Enhanced Photocurrent Response Research Articles

Biomolecule-Assisted, Environmentally Friendly, One-Pot Synthesis of CuS/Reduced Graphene Oxide Nanocomposites with Enhanced Photocatalytic Performance

In this work, we develop a novel environmentally friendly strategy toward one-pot synthesis of CuS nanoparticle-decorated reduced graphene oxide (CuS/rGO) nanocomposites with the use of L-cysteine, an amino acid, as a reducing agent, sulfur donor, and linker to anchor CuS nanoparticles onto the surface of rGO sheets. Upon visible light illumination (λ > 400 nm), the CuS/rGO nanocomposites show pronounced enhanced photocurrent response and improved photocatalytic activity in the degradation of methylene blue (MB) compared to pure CuS. This could be attributed to the efficient charge transport of rGO sheets and hence reduced recombination rate of excited carriers.

Strongly enhanced photocurrent response for Na doped Ta3N5-nano porous structure

Highly ordered nanoporous (nanochannels) Ta2O5 layers were formed by electrochemical anodization of Ta in a glycerol–ethylene glycol mixture containing NH4F and (NH4)2SO4. The layers were then immersed in Na2SO4 and annealed in NH3. This treatment leads to Na-doped Ta3N5 nanostructures that exhibit an apparent band gap of ~1.6eV (compared to 2.1eV for pure Ta3N5). The doped structure, moreover, shows an almost 10 times higher photoelectrochemical current response in the visible range than pure Ta3N5. This finding represents thus a novel pathway to drastically increase the photoresponse of Ta3N5 which can find wide ranging applications in photoelectrochemical devices.

Synthesis and Photoelectrochemical Properties of Fe2O3/ZnFe2O4 Composite Photoanodes for Use in Solar Water Oxidation

α-Fe2O3/ZnFe2O4 composite electrodes are prepared via a simple surface treatment performed on nanoparticulate α-Fe2O3 electrodes. The α-Fe2O3 electrodes are uniformly covered with a solution containing Zn2+ ions which react with Fe2O3 upon heating to form ZnFe2O4. As Zn2+ ions do not completely diffuse into the core of Fe2O3 particles under mild heating conditions, the ZnFe2O4 forms only on the surface of Fe2O3 as a shell layer, resulting in the formation of Fe2O3/ZnFe2O4 composite electrodes. Any unreacted ZnO is removed by dissolution in 1 M NaOH, where ZnFe2O4 and Fe2O3 are stable. X-ray diffraction and energy-dispersive spectroscopy studies show that a crystalline ZnFe2O4 phase forms after the heat treatment and the composite electrode with the best photoelectrochemical performances contains a 1:1 mol ratio of ZnFe2O4 and Fe2O3. The Fe2O3/ZnFe2O4 composite electrode shows a significantly enhanced photocurrent response compared to the bare Fe2O3 electrode because ZnFe2O4 has conduction and valence band...

Observation of Triplet Exciton Formation in a Platinum-Sensitized Organic Photovoltaic Device.

Organic photovoltaics (OPVs) constitute a promising new technology due to their low production costs. However, OPV efficiencies remain low because excitons typically diffuse only ∼5-20 nm during their lifetime, limiting the effective thickness of the light-absorbing layer. One strategy to improve OPVs is to increase exciton lifetimes by converting them into triplet states, which typically persist 10(3)-10(5) times longer than singlet excitons. We present femtosecond transient absorption and steady-state photovoltaic measurements of a model OPV system consisting of diphenyltetracene (DPT) films doped with platinum tetraphenylbenzoporphyrin (Pt(TPBP)). Photoexcitation of Pt(TPBP) creates a singlet excitation that rapidly intersystem crosses to a triplet state before transferring to the DPT host matrix. This transfer is rapid and efficient, occurring in 35 ps with an 85% conversion ratio of porphyrin singlets to DPT triplets. These triplet excitons lead to enhanced photocurrent response that increases with device thickness.

Highly stable CdS-modified short TiO 2 nanotube array electrode for efficient visible-light hydrogen generation

A highly stable photoelectrocatalytic electrode made of CdS-modified short, robust, and highly-ordered TiO 2 nanotube array for efficient visible-light hydrogen generation was prepared via sonoelectrochemical anodization and sonoelectrochemical deposition method. The short nanotube electrode possesses excellent charge separation and transfer properties, while the sonoelectrochemical deposition method improves the combination between CdS and TiO 2 nanotubes, as well as the dispersion of CdS nanoparticles. Different characterization techniques were used to study the nanocomposite electrode. UV–vis absorption and photoelectrochemical measurements proved that the CdS coating extends the visible spectrum absorption and the solar spectrum-induced photocurrent response. Comparing the photoactivity of the CdS/TiO 2 electrode obtained using sonoelectrochemical deposition method with others that synthesized using plain electrochemical deposition, the current density of the former electrode is ∼1.2 times higher that of the latter when biased at 0.5 V. A ∼7-fold enhancement in photocurrent response is obtained using the sonoelectrochemically fabricated CdS/TiO 2 electrode in comparison with the pure TiO 2 nanotube electrode. Under AM1.5 illumination the composite photoelectrode generate hydrogen at a rate of 30.3 μmol h −1 cm −2, nearly 13 times higher than that of pure titania nanotube electrode. Recycle experiments demonstrated the excellent stability and reliability of CdS/TiO 2 electrode prepared by sonoelectrochemical deposition. This composite electrode, with its strong mechanical stability and excellent combination of CdS and TiO 2 nanotubes, offers promising applications in visible-light-driven renewable energy generation.

Liquid phase deposition of mesoporous TiO 2/DNA hybrid film: Characterization and photoelectrochemical investigation

A photoelectroactive TiO 2/DNA hybrid film was synthesized via the liquid phase deposition (LPD) process. Scanning electron microscopic (SEM) characterization showed that the compact TiO 2 film was changed to a mesoporous structure when DNA was present in the deposition solution, which might be the result of TiO 2 particles growing along the backbones of the double-helical structure of DNA molecules. Although UV absorption spectra and cyclic voltammograms indicated that the deposited TiO 2 on the substrate surface was decreased in the presence of DNA, an enhanced photocurrent response was observed. The electrochemical impedance and cyclic voltammetric measurements using K 3[Fe(CN) 6] as a redox probe suggested that the mesoporous film provided a relatively more efficient electron transfer interface, which could improve the photoelectron transfer rate from the semiconducting film to the electrode and reduce the recombination of photoelectrons and holes. This results in an enhanced photocurrent. Even after long-term and continuous UV irradiation, the mesoporous film exhibited a promoted photoelectrochemical response. The promoted photoelectrocatalytic degradation of methylene blue was obtained on the TiO 2/DNA composite film, which is consistent with the enhanced photocurrent, and this demonstrates that DNA behaved as a useful biomaterial for the synthesis of a photoelectroactive hybrid film with improved performance.

Self-organized CdS microstructures by anodization of Cd in chloride containing Na 2S solution

In the present work, we report on the formation of CdS films formed by anodization of a Cd-substrate in an aqueous electrolyte containing Na 2S and NaCl. The presence of NaCl in the electrolyte strongly affects the morphology of the CdS film. Under optimized concentrations of NaCl, self-organized arrangements of CdS microstructures are formed. These structures are characterized by SEM, XRD and XPS. After heat treatment at 150 °C, the self-organized CdS structures show enhanced photocurrent response compared with a non-structured CdS film. Therefore, the self-ordered microstructures of CdS reported in this work bear potential for improved photo-electrochemical applications of CdS.

Effects of surface modification of the individual ZnO nanowire with oxygen plasma treatment

This study examined the effects of an oxygen plasma treatment on the properties of ZnO nanowires with diameters of 80 nm using a single nanowire field effect transistor. After the oxygen plasma treatment, the carrier concentration and mobility of individual ZnO nanowires decreased with a substantial positive shift in the threshold voltage. The shifting was accounted to the surface modification, resulted to the improved gas sensitivity under hydrogen gas exposure and an enhanced photocurrent response time in ultraviolet illumination. The plausible surface mechanisms responsible for these significant changes after the surface modification were suggested by considering the surface analysis and electrical transport mechanism.

Preparation of short, robust and highly ordered TiO2 nanotube arrays and their applications as electrode

The short, robust and highly ordered TiO2 nanotube arrays (TNAs) electrode was prepared by sonoelectrochemical anodization of titanium in HF–H2O electrolyte solution (referred as short TNAs, STNAs). The self-organized arrays of titania nanotubes of approximately 12–65nm in diameter and 75–280nm in length can be synthesized at anodic voltage of 5–20V. The electron transport process within the STNAs electrode was much favorable in comparison with that for the long TNAs electrode synthesized by conventional magnetic agitation technique (referred as long TNAs, LTNAs), as confirmed by the obviously enhanced photocurrent response of STNAs electrode either in 0.02M Na2SO4 electrolyte solution or in different concentrations of glucose solution or under different intensities of UV illumination. To investigate their photoelectrochemical applications, degradation of tetracycline, a typical pharmaceutical and personal care products (PPCPs), was carried out using photoelectrocatalytic (PEC) means, comparing with electrochemical (EC) and photocatalytic (PC) processes. The kinetic constant of the PEC process of STNAs electrode was ∼3.17 times as high as its PC process. The color removal rate of tetracycline by STNAs electrode achieved 81% within 3h, which was 21% higher than that for LTNAs electrode. In degrading tetracycline, 41% of TOC was mineralized using the STNAs electrode against 23% using LTNAs electrode under similar conditions. Such kind of titania nanotubes will have many potential applications in various areas as an outstanding photoelectrochemical material.

Sonoelectrochemical synthesis of highly photoelectrochemically activeTiO2 nanotubes by incorporating CdS nanoparticles

Self-organized anodic TiO2 nanotube arrays (TiO2NTs) are functionalized with CdS nanoparticle based perfusion and depositionthrough a single-step sonoelectrodeposition method. Even controlled at50 °C, CdS nanoparticles with smaller size and more homogeneous distribution are successfullysynthesized in dimethyl sulfoxide (DMSO) under ultrasonic irradiation. Moreover,TiO2 nanotubes can be filled with nanoparticles because of the ultrasonic effect. The CdS incorporatedTiO2NTs (CdS–TiO2NTs)effectively harvest solar light in the UV as well as the visible light (up to 480 nm) region. Compared withpure TiO2NTs, a more than ninefold enhancement in photocurrent response is observed using the CdS–TiO2NTs. Maximum incident photon to charge carrier efficiency (IPCE)values of 99.95% and 9.85% are observed respectively for CdS–TiO2 nanotubesand pure TiO2NTs. The high value of IPCE observed with the CdS–TiO2NTs is attributed to the increased efficiency of charge separation and transport ofelectrons. A schematic diagram is proposed to illustrate the possible process ofCdS formation in nanotubes under sonochemical and electrochemical conditions.

Photoelectrochemical Splitting of Water to H2 and O2 at n-Fe2O3 Nanowires and Nanocrystalline Carbon-Modified (CM)-n-Fe2O3 Thin Films

Iron oxide n-Fe2O3 nanowire thin films were synthesized by thermal oxidation of Fe metal sheet (Alfa Co. 0.25 mm thick) in an electric oven then tested for their photoactivity. The photoresponse of the n-Fe2O3 nanowires was evaluated by measuring the rate of water splitting reaction to hydrogen and oxygen, which was found to be proportional to photocurrent density, jp. The optimized electric oven-made n-Fe2O3 photoelectrodes showed photocurrent densities of 1.32 mA cm-2 at measured potential of 0.0 V/SCE with photoconversion efficiency of 1.69 % at applied potential of 0.70 V vs Eaoc (electrode potential at open circuit conditions) under illumination intensity of 100 mW cm-2 from a Solar simulator with a global AM 1.5 filter. The photoactivity was improved upon incorporation of carbon into the lattice of n-Fe2O3 by flame oxidation at 850{degree sign}C. The carbon modified (CM)-n-Fe2O3 showed enhanced photocurrent response to 3.14 mA cm-2 at a measured potential of 0.0 V/SCE with an efficiency of 2.23% at applied potential of 0.52 V vs Eaoc. The nanocrystalline CM-n- Fe2O3 and n-Fe2O3 nanowires thin films were characterized using photocurrent density measurements under monochromatic light illumination, UV-Vis spectra, X-ray diffraction (XRD) and scanning electron microscopy (SEM).

Natural hybrid organic–inorganic photovoltaic devices

Natural hybrid organic–inorganic photovoltaic devices based on TiO 2 have been realized. Chlorophyll A (from anacystis nidulans algae), chlorophyll B (from spinach), carmic acid (from insect Coccus cacti L.), synthetic trans- β -carotene, natural fresh picked Morus nigra, and their mixtures have been used as an organic photo active layer to fabricate photovoltaic prototypes. In order to reduce the charge’s interfacial recombination, different thicknesses (5–45 nm) of Si layers, subsequently oxidized in air, were inserted between the TiO 2 and chlorophyll B. Scanning electron microscopy of TiO 2 and Si/TiO 2 systems shows the coexistence at least of four classes of nanoparticles of 60, 100, 150 and 250 nm in size. Auger electron spectroscopy of the Si L 2,3V V transition demonstrates the presence of silica and SiO x suboxides. Photocurrent measurements versus radiation wavelength in the range 300–800 nm exhibit different peaks according to the absorption spectra of the organic molecules. All realized photovoltaic devices are suitable for solar light electric energy conversion. Those made of a blend of all organic molecules achieved higher current and voltage output. The Si/TiO 2-based devices containing chlorophyll B exhibited an enhanced photocurrent response with respect to those with TiO 2 only.

Efficient photochemical water splitting and organic pollutant degradation by highly ordered TiO2 nanopore arrays

We synthesized highly ordered TiO2 nanopore arrays (TNPs) with controllable pore size and good uniformity by anodization at low temperature in fluorinated dimethyl sulfoxide (DMSO) solution with a post-sonication treatment. TNPs possess excellent separation and transport properties of photo-generated electron/hole pair and hence reveal enhanced photocurrent response and photochemical properties for water splitting and organic compound degradation. The TNPs present maximum photo-conversion efficiency for water splitting of 0.28% under AM1.5 irradiation. This value compares favorably with a maximum photo-conversion efficiency of 0.21% for TiO2 nanotube arrays (TNAs) under the same conditions. When illuminated with 1.0mWcm−2 UV light, the maximum photo-conversion efficiency can be increased to 22% for TNPs, which is 5% higher than TNAs. The kinetic constant of photoelectrocatalytic (PEC) degradation of methyl orange (MO) for TNPs is found to be 1.27 times as high as that for TNAs when biased at 0.5V. The visible light response of TNPs might be further improved by relevant surface modification technology.

Ion Implantation and Annealing for an Efficient N-Doping of TiO2 Nanotubes

Self-organized anodic titania nanotube layers were doped with nitrogen successfully using ion implantation. Photoelectrochemical measurements combined with XRD measurements show that the damage created by ion bombardment (that leads to a drastic decrease of the photoconversion efficiency) can be “annealed out” by an adequate heat treatment. This results in a N-doped crystalline anatase nanotube structure with strongly enhanced photocurrent response in both the UV and the visible range.

Enhanced photocurrent response in photocells made with platinum-poly-yne/C 60 blends by photoinduced electron transfer

We report photocurrent and photoluminescence measurements on blends of the organometallic polymer Pt-poly-yne and C 60. Due to efficient intersystem crossing, triplet excitons with a binding energy of at least 0.8 eV are created after photoexcitation of Pt-poly-yne. We find that electrons are transferred from Pt-poly-yne onto C 60 by efficient ionization of the triplet exciton. This process gives rise to a photocurrent quantum yield of up to 1–2% in photocells fabricated in a layer structure of indium-tin oxide (ITO)/Pt-poly-yne and 7 wt.% C 60/aluminium. Evidence for the ionization of the strongly bound triplet excitons is provided by studies of luminescence in the blends. There is only partial quenching of the singlet emission band, but complete quenching of the triplet emission band, and the lifetime of the triplet emission at room temperature is reduced from 2 to 0.1 μs upon addition of 7 wt.% C 60.

Resonant-cavity InGaAlAs/InGaAs/InAlAs phototransistors with high gain for 1.3–1.6 μm

We describe the characteristics of resonant-cavity heterojunction bipolar phototransistors (RCHPTs) grown on InP substrates. The active layers are part of a Fabry–Perot cavity in which gold is the top reflector and an InGaAlAs/InAlAs quarter-wave stack is the bottom reflector. The stack and active layer thicknesses are such that the absorption is resonantly enhanced by ∼100% at 1.28 μm in one wafer and 1.52 μm in another. An enhancement in photocurrent response also of ∼100% is obtained at resonance. A dc phototransistor current gain of 5000, corresponding to an optical gain of ∼2500 at resonance, was measured on a three-terminal device in a RCHPT wafer.

Enhancement of photoactivity in pyrite (FeS 2) interfaces by photoelectrochemical processes

A temporary photoelectrochemical oxidation of chloride on a pyrite (FeS 2) electrode surface leads to a considerable enhancement of photocurrent response and a decrease of the dark current. In contrast, passage of a dark current or of a photocurrent in presence of iodide has no significant effect. The surface transformation is accompanied by a significant modification of the photocurrent spectrum of the pyrite material. The improvement of the interface is caused by a photocorrosive transformation of the interface which reduces the concentration of surface recombination centers and leads to a change in the concentration and redistribution of energy levels in the forbidden energy region. XPS studies identify the surface states in the pyrite interface, which are responsible for its poor (photo)electrochemical behaviour and which can be removed during chloride oxidation as FeS groups.