Interfacial Charge Transfer Process Research Articles

Photoluminescence and Photo-Catalytic Activity of Synthesized Nanocrystals

Abtract Intrinsic and extrinsic semiconductor nanostructures have attracted great attention due to their size tunable photo-physical and photo-chemical properties. In the present paper, polyvinyl pyrrolidone (PVP) capped Zn 1-x Eu x S (0.00001≤x≤0.1) nanocrystals have been synthesized by means of a facile chemical synthesis method. Crystallography and morphology of synthesized materials have been deliberated using X-ray diffraction (XRD) and transmission electron microscope (TEM), respectively. Diffraction and electron micrograph studies reveal that the synthesized materials are zinc blende nanocrystals having average particle size ~3nm. Elemental and compositional analyses of the nanocrystals have been done using energy dispersive X-ray fluorescence (EDXRF) technique. Steady state photoluminescence spectra have been recorded for optical characterization of synthesized nanomaterials. Photocatalytic activity potential of synthesized nanomaterials under UV radiation exposure has been investigated using methylene blue (MB) dye as a test contaminant in aqueous media. Photo-physical and photo-chemical behaviour dependence on doping concentration has been described in detail. Moreover, the sophistication of competition between charge carrier recombination and charge carrier trapping followed by the competition between recombination of trapped carriers and interfacial charge transfer processes have been presented in a fantastic and elaborative way by comparative study of photoluminescence and photo-catalytic activity results.

Electronic structure and dynamics of quasi-2D states of vanadyl naphthalocyanine on Au(111)†

We investigate the evolution of the interfacial electronic structure and dynamics of thin films of the organic semiconductor vanadyl naphthalocyanine on Au(111). Using angle-resolved two-photon photoemission, a comprehensive coverage- and excitation-energy-dependent characterisation of the electronic structure and the resulting dynamics of short-lived image potential resonances (IPRs) on Au(111) are presented. The study of these quasi-two-dimensional (quasi-2D) bands is enabled by molecular adsorption and reveals a significant lengthening of their lifetimes. The resonances remain, however, significantly coupled to the continuum of bulk bands of Au(111) even in the presence of the organic adsorbate, giving rise to Fano-like quantum interference and ‘intensity switching’ effects. Coupling to the continuum is also responsible for providing excitation pathways to the image potential manifold above and below optical resonance with the Shockley surface state. The organic semiconductor interface and quasi-2D bands investigated here provide a model for understanding the role of quantum effects in ultrafast dynamics of confined systems and at interfaces such as those that are relevant e.g. for interfacial charge-transfer processes in organic electronics.

The reduction of graphene oxide by zinc powder to produce a zinc oxide-reduced graphene oxide hybrid and its superior photocatalytic activity

We have synthesized for the first time ZnO/rGO hybrids from metal zinc and GO using hydrothermal technique without adding further reducing agent. The photocatalytic property of ZnO–rGO reveals that the hybrid for 50mg of GO has the highest activity, causing a 94% degradation of methyl orange compared to 70% by only ZnO. The consistent quenching and a gradual decrease in the decay life time of the emission at ∼500nm as the rGO content increases indicates the interfacial charge transfer process between ZnO and rGO by the defect states responsible for green emission.

Multiphonon Resonant Raman Scattering and Photoinduced Charge-Transfer Effects at ZnO–Molecule Interfaces

Detailed understanding of the underlying mechanisms of surface-enhanced Raman scattering (SERS) remains challenging for different experimental conditions. In this study, a novel laser-driven photoinduced interfacial charge transfer (CT) was observed based on UV–visible–infrared excitation wavelengths (325, 488, 514, 633, and 785 nm) through surface modification of ZnO nanorods by 4-aminothiophenol (PATP). SERS spectra combined with well-characterized surface morphology and optical spectroscopy indicate that the chemical enhancement occurs at visible-infrared excitation but at ultraviolet excitation (325 nm) multiphonon resonant Raman Scattering (MRRS) results in additional strong enhancements of particular Raman transitions through Cu–ZnO–PATP model. The relationships between the excitation photon energies (3.82, 2.54, 2.41, 1.96, and 1.58 eV), and its Raman shift were discussed. We found the strong dependence of the Raman shifts with the exchanges of excitation photon energies. These results highlight the role of excitation energy in determining the interface enhanced Raman scattering for semiconductor-molecule models. This implies that copper sheet under the ZnO improve the interfacial CT in ZnO-molecule and act as an effective donor for inhibiting reversible CT, and there was a strong interaction, which might be regarded as CT resonance process, between PATP molecules and the ZnO surface. This work not only shows a possibility for further understanding the origin of the SERS mechanism from semiconductor substrates but also for exhibits a situ characterization technique for probing the photoinduced interfacial charge-transfer processes.

Significant Fowler–Nordheim tunneling across ZnO – Nanorod based nanojunctions for nanoelectronic device applications

We demonstrate significant Fowler–Nordheim (FN) tunneling across Al/Al2O3/ZnO metal–insulator–semiconductor (MIS) and Ag/ZnO metal–semiconductor (MS) nanojunctions. The transport properties of ZnO nanostructures in the form of urchins and randomly distributed nanorods were investigated in terms of various conduction mechanism. The minimum voltage necessary for triggering Fowler–Nordheim (FN) tunneling, under forward biasing, was ∼1.2 V and ∼3.4 V; respectively, below which only direct tunneling and thermionic emission events were evident. Mediated through Al2O3 layer, the FN tunneling was more prominent across MIS junction than MS one. The weak FN tunneling across MS junction was owing to interfacial charge transfer process through the atomic scale gapping between adjacent nanostructures. The extent of such type of tunneling is found to be nanostructure morphology dependent and largely rely on the free electrons donated by the native donor defects in the crystal structure of ZnO. The significant FN tunneling across the MIS and MS junctions has a direct relevance in designing nanoscale field emission devices/components working at low voltage with high throughputs.

Effects of Metal Oxide Modifications on Photoelectrochemical Properties of Mesoporous TiO2 Nanoparticles Electrodes for Dye-Sensitized Solar Cells

Mesoporous TiO2 (m-TiO2) nanoparticles were used to prepare the porous film electrodes for dye-sensitized solar cells, and a second metal oxide (MgO, ZnO, Al2O3, or NiO) modification was carried out by dipping the m-TiO2 electrode into their respective nitrate solution followed by annealing at 500 °C. Experimental results indicated that the above second metal oxide modifications on m-TiO2 electrode are shown in all cases to act as barrier layer for the interfacial charge transfer processes, but film electron transport and interfacial charge recombination characteristics under applied bias voltage were dependent significantly on the existing states and kinds of these second metal oxides. Those changes based on second metal oxide modifications showed good correlation with the current-voltage analyses of dye-sensitized solar cell, and all modifications were found to increase the open-circuit photo-voltage in various degrees, while the MgO, ZnO, and NiO modifications result in 23%, 13%, and 6% improvement in cell conversion efficiency, respectively. The above observations indicate that controlling the charge transport and recombination is very important to improve the photovoltaic performance of TiO2-based solar cell.

Surface functionalization of mesoporous antimony doped tin oxide by metalorganic reaction

Electrically conducting mesoporous antimony doped tin oxide was functionalized by the metalorganic reaction with several Grignard reagents. This fast and efficient grafting approach enables a direct connection of the metal atoms with the organic functionalities avoiding the formation of insulating Si–O linkers, which is of special interest for the interfacial charge transfer processes. Using this approach we introduced vinyl, allyl and phenyl groups into the pores of mesoporous ATO, which was confirmed by IR spectroscopy, TGA and nitrogen sorption measurements. We obtained a high loading of organic groups corresponding to about 50–60% of the monolayer surface coverage. The obtained mesoporous inorganic–organic hybrids can serve as a platform for incorporation of electrochemically active species.

Probing the Energetic Distribution of Injected Electrons at Quantum Dot–Linker–TiO2 Interfaces

We have used steady-state and time-resolved emission measurements to characterize interfacial electron transfer, or electron injection, from CdSe quantum dots (QDs) to molecularly linked TiO2 nanoparticles. Electrons were injected from both band-edge and trap states on relatively fast ( 10–8 s) time scales. The quantum yield of electron injection from trap states decreased as the trap-state distribution was shifted, by varying excitation energy, to lower energies. This effect probably arose from a driving-force dependence of the rate constant for electron injection. In contrast, the quantum yield of electron injection from band-edge states was independent of excitation energy. Our results highlight the key role of trapped carriers in interfacial charge-transfer processes of QDs and the influence of the energies and densities of trap states on the efficiencies of such processes.

ORGANIC CuPc COATING INDUCED IMPROVED PHOTOLUMINESCENCE AND PHOTOCONDUCTIVITY OF ZnO NANOWIRES ARRAY

In this work, we investigated the effect of organic CuPc coating on the surface of the ZnO NWs for possible improvement in the photoluminescence, photoconductivity and photoresponse. As a result of surface covering, the UV emission is enhanced by a factor of 7–8 while the green emission is reduced to half. Despite an increase in dark current after the CuPc covering, we obtained a significant improvement in the photocurrent and photoresponse rate. The photocurrent-to-dark current ratio is nearly doubled and the photoresponse process becomes faster for the ZnO/CuPc heterostructure. Improvements in the photoluminescence and photoconductivity for the ZnO/CuPc heterostructure are explained on the basis of modification of surface defects and interfacial charge transfer process.

Sheaf-like-ZnO@Ag nanocomposite materials modified photoanode for low-cost metal-free organic dye-sensitized solid-state solar cells

Sheaf-like-ZnO-Ag nanocomposite material (sheaf-like-(ZnO@Ag)NCM) was prepared and used as photoanode in dye-sensitized solar cell. Eosin-Y adsorbed photoanode was employed, and the overall efficiency (η) was found to be 1.39% under standard AM 1.5G simulated solar irradiation (100 mWcm−2) which is ∼2-fold higher than that of ZnO nanorods (ZnO NRs), owing to the presence of an effective interfacial charge transfer processes through the Ag from the excited state of eosin-Y to ZnO. The obtained efficiency is also compared to the sensitized with N719, whose efficiency (1.8%) is ∼30% higher than that of the cells sensitized with eosin-Y.

Nanocarbon Composites and Hybrids in Sustainability: A Review

There is an ever-growing need to protect our environment by increasing energy efficiency and developing "clean" energy sources. These are global challenges, and their resolution is vital to our energy security. Although many conventional materials, such as metals, ceramics, and plastics, cannot fulfil all requirements for these new technologies, many material combinations can offer synergistic effects that create improved and even new properties. The implementation of nanocarbons, such as graphene and carbon nanotubes, into nanocomposites and, more recently, into the new class of hybrids, are very promising examples. In contrast to classical nanocomposites, where a low volume fraction of the carbon component is mixed into a polymer or ceramic matrix, hybrids are materials in which nanocarbon is coated with a thin layer of the functional compound, which introduces the interface as a powerful new parameter. Based on interfacial charge and energy transfer processes, nanocarbon hybrids have shown increased sensitivities in gas sensors, improved efficiencies in photovoltaics, superior activities in photocatalysts, and enhanced capacities in supercapacitors. This review compares the characteristics and potentials of both nanocarbon composites and hybrids, highlights recent developments in their synthesis and discusses key challenges for their use in various energy applications.

ZnO/anthracene based inorganic/organic nanowire heterostructure: Photoresponse and photoluminescence studies

The effects of surface modification of vertically aligned ZnO nanowires (NWs) with anthracene for the improved photocurrent, photoresponse, and UV photoluminescence have been investigated in this work. The formation of ZnO/anthracene based inorganic/organic NWs heterostructure by surface capping with anthracene solution was confirmed from the high resolution transmission electron microscopy and Fourier transport infrared spectroscopy analyses. After the anthracene capping of ZnO NWs, despite an increase in dark current, we obtained a significant improvement in the photocurrent and photoresponsivity. A sixfold improvement in the UV photocurrent-to-dark current ratio is obtained with capping. Compared to the uncapped NWs, the photoresponse is significantly faster for the ZnO/anthracene system with response and reset times of 1.5 and 1.6 s, respectively. The room temperature photoluminescence spectra show threefold enhanced UV emission with large enhancement in the ratio of UV to green emission intensities. The faster photoresponse and enhanced photocurrent from the ZnO/anthracene heterostructure are explained on the basis of modification of surface defects and interfacial charge transfer process.

Hybridizing photoactive zeolites with graphene: a powerful strategy towards superior photocatalytic properties

Research on zeolitic materials has recently taken off in a new direction, driven by the desire to design nanoscale hierarchical architectures with specific chemical functionalities. From both, a scientific and industrial perspective, photoactive zeolites, such as titanosilicates (TS-1), are among the most interesting candidates and research has focused on maximising their photocatalytic performance by means of nanostructuring and electronic modification. In this work, we employ graphene to create a new class of photocatalysts that truly advances semi-conductor photocatalysis to its next generation. We demonstrate that the photocatalytic activity of TS-1 for the degradation of organic dyes can be enhanced through graphene by more than 25 times. Responsible for this remarkable improvement are (1) a more efficient electron–hole separation via interfacial charge transfer processes and (2) surprising beneficial effects of graphene on the crystal morphology, the porosity and the adsorption behaviour of the zeolite.

TiO2–Au nanocomposite materials embedded in polymer matrices and their application in the photocatalytic reduction of nitrite to ammonia

Titanium dioxide–gold nanocomposite materials ((TiO2–Au)nps) were prepared and embedded in methyl functionalized silicate sol–gel (MTMOS) and Nafion (Nf) matrices. The nanocomposite materials were characterized by UV-vis absorption spectra, scanning electron micrographs (SEM) and high resolution transmission electron micrographs (HRTEM). When gold nanoparticles were deposited on TiO2 ((TiO2–Au)nps) and immobilized in MTMOS silicate sol–gel and Nafion matrices, the band-gap (Ebg) of the TiO2 shifted to lower energy. During the photocatalytic experiment, the (TiO2–Au)nps incorporated polymer matrices improved the photocatalytic reduction of nitrite ions to ammonia, owing to the effective interfacial charge transfer process in the presence of a hole scavenger, oxalic acid. Further increase in the photocatalytic reduction of nitrite to ammonia was observed by incorporating a [Ni(teta)]2+ complex into the MTMOS/(TiO2–Au)nps or Nf/(TiO2–Au)nps film. The immobilization of (TiO2–Au)nps in a functionalized silicate sol–gel or Nafion matrix is advantageous for the preparation of solid-phase photocatalyst film and to design a solid–solution system leading to the physical separation of the catalyst from the solution and the products in contrast compared to the colloidal photocatalyst system. The Aunps deposited on TiO2 act as an e− sink which promotes efficient interfacial electron transfer from TiO2 to the substrate upon irradiation. The ultimate contact between (TiO2–Au)nps and Ni(II) complex in the film efficiently promotes the electron transfer from Aunps to Ni(II) complex leading to the formation of an intermediate Ni(I) complex, which facilitates the efficient catalytic reduction of nitrite to ammonia.

Synthesis, optical and electrochemical properties of ZnO nanorod hybrids loaded with high-density gold nanoparticles

Derived gold nanoparticles supported on ZnO nanorods were generated via the self-assembly of gold salt precursors and subsequent local reduction of AuCl4− ions, which are ion-paired with protonated 3-aminopropyltriethoxysilane and chemically attached to the hydroxyl-rich surfaces of the ZnO nanorods. X-ray diffraction, X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy and transmission electron microscopy were used to characterize the as-prepared gold nanoparticles loaded onto the ZnO nanorod hybrids. Attachment of the gold nanoparticles onto the ZnO nanorods alters the interfacial charge transfer process and facilitates the electron-hole separation at the interface between the gold and the ZnO, resulting in very good electrochemical properties and characteristic optical properties in the UV-visible spectrum and photoluminescence spectrum. This study provides a new opportunity for the tunable attachment of gold nanoparticles onto the functionalized ZnO nanorods and enriches the preparation methods of new nanohybrids with unique structures and novel properties.

Hydrogen evolution from water using semiconductor nanoparticle/graphene composite photocatalysts without noble metals

Semiconductor nanoparticle/graphene composite photocatalysts containing semiconductor CdS or TiO2 nanoparticles are fabricated by one-pot solution methods and their structures are characterized. The photocatalytic hydrogen-generating capabilities of the composite photocatalysts are investigated in the presence of sacrificial reagent and compared with those of the same semiconductor materials with platinum as a co-catalyst under the same conditions. The results obtained by the measurements of time-resolved emission spectra, photocurrent generated response and electrochemical impedance spectra revealed that graphene attached to semiconductor surfaces can efficiently accept and transport electrons from the excited semiconductor, suppressing charge recombination and improving interfacial charge transfer processes. The semiconductor nanoparticle/graphene photocatalysts displayed higher activity for photocatalytic hydrogen evolution, which can be compared with the hydrogen-generating efficiency of systems containing the well-known Pt co-catalyst. This work provides an inexpensive means of harnessing solar energy to achieve highly efficient hydrogen evolution without noble metals.

Charge Trapping and Storage by Composite P3HT/PC60BM Nanoparticles Investigated by Fluorescence-Voltage/Single Particle Spectroscopy

Fluorescence-voltage/single particle spectroscopy (F-V/SPS) was employed to study exciton-hole polaron interactions and interfacial charge transfer processes for pure poly(3-hexylthiophene) (P3HT) nanoparticles (NPs) and composite P3HT/PC(60)BM NPs in functioning hole-injection devices. F-V/SPS data collected on a particle-by-particle basis reveal an apparent bistability in the fluorescence-voltage modulation curves for composite NPs of P3HT and [6,6]-phenyl-C(61)-butyric acid methyl ester (PC(60)BM) that is absent for pure P3HT NPs. A pronounced deep trapping of free electrons photogenerated from the composite P3HT/PC(60)BM NPs at the NP/dielectric interface and hole trapping by fullerene anions in composite P3HT/PC(60)BM NPs under photoexcitation lies at the basis of this finding. The deep electron trapping effect reported here for composite conjugated polymer/fullerene NPs presents an opportunity for future application of these NPs in nanoscale memory and imaging devices.

Strategies developed on the modification of titania for visible light response with enhanced interfacial charge transfer process: an overview

AbstractThe modification of titania by metal / non metal ion doping, coupling with narrow band gap sensitizer, surface flourination, metal deposition, and together with recent ventures on application of {001} facets of anatase titania for visible light response with enhanced charge carrier separation are briefly overviewed.

Optical Properties of Organic Semiconductor Blends with Near‐Infrared Quantum‐Dot Sensitizers for Light Harvesting Applications

AbstractWe report an optical investigation of conjugated polymer (P3HT)/fullerene (PCBM) semiconductor blends sensitized by near‐infrared absorbing quantum dots (PbS QDs). A systematic series of samples that include pristine, binary and ternary blends of the materials are studied using steady‐state absorption, photoluminescence (PL) and ultrafast transient absorption. Measurements show an enhancement of the absorption strength in the near‐infrared upon QD incorporation. PL quenching of the polymer and the QD exciton emission is observed and predominantly attributed to intermaterial photoinduced charge transfer processes. Pump‐probe experiments show photo‐excitations to relax via an initial ultrafast decay while longer‐lived photoinduced absorption is attributed to charge transfer exciton formation and found to depend on the relative ratio of QDs to P3HT:PCBM content. PL experiments and transient absorption measurements indicate that interfacial charge transfer processes occur more efficiently at the fullerene/polymer and fullerene/nanocrystal interfaces compared to polymer/nanocrystal interfaces. Thus the inclusion of the fullerene seems to facilitate exciton dissociation in such blends. The study discusses important and rather unexplored aspects of exciton recombination and charge transfer processes in ternary blend composites of organic semiconductors and near‐infrared quantum dots for applications in solution‐processed photodetectors and solar cells.

Understanding the Role of the Sulfide Redox Couple (S2–/Sn2–) in Quantum Dot-Sensitized Solar Cells

The presence of sulfide/polysulfide redox couple is crucial in achieving stability of metal chalcogenide (e.g., CdS and CdSe)-based quantum dot-sensitized solar cells (QDSC). However, the interfacial charge transfer processes play a pivotal role in dictating the net photoconversion efficiency. We present here kinetics of hole transfer, characterization of the intermediates involved in the hole oxidation of sulfide ion, and the back electron transfer between sulfide radical and electrons injected into TiO(2) nanoparticles. The kinetic rate constant (10(7)-10(9) s(-1)) for the hole transfer obtained from the emission lifetime measurements suggests slow hole scavenging from CdSe by S(2-) is one of the limiting factors in attaining high overall efficiency. The presence of the oxidized couple, by addition of S or Se to the electrolyte, increases the photocurrent, but it also enhances the rate of back electron transfer.