Separation Of Photogenerated Charge Carriers Research Articles

Construction of flexible photoelectrochemical solar cells based on ordered nanostructural BiOI/Bi2S3 heterojunction films.

Ordered 2D nanostructural BiOI nanoflake arrays decorated with Bi2S3 nanospheres have been designed and in situ fabricated for the first time, to form BiOI/Bi2S3 bulk heterojunctions through a soft chemical route. A modified successive ionic layer adsorption and reaction (SILAR) method was developed to fabricate BiOI nanoflake arrays on flexible ITO/PET substrates at room temperature. The degree of transformation of BiOI to Bi2S3 was controlled through the adjustment of exposure time of the BiOI/ITO substrate to thioacetamide (TAA) aqueous solution. The morphologies of BiOI, BiOI/Bi2S3 heterojunctions and Bi2S3 films were examined by scanning electron microscopy (SEM), X-ray powder diffraction (XRD) patterns, and high resolution transmission electron microscopy (HRTEM). The presence of Bi2S3 was further validated through Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Especially, photoelectrochemical measurements demonstrated that such a Bi2S3 decorated BiOI photoanode based cell exhibits significant augments of short-circuit current density (Jsc) and incident photon-to-current conversion efficiency (IPCE, 3 times higher than the pure BiOI photoanode), attributable to the stronger photo-absorption and better photogenerated charge carrier separation and transport efficiency. The surface photovoltage (SPV) measurements further confirmed the importance of BiOI/Bi2S3 heterojunctions in such PEC cells. This solution-based process directly on flexible ITO offers the promise for low-cost, large-area, roll-to-roll application of the manufacturing of the third generation thin-film photovoltaic devices.

Facile synthesis and visible photocatalytic activity of single-crystal TiO2/PbTiO3 heterostructured nanofiber composites

For single-crystal TiO2/PbTiO3 nanofiber composites, anatase TiO2 nanorods grew on the surface of tetragonal perovskite (TP) PbTiO3 nanofibers to form TiO2/PbTiO3 heterostructures.

Synthesis, Characterization, and Photocatalytic Activity of g‐C3N4/KTaO3 Composites under Visible Light Irradiation

Novel graphitic carbon nitride/KTaO3 (g‐C3N4/KTaO3) nanocomposite photocatalysts have been successfully synthesized via a facile and simple ultrasonic dispersion method. Compared to either g‐C3N4 or KTaO3, the composite photocatalysts show significantly increased photocatalytic activity for degradation of Rhodamine B (RhB) under visible light irradiation. The increased photocatalytic performance of the composite could be attributed to the enhanced photogenerated charge carrier separation capacity. Moreover, it is observed that ∙ is the main active species in the photocatalytic degradation of RhB using the g‐C3N4/KTaO3 composite photocatalysts.

Electrochemically grafted polypyrrole changes photoluminescence of electronic states inside nanocrystalline diamond

Hybrid diamond-organic interfaces are considered attractive for diverse applications ranging from electronics and energy conversion to medicine. Here we use time-resolved and time-integrated photoluminescence spectroscopy in visible spectral range (380–700 nm) to study electronic processes in H-terminated nanocrystalline diamond films (NCD) with 150 nm thin, electrochemically deposited polypyrrole (PPy) layer. We observe changes in dynamics of NCD photoluminescence as well as in its time-integrated spectra after polymer deposition. The effect is reversible. We propose a model where the PPy layer on the NCD surface promotes spatial separation of photo-generated charge carriers both in non-diamond carbon phase and in bulk diamond. By comparing different NCD thicknesses we show that the effect goes as much as 200 nm deep inside the NCD film.

Hierarchically CdS Decorated 1D ZnO Nanorods‐2D Graphene Hybrids: Low Temperature Synthesis and Enhanced Photocatalytic Performance

A simple, low‐temperature synthesis approach is reported for planting CdS‐sensitized 1D ZnO nanorod arrays on the 2D graphene (GR) sheet to obtain the ternary hierarchical nanostructures, during which graphene oxide (GO) as the precursor of GR acts as a flexible substrate for the formation of ZnO nanorod arrays. The hierarchical CdS‐1D ZnO‐2D GR hybrids can serve as an efficient visible‐light‐driven photocatalyst for selective organic transformations. The fast electron transport of 1D ZnO nanorods, the well‐known electronic conductivity of 2D GR, the intense visible‐light absorption of CdS, the unique hierarchical structure, and the matched energy levels of CdS, ZnO and GR efficiently boost the photogenerated charge carriers separation and transfer across the interfacial domain of hierarchical CdS‐1D ZnO‐2D GR hybrids under visible light irradiation via three‐level electron transfer process. Furthermore, the superior reusability of ternary hybrids is achieved by controlling the reaction parameters, i.e., using visible light irradiation and holes scavenger to prevent ZnO and CdS from photocorrosion. This work demonstrates a facile way of fabricating hierarchical CdS‐1D ZnO‐2D GR hybrids in a controlled manner and highlights a promising scope of adopting integrative photosensitization and co‐catalyst strategy to design more efficient semiconductor‐based composite photocatalysts toward solar energy capture and conversion.

Superior photocatalytic performance of reduced graphene oxide wrapped electrospun anatase mesoporous TiO2 nanofibers

Reduced graphene oxide (rGO) wrapped with anatase mesoporous TiO2 nanofibers (TNFs) were synthesized, by using the electrospinning technique along with easy chemical methods. The structural and morphological results demonstrate the success of wrapping of nanofibers with rGO. Wrapping with rGO leads to an efficient photogenerated charge carrier separation across the interface of rGO and TNFs. As a result, the photocatalytic activity of the composites is enhanced to 96% compared to only 43% for TNFs alone, in the photo degradation of methyl orange. This simple synthetic method can be applied not only to wrap rGO over other nanostructures of different morphology, but also for enhancing the properties of multifunctional materials.

Enhancing the photocatalytic activity of nanocrystalline TiO2 by co-doping with fluorine and yttrium

Fluorine and yttrium codoped TiO2 nanoparticles were prepared using a simple sol–gel method. The products were characterized with various spectroscopic and analytical techniques to determine their structural, morphological, optical absorption and photocatalytic properties. The results reveal that neither F nor Y doping causes obvious absorption edge shift in TiO2. Photoluminescence (PL) emission spectra present that the PL signal is enhanced, suggesting a decrease of photo-generated charge carrier separation efficiency, after the F or Y doping. The synergistic action by the F and Y doping leads to the highest photocatalytic activity for the degradation of methylene blue solution in the 0.05% (F, Y)-codoped sample (15 times of that over the pure TiO2). With the increase of Y doping level, the photocatalytic performance in the codoped samples increases firstly and then decreases. The photocatalytic activity variations after the F and Y doping were interpreted by the formation of photocatalytic reactive oxygen species induced by the dopings.

High visible light activity of hydrogenated structure-engineered mixed phase titania photocatalyst

The hydrogenation induced band-edge tailing with the resultant red-shift in the absorption spectrum and the creation of oxygen vacancies have incredible impact on the functionality of the monophasic titania photocatalyst in detoxification of aqueous pollutant under solar radiation. The mixed phase titania with optimum phase ratio consistently shows high activity in UV and/or visible radiation flux due to efficient separation of photo-generated charge carriers. We demonstrate that the hydrogenation of mixed phase titania (anatase-to-rutile ratio=0.67) shows 15 times higher photocatalytic activity in the degradation of aqueous probe pollutant under the visible light, than the non-hydrogenated Degussa P25 catalyst.

High short-circuit current density CdTe solar cells using all-electrodeposited semiconductors

CdS/CdTe and ZnS/CdTe n–n heterojunction solar cells have been fabricated using all-electrodeposited semiconductors. The best devices show remarkable high short-circuit current densities of 38.5mAcm−2 and 47.8mAcm−2, open-circuit voltages of 630mV and 646mV and conversion efficiencies of 8.0% and 12.0% respectively. The major strength of these device structures lies in the combination of n–n heterojunction with a large Schottky barrier at the n-CdTe/metal back contact which provides the required band bending for the separation of photo-generated charge carriers. This is in addition to the use of a high quality n-type CdTe absorber layer with high electron mobility. The potential barrier heights estimated for these devices from the current–voltage characteristics exceed 1.09eV and 1.13eV for CdS/CdTe and ZnS/CdTe cells respectively. The diode rectification factors of both devices are in excess of four orders of magnitude with reverse saturation current densities of 1.0×10−7Acm−2 and 4.0×10−7Acm−2 respectively. These all-electrodeposited solar cell device structures are currently being studied and developed as an alternative to the well-known p–n junction structures which utilise chemical bath-deposited CdS. The preliminary material growth, device fabrication and assessment results are presented in this paper.

Facile synthesis and the photo-catalytic behavior of core–shell nanorods

Sodium niobate nanorods (SNRs) have been synthesized by a facile surfactant free hydrothermal method. To explore their potential for photoelectrochemical water splitting under visible light, core–shell nanorods were fabricated by grafting CdS on sodium niobate nanorods. The TEM analysis shows the formation of sodium niobate nanorods which are in the order of 40 ± 5 nm in width and 1300 ± 100 nm in length. The presence of a thin layer on nanorods, as observed in a TEM image, and XRD and SAD analysis, reveals the grafting of hexagonal CdS on orthorhombic sodium niobate nanorods. This was further confirmed by dual band gap values (Eg: 3.6 for sodium niobate and 2.59 eV for CdS) determined from diffuse reflectance data of the CdS–sodium niobate nanorod sample. The CdS–sodium niobate nanorods show drastic enhancement in the current density (Jan: 7.6 mA cm−2 at 0.2 V vs. SHE) when irradiated with monochromatic UV light (300 nm), many folds higher than that observed for bare sodium niobate nanorods (Jan: 2.5 mA cm−2 at 0.2 V vs. SHE), bulk sodium niobate (Jan: 0.6 mA cm−2 at 0.2 V vs. SHE) and CdS. The conduction band (CB) minima calculations show a downhill offset of the CB edges of CdS–sodium niobate. Such a downhill staggered band gap and smooth lattice matched interface, as shown by HRTEM, seem to facilitate an efficient charge separation followed by a photo-generated e− transfer from the CdS CB to the sodium niobate CB and, therefore, appear responsible for the enhancement of the photocurrent density of CdS–sodium niobate nanorods. This is further corroborated by the time resolved photoluminescence decay measurements which show a longer average decay time (〈τ〉) for CdS–sodium niobate nanorods in the order of 8.06 ns than that for sodium niobate nanorods (6.45 ns). Furthermore, better light harvesting efficiency and incident to photon conversion efficiency (23.91% at 300 nm) observed for CdS–sodium niobate nanorods imply a better photo-generated charge carrier separation than those observed for bare sodium niobate nanorods and bulk sodium niobate. The synthesis of CdS modified sodium niobate nanorods, detailed results on the photoelectrochemical behaviour of CdS modified sodium niobate nanorods and underlying mechanism are presented.

Electrodeposition preparation of Ag nanoparticles loaded TiO2 nanotube arrays with enhanced photocatalytic performance

Small silver (Ag) nanoparticles (NPs) loaded TiO2 nanotube arrays (TNAs) were prepared by a two-step method based on anodization method followed by an electrodeposited process. UV–visible diffuse reflectance spectroscopy (UV–vis) and photoluminescence emission spectroscopy (PL) demonstrated that the loaded Ag NPs significantly enhanced the light absorption of TiO2 nanotube arrays in the visible spectral range and improved the separation of photo-generated charge carriers. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX) were also used for the characterization of surface morphology, elemental composition and microstructure of the pure TNAs and Ag NPs decorated TNAs. The results indicate that Ag NPs are uniformly distributed in the TNAs, the deposition process does not damage its ordered tubular structure and the loaded amounts and size of Ag NPs increase with the electrodeposition time. Moreover, the Ag NPs decorated TNAs largely enhanced the photocatalytic degradation of methyl orange (MO) under UV–visible light irradiation. Finally, A possible photocatalytic reaction mechanism of Ag NPs/TNAs has also been proposed.

TiO2–graphene complex nanopaper for paper-based label-free photoelectrochemical immunoassay

A novel strategy was reported for the preparation of TiO2 nanobelts by the acid-assisted hydrothermal method. Complex nanopaper based on the TiO2 nanobelts and graphene was fabricated via a modified paper-making process. The as-prepared TiO2–graphene structure inherited the excellent electron transport of graphene and facilitated the spatial separation of photo-generated charge carrier, therefore resulting in the enhanced photocurrent, and making it a promising candidate for developing photoelectrochemical (PEC) biosensors. Thus, an advanced PEC biosensing platform for the detection of carcinoembryonic antigen was proposed at relatively low applied potential on screen-printed working paper-electrode using the as-prepared TiO2–graphene complex nanopaper. The complex nanopaper with good biocompatibility showed high PEC intensity and satisfactory stability, all of which held great promise for the fabrication of PEC biosensors with improved sensitivity. Furthermore, the TiO2–graphene complex nanopaper based PEC biosensor in this study was anticipated to provide the chance for the design of high-performance PEC biosensors for the detection of other enzymes and biomolecules.

Soft-chemical method for fabrication of SnO–TiO2 nanocomposites with enhanced photocatalytic activity

Abstract

Fabrication of glutathione photoelectrochemical biosensor using graphene–CdS nanocomposites

A novel glutathione (GSH) photoelectrochemical biosensor was fabricated using the newly synthesized graphene-CdS (GR-CdS) nanocomposites. The GR-CdS nanocomposites were prepared by a fast, one-step, aqueous reaction. The as-prepared GR-CdS structure inherited the excellent electron transport of GR and facilitated the spatial separation of photo-generated charge carrier, therefore resulting in the enhanced photocurrent, and making it a promising candidate for developing photoelectrochemical biosensors. The proposed GSH sensor displays satisfactory analytical performance with an acceptable linear range from 0.01 to 1.5 mmol L(-1) with a detection limit of 0.003 mmol L(-1) at a signal-to-noise ratio of 3, and also shows an excellent specificity against anticancer drugs and can be successfully applied for GSH detection in real samples. The as-synthesized GR-CdS nanocomposites exhibited obviously enhanced photovoltaic properties, which could be extended to the detection of other enzymes and biomolecules, thus providing a promising platform for the development of photoelectrochemical biosensors.

Effects of Calcination Temperatures on Photocatalytic Activity of Ordered Titanate Nanoribbon/SnO2Films Fabricated during an EPD Process

Ordered titanate nanoribbon (TNR)/SnO2films were fabricated by electrophoretic deposition (EPD) process using hydrothermally prepared titanate nanoribbon as a precursor. The formation mechanism of ordered TNR film on the fluorine-doped SnO2coated (FTO) glass was investigated by scanning electron microscopy (SEM). The effects of calcination temperatures on the phase structure and photocatalytic activity of ordered TNR/SnO2films were investigated and discussed. The X-ray diffraction (XRD) results indicate that the phase transformation of titanate to anatase occurs at 400°C and with increasing calcination temperature, the crystallization of anatase increases. At 600°C, the nanoribbon morphology still hold and the TiO2/SnO2film exhibits the highest photocatalytic activity due to the good crystallization, unique morphology, and efficient photogenerated charge carriers separation and transfer at the interface of TiO2and SnO2.

Photocatalytic degradation of malathion in aqueous solution using an Au–Pd–TiO2 nanotube film

The extensive use of pesticides has promoted the agricultural production, but a series of subsequent environmental issues have drawn the concern of governments and people worldwide, such as groundwater and surface water pollutions. In order to remove these pollutants, photocatalysis has emerged as a powerful method. In this paper, the photocatalytic degradation of an organophosphorus pesticide malathion was investigated using an Au–Pd co-modified TiO2 nanotube film (Au–Pd–TiO2). This film was fabricated by simultaneously photo-depositing Au and Pd precursors on a self-organized TiO2 nanotube film. Its morphology and structures were well characterized by a scanning electron microscope (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The photocatalytic experiments revealed that the malathion elimination rate increased by 172% when the photocatalyst of the naked TiO2 nanotube film was replaced by Au–Pd–TiO2. Additionally, the amount of H2O2 yielded on the Au–Pd–TiO2 film in 60min was 2.89 times that on the naked TiO2. The enhanced photocatalytic performance could be attributed to both the effective separation of photo-generated charge carriers and the higher synthesis rate of H2O2. The possible photocatalytic mechanism was discussed.

Electric field dependent photogenerated charge carrier separation in bipyridine containing poly(p-phenylene vinylene)

In this paper we review the results of the steady state and transient photoconductivity characteristics of bipyridine-containing poly(p-phenylene-vinylene) derivatives having n=0, 1 and 3 phenylene vinylene subunits separating the bipyridylene vinylene skeleton. It is observed that the photo-induced charge carrier generation and transport properties in this system vary significantly with n, with the results revealing the facile electric-field induced separation of the electron-hole, (e-h) pairs in poly(p-phenylene-vinylene-bipyridylene), n=1, sample relative to n = 0 and n = 3 samples.