High Photoelectrochemical Activity Research Articles

Photocurrent Generation by Immobilized Nostoc Sp. Via Direct Electron Transport

Photosynthesis based energyconversion is an effective way of utilizing the most abundant solar energy for the benefit of mankind. Photosynthetic energy conversion processes have enormous potential towards energy and environmental sustainability. Among the various photosynthetic microorganisms, cyanobacteria possess exciting features for energy conversion. This study focuses on production of direct electricity from a cyanobacterium called Nostoc sp. (NOS). By illuminating with white light (of intensity 76 mW cm-2), the NOS immobilized on multi-walled carbon nanotubes (MWNT) modified electrodes generated a photocurrent density of 30 mA m-2 at an applied potential of 0.2 V (vs. Ag/AgCl). The contribution of different photosynthetic pigments of NOS to the photocurrent generation was analyzed by measuring the current density produced by MWNT/NOS upon illuminating with monochromatic lights of different wavelengths. Phycocyanin was found to be the major pigment greatly contributing to the photocurrent. Further investigation using a set of inhibitors1 revealed that the plastoquinones were primarily responsible for the electron transfer between the cyanobacteria and the electrode. Finally a rudimentary design photosynthetic electrochemical cell2 has been constructed with MWNT/NOS on the anode and MWNT/laccase on the cathode. The cell generated a maximum current density of 250 mA m-2 and a peak power density of 35 mW m-2 without any mediator. With the addition of benzoquinone as a redox mediator, the electricity generation capability was significantly enhanced to a current density of ~2300 mA m-2 and power density of 100 mW m-2, which is the highest among such systems reported thus far in literature. References Torimura M, Miki A, Wadano A, Kano K and Ikeda T (2001) Electrochemical investigation of cyanobacteria Synechococcus sp. PCC7942-catalyzed photoreduction of exogenous quinones and photoelectrochemical oxidation of water. Journal of Electro-analytical Chemistry, 496, 21–28.Calkins J, Umasankar Y, O'Neill H and Ramasamy R (2013) High photo-electrochemical activity of thylakoid–carbon nanotube composites for photosynthetic energy conversion. Energy Environ. Sci., 2013, 6, 1891-1900.

Nanoscale structural disorder in manganese oxide particles embedded in Nafion

High degree of structural disorders in birnessite phase particles in Nafion are responsible for high photo-electrochemical water-oxidation activity.

Realizing high visible-light-induced carriers mobility in TiO2-based photoanodes

Increasing the quantum conversion efficiency in visible-light region is a key issue for the efficient usage of solar energy in the process of photoelectrochemical (PEC) water splitting. Here, based on the N anion and S cation codoped TiO2 thin film photoanodes, we find experimentally and theoretically that the transport ability of visible-light-induced carriers can be notably enhanced by engineering the dopant electronic energy states. The hole transport resistivity is decreased from 6.8 × 104 to 1.3 × 104 Ω m via delocalizing the introduced bands, leading to an evident increase of the internal quantum efficiency from 5% to 20% at the visible-light region of 450–550 nm. Further DFT calculations exhibit that the split of the isolated band is induced by a strong N and S hybridization, which favors to decrease the hole effective mass and to improve the hole mobility by three times. The photocurrent of N − S codoped TiO2 thin film is thus twice that of monodoping cases, the best value for doping modified TiO2 thin film photoanodes. This work may provide not only a design principle but also a candidate for tailoring and optimizing TiO2 towards high PEC activity.

Liquid phase deposition of ZnO film for photoelectrocatalytic degradation of p-nitrophenol

Liquid phase deposition (LPD) technique was developed to prepare zinc oxide (ZnO) thin film with high photoelectrochemical activity. The field emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD) characterization showed that the deposited film was composed of many stick microcrystalline structured ZnO. The photoelectrochemical property of LPD film was analyzed by chronoamperometry and electrochemical impedance spectroscopy (EIS) under UV-light excitation. Using the LPD ZnO film as photoanode, the removal of p-nitrophenol (PNP) was studied by photoelectrocatalytic (PEC) oxidation technique. The influencing factors such as initial concentration of PNP, pH and bias potential were systematically investigated. Under optimized conditions, PNP was effectively degraded by photoelectrocatalysis on the LPD ZnO film.

Identifying champion nanostructures for solar water-splitting

Charge transport in nanoparticle-based materials underlies many emerging energy-conversion technologies, yet assessing the impact of nanometre-scale structure on charge transport across micrometre-scale distances remains a challenge. Here we develop an approach for correlating the spatial distribution of crystalline and current-carrying domains in entire nanoparticle aggregates. We apply this approach to nanoparticle-based α-Fe₂O₃ electrodes that are of interest in solar-to-hydrogen energy conversion. In correlating structure and charge transport with nanometre resolution across micrometre-scale distances, we have identified the existence of champion nanoparticle aggregates that are most responsible for the high photoelectrochemical activity of the present electrodes. Indeed, when electrodes are fabricated with a high proportion of these champion nanostructures, the electrodes achieve the highest photocurrent of any metal oxide photoanode for photoelectrochemical water-splitting under 100 mW cm(-2) air mass 1.5 global sunlight.

MWCNT/WO3 nanocomposite photoanode for visible light induced water splitting

The Multi-walled carbon nanotube (MWCNT)/WO{sub 3} nanocomposite thin films with different MWCNT’s weight percentages were prepared by sol–gel method as visible light induced photoanode in water splitting reaction. Weight percentage of MWCNT in the all nanocomposite thin films was confirmed by TGA/DSC analysis. According to XPS analysis, oxygenated groups at the surface of the MWCNT and stoichiometric formation of WO{sub 3} thin films were determined, while the crystalline structure of the nanocomposite samples was studied by XRD indicating (0 0 2) peak of MWCNT in the monoclinic phase of WO{sub 3}. The influence of different weight percentage (wt%) of MWCNT on WO{sub 3} photoactivity showed that the electron conductivity, charge transfer and electron life time had improved as compared with the pure WO{sub 3}. Based on linear sweep voltammetry and chronoamperometry measurements, the (1 wt%) MWCNT/WO{sub 3} nanocomposite thin films photoanode has a maximum photocurrent density of ∼4.5 A/m{sup 2} and electron life time of about 57 s. - Graphical abstract: Photocurrent density versus time at constant potential (0.7 V) for the WO{sub 3} films containing different MWCNT weight percentages annealed at 400 °C under 1000 Wm{sup −2} visible photo-illumination. Display Omitted - Highlights: • MWCNT/ WO{sub 3} nanocomposite thinmore » films were synthesized using sol–gel derived method. • TGA/DSC confirmed the weight percentage of MWCNT in the all nanocomposite thin films. • XPS analysis revealed that WO{sub 3} was attached on the oxygenated group of MWCNT surface. • The Highest Photoelectrochemical activity is achieved for (1 wt%)MWCNT/WO{sub 3} thin film.« less

MoS2/CdS Heterojunction with High Photoelectrochemical Activity for H2 Evolution under Visible Light: The Role of MoS2

MoS2/CdS p-n heterojunction films with high photoelectrochemical activity for H2 evolution under visible light were successfully prepared by electrodeposition followed by chemical bath deposition. The films were characterized by X-ray diffraction, scanning electron microscopy, UV–visible absorption, X-ray photoemission spectroscopy, photoluminescence, and photoelectrochemical response. The MoS2/CdS heterostructure shows much higher visible-light photoelectrocatalytic activity and higher stability toward the water splitting than pure CdS film. The MoS2/CdS film with an optimal ratio of 0.14% exhibits the highest photocurrent of 28 mA/cm2 and the highest IPCE of ca. 28% at 420 nm at 0 V vs Ag/AgCl. The critical role of MoS2 in the MoS2/CdS film was investigated. The improved photoelectrochemical performance of the MoS2/CdS heterojunction film was attributed to the visible light absorption enhanced by MoS2 and the formation of a p-n junction between CdS and MoS2, which accelerates the effective separation of...

Nitrogen doped TiO2 nanotube arrays with high photoelectrochemical activity for photocatalytic applications

Nitrogen doped TiO2 nanotube arrays (N-TNAs) were prepared by immersing TNAs in 1M NH3·H2O solution and then annealing in different temperatures. The morphology, structure and composition of the N-TNAs were characterized by field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and UV–vis spectroscopy, respectively. Effects of annealing temperatures on structure, photocatalytic properties, and the crystal structure transformation process of the N-TNAs were discussed. Photocatalytic properties of the N-TNAs were evaluated in term of the degradation of methyl orange (MO) under UV light and visible light, and the photocurrent of N-TNAs were tested by electrochemical workstation. The XPS results showed that the N-TNAs were achieved by interstitial doping and substitutional doping, and the FESEM results showed the morphology was not changed after doping process. Compared with the pure TNAs, the N-TNAs annealed at 500̊C for 2h with a mixed phase of anatase and rutile exhibited higher photocatalytic degradation activity to MO. Furthermore, the photocatalytic mechanism of organic pollutants degradation (MO) was discussed based on our experiments.

High photo-electrochemical activity of thylakoid–carbon nanotube composites for photosynthetic energy conversion

Spinach thylakoids were immobilized onto multiwalled carbon nanotubes using a molecular tethering chemistry. The resulting thylakoid–carbon nanotube composites showed high photo-electrochemical activity under illumination. Multiple membrane proteins have been observed to participate in direct electron transfer with the electrode, resulting in the generation of photocurrents, the first of its kind reported for natural photosynthetic systems. Upon inclusion of a mediator, the photo-activity was enhanced. The major contributor to the photocurrent was the light-induced water oxidation reaction at the photosystem II complex. The thylakoid–MWNT composite electrode yielded a maximum current density of 68 μA cm−2 and a steady state current density of 38 μA cm−2, which are two orders of magnitude larger than previously reported for similar systems. The high electrochemical activity of the thylakoid–MWNT composites has significant implications for both photosynthetic energy conversion and photofuel production applications. A fuel cell type photosynthetic electrochemical cell developed using a thylakoid–MWNT composite anode and laccase cathode produced a maximum power density of 5.3 μW cm−2, comparable to that of enzymatic fuel cells. The carbon based nanostructured electrode has the potential to serve as an excellent immobilization support for photosynthetic electrochemistry based on the molecular tethering approach as demonstrated in this work.

Cobalt-bilayer catalyst decorated Ta3N5 nanorod arrays as integrated electrodes for photoelectrochemical water oxidation

Ta3N5 nanorod arrays were fabricated by nitridation of fluorine-containing tantalum oxide (F–Ta2O5) nanorod arrays grown in situ on Ta substrates by a one-pot vapour-phase hydrothermal induced self-assembly technique. In this protocol, the in situ generation and the morphology of arrays elaborately adjusted by reaction time, play a vital role in the formation of the F–Ta2O5 nanorod arrays and a highly conductive interlayer between the nanorods and the substrate. Due to the shape anisotropy, ordered hierarchical structure and high surface area, a high photoelectrochemical activity was achieved by the optimum Ta3N5 nanorod photoelectrode with a photocurrent density of 1.22 mA cm−2 under AM 1.5G irradiation at 1.23 V vs. RHE (reversible hydrogen electrode). Furthermore, a higher and more stable photocurrent was demonstrated by combining the highly active Ta3N5 nanorods with stable Co3O4/Co(OH)2 (Co3O4/Co(II)) bilayer catalysts when compared with that demonstrated for Co(II)/Ta3N5 and Co3O4/Ta3N5 photoelectrodes, exhibiting that not only is the onset potential negatively shifted but also the photocurrent and the stability are significantly improved, which is correlated to an order of magnitude reduction in the resistance to charge transfer at the Ta3N5/H2O interface. Specifically, about 92% of the initial stable photocurrent remains after long-term irradiation at 1.23 V vs. RHE. At 1.23 V vs. RHE, the photocurrent density of Co3O4/Co(II)/Ta3N5 arrays reached 3.64 mA cm−2 under AM 1.5G simulated sunlight at 100 mW cm−2, and a maximum IPCE of 39.5% was achieved at 440 nm. This combination of catalytic activity, stability, and conformal decoration makes this a promising approach to improve the photoelectrochemical performance of photoanodes in the general field of energy conversion.

A Three‐Dimensional Branched Cobalt‐Doped α‐Fe2O3 Nanorod/MgFe2O4 Heterojunction Array as a Flexible Photoanode for Efficient Photoelectrochemical Water Oxidation

Hitting water with nanotrees: A three-dimensional branched heterojunction array for water oxidation based on cobalt-doped α-Fe2O3 nanorod cores and MgFe2O4 shells has been synthesized. Such an architecture not only has a large surface area and excellent light absorption properties, and shows fast charge transport, but also exhibits a very high photoelectrochemical activity for water oxidation and a high stability.

Magnetic loading of TiO 2/SiO 2/Fe 3O 4 nanoparticles on electrode surface for photoelectrocatalytic degradation of diclofenac

A novel magnetic nanomaterials-loaded electrode developed for photoelectrocatalytic (PEC) treatment of pollutants was described. Prior to electrode fabrication, magnetic TiO 2/SiO 2/Fe 3O 4 (TSF) nanoparticles were synthesized and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and FT-IR measurements. The nanoparticles were dispersed in ethanol and then immobilized on a graphite electrode surface with aid of magnet to obtain a TSF-loaded electrode with high photoelectrochemical activity. The performance of the TSF-loaded electrode was tested by comparing the PEC degradation of methylene blue in the presence and absence of magnet. The magnetically attached TSF electrode showed higher PEC degradation efficiency with desirable stability. Such a TSF-loaded electrode was applied to PEC degradation of diclofenac. After 45 min PEC treatment, 95.3% of diclofenac was degraded on the magnetically attached TSF electrode.

Controlled Synthesis of Vertically Aligned Hematite on Conducting Substrate for Photoelectrochemical Cells: Nanorods versus Nanotubes

This paper describes two different processes to synthesize vertically aligned hematite nanorod and nanotube arrays, respectively, on a conductive substrate by the electrochemical deposition method with the help of an anodized aluminum oxide nanotemplate. The two types of nanostructured hematite were used as the photoanode for photoelectrochemical cells. The hematite nanotubes exhibited much higher photoelectrochemical activity than the hematite nanorods, including an improved photocurrent density, more negative onset potential, better photon harvesting, and better charge carrier transfer ability. The observed behavior may offer new information to enhance the photocatalytic ability of hematite, which is considered to be one of the best photoanode materials in the research field of photoelectrochemical cells.

Anodically Fabricated Sr-doped TiO2 Nanotube Arrays for Photoelectrochemical Water Splitting Applications

ABSTRACTThe present work reports the synthesis of self-organized strontium-doped titania nanotubes arrays as a potential material for photocatalytic water splitting. Electrochemical anodization process was used to grow such material under various electrochemical conditions. The effect of dopant concentration on the morphology and photoelectrochemical properties of the material was investigated. The microstructure, morphology and composition of as-prepared and heat treated nanotubes were characterized by field emission scanning electron microscopy (FESEM), x-ray diffraction (XRD), transmission electron microscopy (TEM) and x-ray photoelectron spectroscopy (XPS). The results showed that increasing the dopant concentration up to its solubility limit results in higher photoelectrochemical activity. A preliminary proof of concept of the photocatalytic activity of the fabricated material was estimated in terms of the use of such material as a photoanode for photoelectrochemical water splitting.

Nanocrystalline WO 3 film with high photo-electrochemical activity prepared by polymer-assisted direct deposition

Highly photoactive, nanocrystalline WO3 films were fabricated by an economic solution process based on polymer-assisted direct deposition. The optimized film exhibited a photocurrent of 2.3 mA cm−2 (at 1.23 V vs. reversible hydrogen electrode) under simulated solar illumination (air mass 1.5 global) and a maximum incident photon to current efficiency of 71% at an incident wavelength of 340 nm. The observed performance represents one of the highest values reported for WO3. High crystallinity, porosity and optimum thickness of nanocrystalline films were identified as the critical variables that induce the high photoactivity.

Visible photoenhanced current–voltage characteristics of Au : TiO2 nanocomposite thin films as photoanodes

In this investigation, the effect of annealing temperature and concentration of gold nanoparticles on the photoelectrochemical properties of sol–gel deposited Au : TiO2 nanocomposite thin films is studied. Various gold concentrations have been added to the TiO2 thin films and their properties are compared. All the deposited samples are annealed at different temperatures. The optical density spectra of the films show the formation of gold nanoparticles in the films. The optical bandgap energy of the Au : TiO2 films decreases with increasing Au concentration. The crystalline structure of the nanocomposite films is studied by x-ray diffractometry indicating the formation of gold nanocrystals in the anatase TiO2 nanocrystalline thin films. X-ray photoelectron spectroscopy reveals that the presence of gold in the metallic state and the formation of TiO2 are stoichiometric. The photoelectrochemical properties of the Au : TiO2 samples are characterized using a compartment cell containing H2SO4 and KOH as cathodic and anodic electrolytes, respectively. It is found that the addition of Au nanoparticles in TiO2 films enhances the photoresponse of the layer and the addition of gold nanocrystals with an optimum value of 5 mol% resulted in the highest photoelectrochemical activity. Moreover, the photoresponse of the samples is also enhanced with an increase in the annealing temperature.

Photoelectrochemical activity of liquid phase deposited TiO 2 film for degradation of benzotriazole

TiO 2 film deposited on glassy carbon electrode surface was prepared via the liquid phase deposition (LPD). The deposited TiO 2 film before and after calcination was characterized with scanning electron microscopy (SEM) and X-ray diffraction (XRD). Based on the high photoelectrochemical activity of calcined LPD TiO 2 film, the photoelectrocatalytic degradation of benzotriazole (BTA) was investigated. Compared with the electrochemical oxidation process, direct photolysis or photocatalysis for treatment of BTA, a synergetic photoelectrocatalytic degradation effect was observed using the LPD TiO 2 film-coated electrode. Various factors influencing the photoelectrocatalytic degradation of BTA such as film calcination, applied bias potential, pH value, supporting electrolyte concentration and initial concentration of BTA were investigated. The COD removal for BTA solution was analyzed to evaluate the mineralization of the PEC process. Based on the degradation experimental results, a possible photoelectrocatalytic degradation mechanism for BTA was proposed.

Enhanced photoelectrochemical properties of F-containing TiO 2 sphere thin film induced by its novel hierarchical structure

The novel nanostructured F-containing TiO 2 (F-TiO 2) sphere was directly synthesized on the surface of Ti foil in the solution of NH 4F and HCl by one-step hydrothermal approach under low-temperature condition. The samples were characterized respectively by means of field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The results showed that the F-TiO 2 sphere was hierarchical structure, which composed of porous octahedron crystals with one truncated cone, leading to a football-like morphology. XPS results indicated that F − anions were just physically adsorbed on the surface of TiO 2 microspheres. The studies on the optical properties of the F-TiO 2 were carried out by UV–vis light absorption spectrum. The surface fluorination of the spheres, the unique nanostructure induced accessible macropores or mesopores, and the increased light-harvesting abilities were crucial for the high photoelectrochemical activity of the synthesized F-TiO 2 sphere for water-splitting. The photocurrent density of the F-TiO 2 sphere thin film was more than two times than that of the P25 thin film. Meanwhile, a formation mechanism was briefly proposed. This approach could provide a facile method to synthesize F-TiO 2 microsphere with a special morphology and hierarchical structure in large scale.

Synthesis of F-Doped Flower-like TiO2 Nanostructures with High Photoelectrochemical Activity

We report on a novel and facile approach for the direct growth of F-doped flower-like TiO(2) nanostructures on the surface of Ti in HF solutions under low-temperature hydrothermal conditions. The influence of the experimental parameters such as temperature, reaction duration, and the HF concentration on the morphology and photoelectrocatalytic activity of the formed F-doped flower-like TiO(2) nanostructures was systematically studied. The presence of HF and the reaction time play an important role in the formation of the F-doped flower-like TiO(2) nanostructures. The synthesized novel F-doped TiO(2) flower-like nanomaterials possess good crystallinity and exhibit high photoelectrochemical activity for water-splitting and photodegradation of organic pollutants compared with P-25, which is currently considered to be one of the best commercial TiO(2) photocatalysts. The approach described in this study provides a simple and novel method to synthesize F-doped TiO(2) nanostructured materials that are ready for practical applications such as the photodegradation of wastewater.

Enhancement of Photoelectrochemical Hydrogen Production from Hematite Thin Films by the Introduction of Ti and Si

Doped and undoped hematite (α-Fe2O3) thin film electrodes for photoelectrochemical hydrogen production have been prepared using reactive magnetron sputtering. We present an extensive characterization of the photoelectrochemical, structural, electrical, and optical properties of the α-Fe2O3 films to assess the mechanisms by which the dopants influence photoelectrode performance. The Si-doped and Ti-doped α-Fe2O3 exhibited much higher photoelectrochemical activity than the undoped material. The Ti-doped α-Fe2O3 showed the largest photocurrent as well as the lowest onset potential, the highest electrical conductivity, the lowest activation energy, and the highest charge carrier density. The crystallographic orientation of the films does not appear to be a dominant factor affecting the photocurrent. The changes in conductivity with doping are insufficient to explain the observed changes in the photoelectrochemical activity. The proposed mechanism for the enhanced photocurrent is reduction of recombination due to an improvement of the charge-transfer rate coefficient at the surface and also possibly passivation of the grain boundaries by the dopants. A highly disordered surface and higher grain boundary recombination due to small crystals may explain the lower photocurrent of the Si-doped α-Fe2O3 compared to the Ti-doped material.