Photoelectrochemical Studies of CdS Nanoparticle Modified Electrodes: Absorption and Photocurrent Investigations

Electronic Structure of Colloidal 2H-MoS2 Mono and Bilayers Determined by Spectroelectrochemistry

The electronic structure of mono and bilayers of colloidal 2H-MoS₂ nanosheets synthesized by wet-chemistry using potential-modulated absorption spectroscopy (EMAS), differential pulse voltammetry, and electrochemical gating measurements is investigated. The energetic positions of the conduction and valence band edges of the direct and indirect bandgap are reported and observe strong bandgap renormalization effects, charge screening of the exciton, as well as intrinsic n-doping of the as-synthesized material. Two distinct transitions in the spectral regime associated with the C exciton are found, which overlap into a broad signal upon filling the conduction band. In contrast to oxidation, the reduction of the nanosheets is largely reversible, enabling potential applications for reductive electrocatalysis. This work demonstrates that EMAS is a highly sensitive tool for determining the electronic structure of thin films with a few nanometer thicknesses and that colloidal chemistry affords high-quality transition metal dichalcogenide nanosheets with an electronic structure comparable to that of exfoliated samples.

Introducing visible-light sensitivity into photocatalytic CeO2 nanoparticles by hybrid particle preparation exploiting plasmonic properties of gold: enhanced photoelectrocatalysis exemplified for hydrogen peroxide sensing

In this report we combine the catalytic properties of CeO₂ nanoparticles with their transduction ability for photoelectrochemical sensing. This study highlights the usage of CeO₂ providing catalytic activity towards H₂O₂, but only with a limited excitation range in the UV for the construction of a sensing system. In order to improve the photoelectrocatalysis of CeO₂ nanoparticles by extending their excitation to visible light, Au/CeO₂ core/shell hybrid nanoparticles have been synthesized. The hybrid nanoparticles are fixed on electrodes, allowing for the generation of photocurrents, the direction of which can be controlled by the electrode potential (without bias). The application of the hybrid nanoparticles results in an enhanced photocurrent amplitude under white light illumination as compared to the pure CeO₂ nanoparticles. Wavelength-dependent measurements confirm the participation of the Au core in the signal transduction. This can be explained by improved charge carrier generation within the hybrid particles. Thus, by using a plasmonic element the photoelectochemical response of a catalytic nanoparticle (i.e. CeO₂) has been spectrally extended. The effect can be exploited for sensorial hydrogen peroxide detection. Here higher photocatalytic current responses have been found for the hybrid particles fixed to gold electrodes although the catalytic reduction has been comparable for both types of nanoparticles. Thus, it can be demonstrated that Au/CeO₂ core-shell nanoparticles allow the utilization of visible light for photoelectrochemical hydrogen peroxide (H₂O₂) detection with improved sensitivity under white light illumination or application of such particles with only visible light excitation, which is not possible for pure CeO₂. With help of the layer-by-layer (LbL) technique for nanoparticle immobilization, the electrode response can be adjusted and with a 5 layers electrode a low detection limit of about 3 μM H₂O₂ with a linear detection range up to 2000 μM is obtained.

Photoluminescence Lifetime Based Investigations of Linker Mediated Electronic Connectivity Between Substrate and Nanoparticle

The evolution of systems based on nanoparticles as the main component seems to be a self-accelerating process during the last five decades. Hence, an overview across this field gets more and more challenging. It is sometimes rewarding to focus on the fundamental physical phenomenon of the electronic interconnection between the different building blocks of the obtained devices. Therefore, the investigation of charge transport among the utilized particles and their substrate is one of the mandatory steps in the development of semiconductor nanoparticle based devices like e.g., sensors and LEDs. The investigation of the influence of tunneling barriers on the properties of nanoparticle-functionalized surfaces is a challenging task. The different basic influences on the charge transport dynamics are often difficult to separate from each other. Non-invasive and easily viable experiments are still required to resolve the charge distributing mechanisms in the systems. In the presented work, we want to focus on thin and transparent indium tin oxide (ITO) layers covered glass slides since this substrate is frequently utilized in nanoelectronics. CdSe/CdS nanorods (NRs) are applied as an optically addressable probe for the electronic surface states of the conductive glass. The presented experimental design provides the proof of electronic interconnections in ITO coated glass/linker/NR electrodes via easy reproducible functionalization and polishing experiments. UV/Vis absorption and photoluminescence (PL) lifetime measurements revealed changes in the optical properties caused by differences in the charge carrier dynamics between the system. Our work is focused on the modification of charge carrier dynamics due to the application of linker molecules with different functional groups like (3-mercaptopropyl)methoxysilane (MPTMS) and (3-aminopropyl)trimethoxysilane (APTMS). The presented observations are explained with a simple kinetic model.

Opportunities and challenges for electrochemistry in studying the electronic structure of nanocrystals

We review the state-of-the-art of determining the electronic structure of nanocrystals in thin films by electrochemistry and emphasize the benefits of correlating electrochemical with spectroscopic methods to this end.

Electronic transport in CdSe nanoplatelet based polymer fibres

One of the most significant objectives in the field of nanotechnology is the transfer of specific properties of smaller nanoparticle building blocks into larger units. In this way, nanoscopic properties can be linked to the macroscopic addressability of larger systems. Such systems might find applications in fields like photoelectrochemical sensing or solar energy harvesting. Our work reports on the novel synthesis of hybrid semiconductor/polymer fibres, which are based on stacks of 4 monolayer (ML) thick CdSe nanoplatelets (NPLs) encapsulated into a polymer shell. The polymer encapsulation not only enables the water transfer of the NPL stacks but also allows the preparation of photoelectrodes by linking the fibres to surface modified indium tin oxide (ITO) glass slides. By applying electrochemical techniques like intensity modulated photocurrent spectroscopy (IMPS), it was possible to prove the motion of charge carriers inside the nanoplatelet stacks and by this the electronic addressibility of them.

Stabilizing black phosphorus nanosheets via edge-selective bonding of sacrificial C60 molecules

Few-layer black phosphorus (BP) with an anisotropic two-dimensional (2D)-layered structure shows potential applications in photoelectric conversion and photocatalysis, but is easily oxidized under ambient condition preferentially at its edge sites. Improving the ambient stability of BP nanosheets has been fulfilled by chemical functionalization, however this functionalization is typically non-selective. Here we show that edge-selective functionalization of BP nanosheets by covalently bonding stable C60 molecules leads to its significant stability improvement. Owing to the high stability of the hydrophobic C60 molecule, C60 functions as a sacrificial shield and effectively protects BP nanosheets from oxidation under ambient condition. C60 bonding leads to a rapid photoinduced electron transfer from BP to C60, affording enhanced photoelectrochemical and photocatalytic activities. The selective passivation of the reactive edge sites of BP nanosheets by sacrificial C60 molecules paves the way toward ambient processing and applications of BP.

Large Area α-Cu2S Particle-Stacked Nanorod Arrays by Laser Ablation in Liquid and Their Strong Structurally Enhanced and Stable Visible Photoelectric Performances

A flexible route is developed for fabrication of large area α-Cu₂S nanorod arrays (NRAs) on the basis of one-step laser ablation of a copper foil in CS₂ liquid. It has been demonstrated that the obtained products are the high-temperature phase α-Cu₂S and consist of the nanorods vertically standing on the Cu foil, exhibiting the array. The nanorods were about 1 μm in length and around 100 nm in thickness and built by stacking the nearly spherical and ⟨110⟩-oriented nanoparticles (NPs) up. Such array can be peeled off from the foil and remain freestanding. Further, it has been found that the ablation duration, the laser power, and the foil surface state are crucial to the formation of the Cu₂S NRA. The formation of such oriented NP-stacked Cu₂S NRAs is attributed to the laser-induced generation of α-Cu₂S NPs and the NPs' deposition/oriented connection growth on the surface-vulcanized copper foil. Importantly, the visible photocurrent response of the α-Cu₂S NRAs is 8 times higher than that of the Cu₂S NPs' film with the equivalent thickness and also larger than that of previously reported Cu₂S, showing significantly enhanced photoelectric performances. As an application, such NRAs have exhibited markedly enhanced visible photocatalytic activity and highly stable recycling performances, compared with the α-Cu₂S NPs. Further studies have revealed that the enhanced performances are attributed to the structurally enhanced light trapping effect of the NRAs as well as short and smooth carrier diffusion path in the oriented NP-stacked nanorods. This work provides a new and simple method for fabrication of the large area Cu₂S NRAs with high and stable photoelectric performances.

The Photoelectrochemistry of Assemblies of Semiconductor Nanoparticles at Interfaces

The application of photoelectrochemical methods presents the researcher with a powerful set of versatile tools by which photoactive materials, such as semiconductor quantum dots, at conductive interfaces may be interrogated. While the range of photoelectrochemical techniques available is quite large, it is surprising that very few have found their way into common usage within the nanoparticle community. Here a number of photoelectrochemical techniques and the principles upon which they are based are introduced. A short discussion on the criticality of ensuring the nanoparticles are reliably anchored to the substrate is followed by an introduction to the basic set of equipment required in order to enable the investigator to undertake such experiments. Subsequently the four techniques of transient photocurrent response to square wave illumination, photocurrent spectroscopy, intensity modulated photocurrent spectroscopy and intensity modulated photovoltage spectroscopy are introduced. Finally, the information that can be acquired using such techniques is provided with emphasis being placed on a number of case studies exemplifying the application of photoelectrochemical techniques to nanoparticles at interfaces, in particular optically transparent electrodes.

Quantum-dot-based photoelectrochemical sensors for chemical and biological detection

Quantum-dot-based photoelectrochemical sensors are powerful alternatives for the detection of chemicals and biochemical molecules compared to other sensor types, which is the primary reason as to why they have become a hot topic in nanotechnology-related analytical methods. These sensors basically consist of QDs immobilized by a linking molecule (linker) to an electrode, so that upon their illumination, a photocurrent is generated which depends on the type and concentration of the respective analyte in the immediate environment of the electrode. The present review provides an overview of recent developments in the fabrication methods and sensing concepts concerning direct and indirect interactions of the analyte with quantum dot modified electrodes. Furthermore, it describes in detail the broad range of different sensing applications of such quantum-dot-based photoelectrochemical sensors for inorganic and organic (small and macro-) molecules that have arisen in recent years. Finally, a number of aspects concerning current challenges on the way to achieving real-life applications of QD-based photochemical sensing are addressed.

Micron-sized hexagonal single-crystalline rods of metal nitride clusterfullerene: preparation, characterization, and photoelectrochemical application

Micron-sized hexagonal single-crystalline Sc(3)N@C(80) rods have been successfully prepared for the first time by a liquid-liquid interfacial precipitation (LLIP) method with the first utilization of p-xylene as the solvent dissolving Sc(3)N@C(80). The effect of the concentration of the Sc(3)N@C(80) solution on the size and length of the Sc(3)N@C(80) rods has been studied, indicating that the length of Sc(3)N@C(80) rods can be readily controlled by varying the concentration of the Sc(3)N@C(80) solution. The crystal structure of the Sc(3)N@C(80) rods has been investigated by XRD and the electron diffraction patterns, pointing to a hexagonal system. The growth kinetics of the Sc(3)N@C(80) rods has been studied by monitoring the morphology evolution of the Sc(3)N@C(80) crystals, and a plausible mechanism is proposed, featuring an intermediate hexagonal star-shaped prism structure with grooves. Raman spectroscopic characterization confirmed that the Sc(3)N@C(80) rods are composed of monomeric pristine Sc(3)N@C(80) molecules and no polymerization has occurred in the crystal lattice, and a significant Raman enhancement in the low-energy region is observed. According to the UV-vis-NIR absorption spectroscopic study of the Sc(3)N@C(80) rods, where much broader and stronger absorptions in the visible and near-infrared regions than that of the Sc(3)N@C(80) solution were revealed, we conclude that the electronic structure of the Sc(3)N@C(80) molecule is largely perturbed upon formation of micron-sized single-crystalline rods because of the strong intermolecular π-π interactions. Finally photoelectrochemical application of the Sc(3)N@C(80) rods was studied based on a Sc(3)N@C(80) rods-modified ITO electrode prepared by electrophoretic deposition and revealed a higher photocurrent response than that obtained in the Sc(3)N@C(80) films drop-coated onto an ITO electrode.

BLUE-GREEN LUMINESCENCE OF CHEMICALLY SYNTHESIZED MWCNT/CdS NANOHYBRID STRUCTURE

A simple inexpensive wet chemical technique at room temperature to prepare hybrid structure of multiwalled carbon nanotubes (MWCNT) and cadmium sulfide ( CdS ) nanoparticles has been reported in this paper. Cadmium sulfide nanocrystals of average size 5 nm have been synthesized and attached with the surfaces of MWCNTs. The hybrid material is characterized by high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and Raman spectroscopy. Interesting optical properties of the composite are revealed through UV–visible and photoluminescence (PL) spectroscopy. Significant blue-green PL emission covering a region from 450–600 nm wavelength has been observed when excited by UV radiation of 220–240 nm wavelength. Sharp emission peak has been obtained and this may find wide applications in optical sensors and optoelectronic devices.

Ferrocene-coated CdSe/ZnS quantum dots as electroactive nanoparticles hybrids

Electrochemical properties of core-shell CdSe/ZnS quantum dots (QDs) in a non-aqueous solution are presented. Cathodic reduction and anodic oxidation processes involving the QD HOMO and LUMO levels as well as defect states were identified by cyclic voltammetry. The electrochemical bandgap was estimated from the anodic and cathodic redox peaks and found to match well with the optical bandgap estimated from the absorption spectrum. The trioctylphosphine oxide ligands on the surface of the QDs were exchanged to electroactive ferrocenyl thiols and the resulting material was characterized by NMR and optical spectroscopy. Cyclic voltammetry showed that the redox potentials of the QDs are modified due to the presence of ferrocene on the surface of the QD. The QD oxidation peak decreased and the reduction peak shifted to more negative potentials. The concurrent shift of the ferrocene redox peaks indicates that the system displays features of a 'molecular hybrid', where both the QD and the ligand influence each other.

CdSe sensitized thin aqueous films: probing the potential distribution inside multilayer assemblies

Ultrathin polypeptide multilayer films are assembled by the sequential electrostatic adsorption of monolayers of poly-l-lysine and poly-l-glutamic acid onto carboxylic acid terminated alkanethiol-modified gold surfaces. The polypeptide multilayer films are hydrophilic, can incorporate electroactive species such as ferri/ferrocyanide, and are stable when immersed in organic solvents such as 1,2-dichloroethane. Cadmium selenide quantum dots stabilized by negatively charged citrate groups are electrostatically attached to the multilayer film assembly in order to act as photoactive species. Photocurrent responses originating from the CdSe sensitized ultrathin multilayer film are investigated as functions of the applied potential, the thickness of the film and the presence of quenchers in the organic phase. A theoretical model is proposed in order to analyze the kinetics of the photoinduced electron-transfer reactions and to probe the potential distribution within the film.

Photocatalytic polypyrrole-TiO2-nanoparticles composite thin film generated at the air-water interface

Herein, we report a new method of generation of TiO(2) nanoparticles (NPs) incorporated thin films of polypyrrole (PPy) at the air-water interface. Aqueous TiO(2) NPs when treated with H(2)O(2) and left in a chamber of pyrrole vapor resulted in the formation of a film at the interface, in addition to bulk precipitate. Spectroscopic, X-ray diffraction, and electron microscopic measurements establish the formation of a thin film of PPy with the incorporation of TiO(2) NPs. The TiO(2)-containing PPy films when transferred onto glass substrates were able to photo catalyze the decomposition of aqueous organic dyes: methyl orange and methylene blue. Further, these films could also photo catalyze the oxidation of iodide to triiodide ions in aqueous potassium iodide solution. We find that the PPy-TiO(2) composite films catalyze the reactions in the presence of light more efficiently than a suspension of TiO(2) NPs.

Size-Dependent Electrochemical Behavior of Thiol-Capped CdTe Nanocrystals in Aqueous Solution

Electrochemical studies of thiol-capped CdTe nanocrystals in aqueous solution have demonstrated several distinct oxidation and reduction peaks in the voltammograms, with the peak positions being dependent on the size of the nanocrystals. While the size dependence of the reduction and one of the oxidation potentials can be attributed to altering the energetic band positions owing to the quantum size effect, an extraordinary behavior was found for the oxidation peak observed at less positive potentials. In contrast to a prediction based on the quantum size effect, this peak moves to more negative potentials as the nanocrystals' size decreases. Moreover, the contribution of the charge associated with this peak compared to the total charge passed during the nanocrystal oxidation correlates well with the photoluminescence (PL) efficiency of individual fractions of the CdTe nanocrystals. These experimental observations allow a peak to be assigned to the oxidation of Te-related surface traps. The intra-band-gap energy level assigned to these Te-related trap states shifts toward the top of the valence band as the nanocrystal size increases, thus allowing the higher photostability of the larger nanocrystals to be explained. At a certain nanocrystal size, the trap level can even move out of the band gap.

Electrochemical and optical properties of two dimensional electrostatic assembly of Au nanocrystals

The spectroscopic and electrochemical properties of two-dimensional electrostatic assembly of Au nanocrystals are examined on poly-L-lysine (pLys) modified gold electrodes. The surface preparation for the nanoparticle deposition involved the self-assembly of a monolayer of 11-mercaptoundecanoic acid on the electrode surface, followed by the electrostatic deposition of pLys from aqueous solution. The polyelectrolyte layer acts as the electrostatic anchor for the Au particles. Electrostatically stabilised Au particles were prepared by homogeneous reduction in the presence of citrate, yielding monodispersed colloidal suspension with an average diameter of 18 +/- 2 nm. After 4 h of deposition, the citrate-stabilised particles reach a maximum surface density of (8.2 +/- 0.1) x 10(10) particles cm(-2), with an average edge-to-edge distance of 25 nm. The particle surface density was estimated from scanning electron micrographs. Kelvin probe measurements were employed for examining changes in surface dipole introduced by the 2D array of nanocrystals. From simple electrostatic arguments, the apparent static dipole moment per particle was estimated of the order of 2700 D. The strong interaction between the nanocrystals and the pLys layer is responsible for the surface charge displacement, leading to changes in the surface dipole of 0.35 eV. These electrostatic interactions also manifest itself by the red shift of the plasmon resonance of the assembly with respect to the aqueous colloidal suspension. Analysis of the spectral broadening was attempted within the framework of the so-called coherent-potential approximation. Finally, electrochemical studies in 1,2-dichloroethane show a large electronic overlap between the nanocrystals and the metal substrate. Results obtained from electrochemical impedance spectroscopy strongly suggest that the electrostatic assembly of nanocrystal behaves like a 2D array of randomly distributed spherical nanoelectrodes.