycondensation reaction, yielding HBT-COF with an enhanced internal D-A effect and hydrophilicity. Interestingly, the photocatalytic H2 production rate of HBT-COF reaches 19.00 μmol h-1, which is 5 times higher than that of BT-COF (3.40 μmol h-1) under visible light irradiation. The increase in photocatalytic activity of HBT-COF is rationally attributed to finely tuned energy levels and improved wettability, which in turn leads to broadened visible light absorption, efficient photoinduced charge separation and transfer, and enhanced interactions between the COF catalyst and reaction substrates. The present results demonstrate that a subtle structural modification can significantly modulate the band structure and interfacial property, thus providing a feasible strategy for the optimization of COF-based photocatalytic systems.

The enhanced performance of Cr(VI) photoreduction and antibiotic removal on 2D/3D TiO2/ZnIn2S4 nanostructures

In this study, 2D/3D TiO2/ZnIn2S4 nanostructures with TiO2 sheet cluster embedded into ZnIn2S4 micro flowers were fabricated via hydrothermal method. The matched band structure, the enlarged surface area and the efficient photo-induced charge transfer offered by effective heterostructures formed between the two components endowed the TiO2/ZnIn2S4 nanoarchitecture with excellent photocatalytic Cr(VI) reduction and tetracycline hydrochloride (TC) degradation performance. Especially, the Cr(VI) photoreduction efficiency of 50% TiO2/ZnIn2S4 was 8.8 times higher compared to pure ZnIn2S4. The enhanced separation efficiency of photo-excited charge carriers was induced by the matched band structure, the enlarged surface area and the strong interaction between TiO2 and ZnIn2S4. The key roles of ·O2− was confirmed via trapping experiments. Otherwise, the pathway of TC degradation was investigated. The proposed mechanism during photocatalysis process was also discussed according to the photocatalytic and characterization results.

Site-Selective Loading of Single-Atom Pt on TiO2 for Photocatalytic Oxidation and Reductive Hydrodefluorination

Separating the redox sites of photocatalysts is one of most promising strategies to promote an efficient photoinduced charge transfer of semiconductor photocatalysis. Herein, we present a site-sele...

Covalent Functionalization of Single-Walled Carbon Nanotubes by the Bingel Reaction for Building Charge-Transfer Complexes.

Functionalization of nanotubes with donor and acceptor partners by the Bingel reaction leads to the formation of charge-transfer dyads, which can operate in organic photovoltaic devices. In this work, we theoretically examine the mechanism of the Bingel reaction for the (6,5)-chiral, (5,5)-armchair, and (9,0)-zigzag single-walled carbon nanotubes (SWCNTs), and demonstrate that the reaction is regioselective and takes place at the perpendicular position of (6,5)- and (5,5)-SWCNTs, and the oblique position of (9,0)-SWCNT. Further, we design computationally the donor-acceptor complexes based on (6,5)-SWCNT coupled with partners of different electronic nature. Analysis of their excited states reveals that efficient photoinduced charge transfer can be achieved in the complexes with π-extended analogue of tetrathiafulvalene (exTTF), zinc tetraphenylporphyrin (ZnTPP), and tetracyanoanthraquinodimethane (TCAQ). The solvent can significantly affect the population of the charge-separated states. Our calculations show that electron transfer (ET) occurs in the normal Marcus regime on a sub-nanosecond time scale in the complexes with exTTF and ZnTPP, and in the inverted Marcus regime on a picosecond time scale in the case of the TCAQ derivative. The ET rate is found to be not very sensitive to the degree of functionalization of the nanotube.

Surface hydroxyl groups functionalized graphite carbon nitride for high efficient removal of diquat dibromide from water

In the study, we present a quick potassium hydroxide heat treatment approach to optimize the “melon” framework of graphite carbon nitride and modify the surface properties by functionalization of hydroxyl groups. The hydroxyl groups functionalized g-CN samples have been applied as bifunctional materials for efficient elimination of diquat dibromide herbicide through synergistic adsorption/photodegradation processes. The structural characterizations of the as-obtained samples, combined with the detailed diquat dibromide herbicide adsorption study, reveal that the surface hydroxyl groups are the active sites for the diquat dibromide adsorption, which account for the much enhanced saturation adsorption capacities of 159.3 mg g−1 at 25 °C and pH = 7 (more than 110 times improvement compared with pristine carbon nitride). Furthermore, the grafted surface hydroxyl groups and optimized planar structures endow the functionalized samples with the advantageous properties of efficient photoinduced charge transfer and separation, low interface resistance, and high photoresponse. Consequently, the deep mineralization of diquat dibromide herbicide was achieved over the bifunctional materials (total removal ratios were ~ 97.1% after 240 min visible-light irradiation). This work not only demonstrates the feasibility of hydroxyl groups functionalized graphite carbon nitride for elimination of herbicide pollutants but also offers new insights to better design efficient and durable materials for environmental remediation.

Crystal-Amorphous Core-Shell Structure Synergistically Enabling TiO2 Nanoparticles' Remarkable SERS Sensitivity for Cancer Cell Imaging.

Exploring novel surface-enhanced Raman scattering (SERS) active materials with high detection sensitivity, excellent biocompatibility, low biotoxicity, and good spectral stability is urgently required for efficacious cancer cell diagnosis. Herein, black TiO2 nanoparticles (B-TiO2 NPs) with crystal-amorphous core-shell structure are successfully developed. Remarkable SERS activity is derived from the synergistic effect of the promising crystal-amorphous core-shell structure. Abundant excitons can be generated by high-efficiency exciton transitions in the crystal core, a feature that provides sufficient charge source. Significantly, the novel crystal-amorphous heterojunction enables the efficient exciton separation at the crystal-amorphous interface, which can effectively facilitate charge transfer from the crystal core to the amorphous shell and results in exciton enrichment at the amorphous shell. Kelvin probe force microscopy (KPFM) confirms the Fermi level of the amorphous layer shifting to a relatively low position compared to that of the crystal core, allowing efficient photoinduced charge transfer (PICT) between the amorphous shell and probe molecules. The first-principles density functional theory (DFT) calculations further indicate that the amorphous shell structure possesses a narrow band gap and a relatively high electronic density of state (DOS), which can effectively promote vibration coupling with target molecules. Moreover, MCF-7 drug-resistant (MCF-7/ADR) breast cancer cells can be quickly and accurately diagnosed based on the high-sensitivity B-TiO2-based SERS bioprobe. To the best of our knowledge, this is the first time the crystal-amorphous core-shell heterojunction enhancement of the TiO2-molecule PICT process, which widens the application of semiconductor-based SERS platforms in precision diagnosis and treatment of cancer, has been investigated.

Surface Engineered ZnO-Humic/Citrate Interfaces: Photoinduced Charge Carrier Dynamics and Potential Application for Smart and Sustained Delivery of Zn Micronutrient

Nanoscale science and nanotechnology are expected to bring about revolutionary advances in the agrochemical industry. Smart delivery of micronutrients enabling sustained release is one such innovative approach to combat the poor nutrient uptake efficiency of conventional micronutrient sources. Herein, we report a bottom-up surface fabrication strategy to modify nanosized zinc oxide (ZnO NPs) surfaces at an atomic scale for sustained release of Zn micronutrients. A large sized environmentally relevant macromolecule humic acid (HA) and a comparatively smaller biologically pertinent citrate ligand were used to functionalize ZnO NPs (30 nm), and the resultant nanohybrids were characterized using electron microscopy, diffraction, and spectroscopic techniques. Functionalization with citrate ligand turns the NP smaller than its original size. The resultant modification in the dissolution characteristics of ZnO NPs leads to sustained release of Zn over a broad pH range. Functionalization with HA causes minimum alteration of NP sizes. But dissolution patterns of NPs examined separately in the presence of HA depicts stabilization of colloidal suspensions. Additionally, picosecond-resolved lifetime measurements reveal efficient photoinduced charge transfer at the nanoheterojunction interfaces. To evaluate the impact of newly developed Zn delivery system on Zn bioavailability, their impacts on the growth of wheat plants (Triticum aestivum) was tested. Results reveal considerable growth enhancements of plants treated with functionalized NPs. The micronutrient nanoformulation requires no auxiliaries and is applicable to plant systems in aqueous dispersions. Our study reinstates the importance of “convergence thinking” or coming together of technologies as a key to creating a fundamental framework toward environmental sustainability.

Enhanced Photoelectrochemical Water Splitting with Er- and W-Codoped Bismuth Vanadate with WO3 Heterojunction-Based Two-Dimensional Photoelectrode.

A novel two-dimensional (2D) heterojunction photoelectrode composed of WO3 and (Er,W):BiVO4 is proposed for water oxidation with efficient photoinduced charge carrier separation and transfer. Er stoichiometric along with W nonstoichiometric codoping was introduced to simultaneously manage vacancy creation during substitutional doping, enhance light absorption, and reduce overall impedance. It was found that Er3+ is substituted at the Bi3+ sites in the BiVO4 lattice to provide expanded light absorption from 400 to 680 nm. The fabricated WO3/(Er,W):BiVO4 electrode shows photocurrent densities of 4.1 and 7.2 mA cm-2 at 1.23 and 2.3 V (vs reversible hydrogen electrode, RHE), respectively, under a 1 sun illumination in K2HPO4 electrolyte. This electrode has shown remarkably high charge separation efficiency of 93% at 1.23 V (vs RHE). With the addition of a standard surface catalyst (i.e., Co-Pi), the WO3/(Er,W):BiVO4/Co-Pi electrode exhibits the highest photocurrent of 5.6 ± 0.3 mA cm-2 at 1.23 V (vs RHE), nearing the theoretical limit (i.e., 7.5 mA cm-2) while retaining 98% of the photoelectrochemical cell performance after 3 h. By concomitantly doping the Bi3+ and V5+ sites to enhance absorption, this study demonstrates for the first time a planar WO3/BiVO4 heterojunction that reaches 88% of the record-high performance of its nanostructured counterpart. Through a detailed characterization of the electrodes, it is concluded that the stoichiometric Er and nonstoichiometric W codoping extend light absorption region and improve charge separation efficiency by reducing bulk resistance. The photoactive materials with 2D morphology were synthesized using a facile ultrasonic spray-coating technique without any complex process steps and thus it can be scaled for commercial development.

Two-Dimensional Amorphous TiO2 Nanosheets Enabling High-Efficiency Photoinduced Charge Transfer for Excellent SERS Activity.

Substrate-molecule vibronic coupling enhancement, especially the efficient photoinduced charge transfer (PICT), is pivotal to the performance of nonmetal surface-enhanced Raman scattering (SERS) technology. Here, through developing novel two-dimensional (2D) amorphous TiO2 nanosheets (a-TiO2 NSs), we successfully obtained an ultrahigh enhancement factor of 1.86 × 106. Utilizing the Kelvin probe force microscopy (KPFM) technology, we found that these 2D a-TiO2 NSs possessed more positive surface potential than their 2D crystalline counterpart (c-TiO2 NSs). First-principles density functional theory (DFT) was used to further reveal that the low coordination number of surface Ti atoms and the large amount of surface oxygen defects endowed the 2D a-TiO2 with high electrostatic potential, which allowed significant charge transfer from the adsorbed molecule to the 2D a-TiO2 and facilitated the formation of a stable surface charge-transfer (CT) complex. Significantly, comparing with the 2D c-TiO2, the smaller band gap and higher electronic density of states (DOS) of the 2D a-TiO2 effectively enhanced the vibronic coupling of resonances in the substrate-molecule system. The strong vibronic coupling within the CT complex obviously enhanced the PICT resonance and lead to the remarkable SERS activity of a-TiO2 NSs. To the best of our knowledge, this is the first report on the remarkable SERS activity of 2D amorphous semiconductor nanomaterials, which may bring the cutting edge of development of stable and highly sensitive nonmetal SERS technology.

Construction of a Stable Ru-Re Hybrid System Based on Multifunctional MOF-253 for Efficient Photocatalytic CO2 Reduction.

Using the open N,N'-chelating sites of MOF-253 (Al(OH)(dcbpy), dcbpy = 2,2'-bipyridine-5,5'-dicarboxylic acid) to coordinate with Re(I), a linker anchored Re complex MOF-253-Re(CO)3Cl active for photocatalytic CO2 reduction was obtained. Unlike the homogeneous bipyridine containing Re complexes which produce CO during photocatalytic CO2 reduction, formate was obtained as the main CO2 reduction product over the as-obtained MOF-253-Re(CO)3Cl. The linker anchored MOF-253-Re(CO)3Cl showed superior photocatalytic performance compared to its homogeneous counterpart since the usual formation of the bimolecular Re intermediate leading to the deactivation of the homogeneous Re complex was significantly inhibited in the MOF supported Re complex. To enhance its light absorption, a linker anchored Ru sensitizer was simultaneously constructed in MOF-253-Re(CO)3Cl (Ru-MOF-253-Re). The total TON(TON is defined as mole of the evolved H2, CO, and HCOO- over per amount of Rhenium) for CO2 reduction (28.8 in 4 h) over the as-obtained Ru-MOF-253-Re system is comparable or even superior to most already reported Re carbonyl complexes featuring bpy ligands and the Ru-Re bimetallic supramolecular systems constructed via the covalent bond under similar reaction conditions. The enhanced photocatalytic CO2 reduction over the Ru-MOF-253-Re can be ascribed to the improved visible light absorption and the existence of an efficient photoinduced charge transfer from Ru sensitizer to Re catalytic center, as evidenced from the transient absorption studies. The use of MOF-253 as a metalloligand and support to assemble the Ru-Re system as well as a mediator to promote the charge transfer from Ru sensitizer to Re catalytic center resembles the construction of Ru-Re supramolecular structures using covalent bonds, but is more facile in preparation and provides more flexibility. This study demonstrates the possibility of using MOFs with open coordination sites as a platform for the construction of a stable multifunctional hybrid system for artificial photosynthesis.

Redox-driven porphyrin based systems for new luminescent molecular switches.

In this work, we explore the possibility of tuning the fluorescence intensity of two porphyrin systems through the electrochemical oxidation of an appended ruthenium acetylide bridge. Two electrochemically switchable systems, a dyad (ZnP-Ru, 3) and a triad (ZnP-Ru-P2H, 5), were prepared and investigated. In the ZnP-Ru dyad, the fluorescence of the zinc porphyrin was switched reversibly between the ON and OFF state upon the oxidation of the ruthenium unit, the most probable quenching process involved after oxidation being the electron transfer from the singlet excited state of ZnP to the oxidized ruthenium center. In the ZnP-Ru-P2H triad, we show that both porphyrins' fluorescence are highly quenched independent of the redox state of the ruthenium bridge owing to the efficient photoinduced charge transfer within the ruthenium complex.

Photoinduced Charge Separation in Semiconductor‐Quantum‐Dot/Organic‐Molecule Hybrids

AbstractSemiconductor colloidal quantum dot/organic molecule nanohybrids are gaining momentum due to the relative ease with which an efficient photoinduced charge transfer in the desired direction can be achieved between the dot and the molecule. At the same time analysis of the experimental studies of such systems, and in particular transient absorption spectroscopy data, is a challenging task because of the statistical nature of hybrid formation and the complex kinetics of the photoreactions. This Minireview aims to find common ground for comparing the photoinduced charge‐separation reactions in quantum dot/organic molecule hybrids. It also discusses a common set of parameters which would help to compare different quantum dot/molecule hybrids as well as progress in developing design principles to achieve controllable photoinduced charge separation in such hybrids.

Effect of laser annealing of a C60–ZnO heterogeneous system on its structural and photoluminescent characteristics

The structural and photoluminescent properties of a heterogeneous system composed of a thin layer of C60 fullerene molecules and nanostructured ZnO film have been studied. It has been shown that this system, which is intrinsic for a particularly highly efficient photoinduced charge transfer, can lay the foundation for the design of a new generation of organic photovoltaic elements.

A graphene-phthalocyanine hybrid as a next photoactive layer

Highly dispersible graphene was hybridized with a phthalocyanine derivative via covalent linkage to create a new type of donor-acceptor hybrid exhibiting photoinduced charge separation. Graphene is beneficial for the hybridization due to its easy functionalization and the unique electron accepting ability. A phthalocyanine (PC) derivative, pyrazine-based silicon phthalocyanine dichloride, possessing rich π-conjugated macrocycle was used for the electron donating purpose. The hybrid molecule showed a significant decrease in fluorescence of π-conjugated PC, implying an efficient photoinduced charge transfer occurring within the covalently-linked molecular structure consisting of “donor-acceptor”. The graphene-PC hybrid molecule has also been demonstrated as an active layer material in organic photovoltaic device as well. Thus our results indeed underline the efficacy of the molecular level heterojunction of the covalently linked graphene-PC molecule in a single cell and will open new opportunities ahead for active layers research to go beyond conventional bulk-heterojunction (BHJ).

Synthesis and photophysical properties of extended π conjugated naphthalimides.

A series of π conjugated naphthalimide derivatives having an imide group as an acceptor conjugated with a methoxy arylethynyl or a methoxyphenyl triazole as a donor were prepared by Sonogashira coupling or "click" chemistry. Their photophysical properties were investigated by steady state and time resolved fluorescence spectroscopy and modelled by TD-DFT calculations. Compound Naphth-AlkyneOMe has a high fluorescence quantum yield and displays efficient photoinduced charge transfer in solution as well as in the powder state. Compound Naphth-TriazoleOMe exhibits a very high Stokes shift and its fluorescence quantum yield is low, which can be rationalized by theoretical calculations.

High-performance 1D type-II TiO2@ZnO core-shell nanorods arrays photoanodes for photoelectrochemical solar fuel production

In this paper, vertically aligned one-dimensional (1D) TiO2@ZnO core-shell nanorods arrays (NRs) were prepared by a facile two-step hydrothermal method. The as-prepared TiO2@ZnO nanorods arrays were used as photoanode and exhibited high photoelectrochemical water splitting performance. A maximum photocurrent density of TiO2@ZnO nanorods arrays achieves to 2.02mAcm−2 at 1.7V vs. RHE, which is 2 times higher than that of pure TiO2 nanorods arrays. In addition, a significant cathodic shift in the onset potential and a stable photocurrent during 6000s measurement are observed in TiO2@ZnO nanorods arrays. TiO2@ZnO core-shell NRs electrode shows a maximal conversion efficiency of 0.48% at 0.55V vs. RHE, which is 2.7 times higher than that of the bare TiO2 NRs electrode. The improvement of photoelectrochemical properties can be attributed to the ingenious core-shell structure that enhances the interfacial interaction based on the type II heterojunction. An efficient interfacial charge separation and photoinduced charge transfer are achieved in 1D type-II TiO2@ZnO core-shell nanorods arrays.

CdIn2S4/g-C3N4 heterojunction photocatalysts: enhanced photocatalytic performance and charge transfer mechanism

CdIn2S4/g-C3N4 heterojunction with excellent photocatalytic activities have been synthesized. The enhanced photocatalytic performance might be ascribed to the efficient photoinduced charge transfer derived from the heterojunction structure.

Efficient photoinduced charge transfer in chemically-linked organic-metal Ag-P3HT nanocomposites

A novel nanocomposite of P3HT and Ag nanoparticles (Ag-P3HT) has been synthesized by laser ablation. The fluorescence wavelength of the nanocomposite can be tuned by varying the ablation duration. The steady-state emission maximum at 580 nm for pristine P3HT shows significant blueshift to wavelengths ranging from 520 to 550 nm for Ag-P3HT. NMR, FTIR and XPS spectroscopy confirm that chemical links are formed between P3HT and silver nanoparticles (NPs) in the nanocomposite. For the sample with an emission maximum at 530 nm, 1H NMR spectra indicate that 66% of the P3HT thiophene ring protons are replaced by Ag NPs. Time-resolved spectroscopy demonstrates that charge transfer efficiency increases for Ag-P3HT and this is attributed to the enhanced intimate interfacial contact between the chemically-bonded metal NPs and polymer chains. The synthetic route outlined provides a one-pot and green strategy for producing metal-organic polymeric materials, with controlled luminescence wavelengths and efficient photoinduced charge transfer that meet the requirement for developing high-performance organic light-emitting and photovoltaic devices.