Photoelectrocatalytic Cell Research Articles

Photoelectrocatalytic reduction of CO2 to methanol over a photosystem II-enhanced Cu foam/Si-nanowire system

A solar-light double illumination photoelectrocatalytic cell (SLDIPEC) was fabricated for autonomous CO2 reduction and O2 evolution with the aid of photosystem II (PS-II, an efficient light-driven water-oxidized enzyme from nature) and utilized in a photoanode solution. The proposed SLPEC system was composed of Cu foam as the photoanode and p-Si nanowires (Si-NW) as the photocathode. Under solar irradiation, it exhibited a super-photoelectrocatalytic performance for CO2 conversion to methanol, with a high evolution rate (41.94mmol/hr), owing to fast electron transfer from PS-II to Cu foam. Electrons were subsequently trapped by Si-NW through an external circuit via bias voltage (0.5V), and a suitable conduction band potential of Si (−0.6eV) allowed CO2 to be easily reduced to CH3OH at the photocathode. The constructed Z-scheme between Cu foam and Si-NW can allow the SLDIPEC system to reduce CO2 (8.03mmol/hr) in the absence of bias voltage. This approach makes full use of the energy band mismatch of the photoanode and photocathode to design a highly efficient device for solving environmental issues and producing clean energy.

Enhanced photopromoted electron transfer over a bilayer WO3 n–n heterojunction prepared by RF diode sputtering

A bilayer WO3 photoelectrode was obtained by radio frequency (RF) plasma sputtering in a reactive 40%O2/Ar atmosphere by depositing two successive WO3 coatings on a tungsten foil at two different total gas pressures (3 Pa and 1.7 Pa, respectively), followed by calcination at 600 °C. Two monolayer samples deposited at each of the two pressures and a bilayer sample deposited at inverted pressures were also prepared. Their photoelectrocatalytic (PEC) activity was evaluated by both Incident Photon-to-Current Efficiency (IPCE) measurements and separate evolution of H2 and O2 by water splitting in a two-compartment PEC cell. SEM analysis revealed that the photoanodes have a nanostructured porous double layer surmounting a columnar basement (Staffa-like morphology, after the name of the Scottish island). Mott–Schottky analysis showed that the single layer deposited at 3 Pa has a conduction flat band potential 0.1 V more positive than that deposited at 1.7 Pa. The equivalent n–n heterojunction at the interface of the double-layer creates a built-in electric field that facilitates the photopromoted electron transfer toward the lower lying conduction band material, while the columnar innermost layer introduces percolation paths for efficient electron transport toward the conductive tungsten foil. Both phenomena contribute to decrease the interfacial charge transfer resistance (Rct) and lead up to a ca. 30% increase in the PEC performance compared to the monolayer and the inverted bilayer coatings and to a 93% faradaic efficiency, which is among the highest reported so far for WO3 photoanodes. Upon methanol addition an outstanding 4-fold photocurrent density increase up to 6.3 mA cm−2 was attained over the bilayer WO3 photoanode, much larger than the usually observed current doubling effect.

Photoelectrocatalytic reduction of CO2 to methanol over the multi-functionalized TiO2 photocathodes

The photoelectrocatalytic (PEC) reduction of CO2 to chemical fuels was achieved by utilizing the multi-functionalized TiO2 film photocathodes. On the surfaces of TiO2 films, Pd nanoparticles were introduced to catch protons, molecules of Eosin Y disodium salt were embedded to absorb solar energy, and four different amine ligands were immobilized to capture and activate CO2. In the novel PEC cells, multi-functionalized TiO2 and Co-Pi/W:BiVO4 films were served as photocathode and the counter electrode, respectively. It was found that methanol was the major liquid product. Besides, some ethanol and acetone can be detected as well. The maximum formation rate and the highest selectivity of methanol were 43.6μMcm−2h−1 and approximately 100%, respectively. The Faradaic efficiencies of the PEC cells can exceed 1000% with the aid of photoelectrons. The 13CO2 labeling experiments verify that methanol and carbon monoxide are derived from CO2.

Analysis of the factors controlling performances of Au-modified TiO2 nanotube array based photoanode in photo-electrocatalytic (PECa) cells

The efficiency of photo-electrocatalytic (PECa) devices for the production of solar fuels depends on several limiting factors such as light harvesting, charge recombination and mass transport diffusion. We analyse here how they influence the performances in PECa cells having a photo-anode based on Au-modified TiO2 nanotube (TNT) arrays, with the aim of developing design criteria to optimize the photo-anode and the PECa cell configuration for water photo-electrolysis (splitting) and ethanol photo-reforming processes. The TNT samples were prepared by controlled anodic oxidation of Ti foils and then decorated with gold nanoparticles using different techniques to enhance the visible light response through heterojunction and plasmonic effects. The activity tests were made in a gas-phase reactor, as well as in a PECa cell without applied bias. Results were analysed in terms of photo-generated current, H2 production rate and photoconversion efficiency. Particularly, a solar-to-hydrogen efficiency of 0.83% and a Faradaic efficiency of 91% were obtained without adding sacrificial reagents.

CO2 Synergistic Reduction in a Photoanode-Driven Photoelectrochemical Cell with a Pt-Modified TiO2 Nanotube Photoanode and a Pt Reduced Graphene Oxide Electrocathode

CO2 synergistic reduction in a photoanode-driven photoelectrocatalytic (PEC) cell was conducted with a Pt-modified TiO2 nanotube (Pt-TNT) photoanode and a Pt-modified reduced graphene oxide (Pt-RGO) electrocathode to reduce energy consumption and increase CO2 PEC reduction efficiency. The carbon atom conversion rate of CO2 reduction under PEC conditions was 2.3 times higher than that of the total rate under photocatalytic and electrocatalytic conditions. Synergistic CO2 reduction in the PEC cell was mainly due to the use of the photoanode, which played a dual role during CO2 reduction: (1) anode photovoltage compensated and conferred more negative cathode potential for CO2 reduction and (2) anode water decomposition provided protons and electrons for cathode CO2 reduction. System current density, product generation rate of CO2 reduction, and carbon atom conversion rate increased first and then decreased with increasing deposition amount of Pt on TNT. The optimal photocatalytic activity of the Pt-TNT anode...

Photo-assisted electrocatalysis of CdS[sbnd]MoS2 hybrid for hydrogen evolution reaction: Morphology-dependent photoelectroactivity of p–n junction photocathode under bias potential

The structure design of hydrogen electrodes catalyst is important for the photoelectrocatalytic activity for hydrogen evolution reaction (HER). An oriented nanorod array on transparent conductive substrates would be the most desirable nanostructure in preparing photoelectrocatalytic cells (PECs) because of its efficient charge separation and transport properties as well as superior light harvesting efficiency. We report a bifunctional CdS@MoS2 core–shell nanorod arrays which exhibit both photo- and electrocatalytic activity for HER. The CdS@MoS2 core–shell nanorod arrays are prepared by a simple hydrothermal method and a followed electrodeposition. The characterization results indicate that CdS nanorods are covered by the MoS2 (∼30 nm), and the as prepared heterostructure markedly improves the separation and transfer of charge carriers. The photo- and electrocatalytic activity are characterized by cathodic polarization measurement. The results show that the nanorod array exhibits much better photo- and electrochemical performance than its counterpart of bilayer structure where no photocurrent is detected, suggesting that the photoresponse of photocathode is structure-dependent. Photocurrent derives of the hopping of photogenerated electrons in the vertical direction of CdS@MoS2 nanorods where p–n junction is probably reverse biased the P region, leading to the depletion region enhanced, and thus, suppresses charge recombination. This work proposes a new idea for designing the nanostructure of cathode with enhanced electrocatalytic activity by photo-assistance.

Directing photocatalytic and photoelectrocatalytic performance of TiO2 by using TEA and NH4F as doping precursors

Triethylamine (TEA) and NH4F-modified TiO2 powders and thin films were prepared by combining sol–gel and hydrothermal processes. Modification with TEA results in increased specific surface area and induces energy states below the conduction band of TiO2. On the other hand, the use of NH4F decreases the band-gap, displacing the valence band. Employing radical-scavenging agents, it was found that formation of O 2 * is preferred in TEA-modified TiO2, whereas generation of both O 2 * and OH* results from simultaneous modification. Furthermore, the photocatalytic degradation rate was directly proportional to their specific surface area. However, this trend was reversed in a photoelectrocatalytic cell, due to the fact that the photogenerated electrons are rapidly transported to the rear contact, which restrains their transfer to the dissolved oxygen to generate O 2 * . Therefore, the presence of OH* radicals and direct charge transfer processes appears to play a key role in the photoelectrocatalytic process.

Nanoscale Engineering in the Development of Photoelectrocatalytic Cells for Producing Solar Fuels

Engineering at the nanoscale level is a key aspect for the design of novel devices for sustainable energy to address the changeover from fossil fuels to renewable energy sources. This perspective paper, after introducing this topic, analyses the design and development of photoelectrocatalytic cells for producing solar fuels. To overcome limitations in the design of photoelectrocatalytic cells, a different one is proposed which eliminates the need of having a liquid electrolyte, where the electrodes are immersed. This cell design requires specific characteristics in the related electrodes/materials, which in combination with the different operation conditions, determine the need to investigate new fundamental aspects in the area. Some of the aspects analyzed regard (i) the role of nanostructure for visible light absorption of the semiconductor used, (ii) the need to use catalytic concepts (photoelectro-catalysis rather than photoelectro-chemistry), (iii) the mobility of charge carriers and relation with electrode characteristics, and (iv) space charge and Helmholtz layer.

Turning Perspective in Photoelectrocatalytic Cells for Solar Fuels.

The development of new devices for the use and storage of solar energy is a key step to enable a new sustainable energy scenario. The route for direct solar-to-chemical energy transformation, especially to produce liquid fuels, represents a necessary element to realize transition from the actual energy infrastructure. Photoelectrocatalytic (PECa) devices for the production of solar fuels are a key element to enable this sustainable scenario. The development of PECa devices and related materials is of increasing scientific and applied interest. This concept paper introduces the need to turn the viewpoint of research in terms of PECa cell design and related materials with respect to mainstream activities in the field of artificial photosynthesis and leaves. As an example of a new possible direction, the concept of electrolyte-less cell design for PECa cells to produce solar fuels by reduction of CO2 is presented. The fundamental and applied development of new materials and electrodes for these cells should proceed fully integrated with PECa cell design and systematic analysis. A new possible approach to develop semiconductors with improved performances by using visible light is also shortly presented.

Electrolyte-less design of PEC cells for solar fuels: Prospects and open issues in the development of cells and related catalytic electrodes

The possibilities and prospects offered from a new electrolyte-less (EL) cell design for photoelectrocatalytic (PEC) cells to produce solar fuels, particularly by reduction of CO2, are discussed in this perspective contribution. The concept of EL configuration is presented evidencing the several potential advantages, but also the need to develop novel specific nanostructured electrodes. Some aspects of the preparation of photoanode and electrocatalysts for EL-PEC solar cells are discussed, with focus on thin films based on arrays of vertically aligned TiO2 nanotubes and nanocarbon-based gas diffusion catalytic electrodes for CO2 reduction, respectively. New possibilities for design of cells, particularly suitable for medium-high temperature of operations (150–200°C), necessary for process intensification and reduction of the costs, are also presented. These new type of cells require the development of new types of semiconductors, which are briefly outlined.

Photoelectrocatalytic decomposition of ethylene using TiO2/activated carbon fiber electrode with applied pulsed direct current square-wave potential

Removing ethylene (C2H4) from the atmosphere of storage facilities for fruits and vegetable is one of the main challenges in their postharvest handling for maximizing their freshness, quality, and shelf life. In this study, we investigated the photoelectrocatalytic (PEC) degradation of ethylene gas by applying a pulsed direct current DC square-wave (PDCSW) potential and by using a Nafion-based PEC cell. The cell utilized a titanium dioxide (TiO2) photocatalyst or γ-irradiated TiO2 (TiO2*) loaded on activated carbon fiber (ACF) as a photoelectrode. The apparent rate constant of a pseudo-first-order reaction (K) was used to describe the PEC degradation of ethylene. Parameters of the potential applied to the PEC cell in a reactor that affect the degradation efficiency in terms of the K value were studied. These parameters were frequency, duty cycle, and voltage. Ethylene degradation by application of a constant PDCSW potential to the PEC electrode of either TiO2/ACF cell or TiO2*/ACF cell enhanced the efficiency of photocatalytic degradation and PEC degradation. Gamma irradiation of TiO2 in the electrode and the applied PDCSW potential synergistically increased the K value. Independent variables (frequency, duty cycle, and voltage) of the PEC cell fabricated from TiO2 subjected 20 kGy γ radiation were optimized to maximize the K value by using response surface methodology with quadratic rotation–orthogonal composite experimental design. Optimized conditions were as follows: 358.36Hz frequency, 55.79% duty cycle, and 64.65V voltage. The maximum K value attained was 4.4×10−4min−1.

A Cu foam cathode used as a Pt–RGO catalyst matrix to improve CO2 reduction in a photoelectrocatalytic cell with a TiO2 photoanode

A Cu foam cathode was used as a Pt–RGO catalyst matrix for CO2 reduction in a photoelectrocatalytic cell with a TiO2 nanotube photoanode.

Electrocatalytic conversion of CO2 to produce solar fuels in electrolyte or electrolyte-less configurations of PEC cells.

The electrocatalytic reduction of CO2 is studied on a series of electrodes (based on Cu, Co, Fe and Pt metal nanoparticles deposited on carbon nanotubes or carbon black and then placed at the interface between a Nafion membrane and a gas-diffusion-layer electrode) on two types of cells: one operating in the presence of a liquid bulk electrolyte and the other in the absence of the electrolyte (electrolyte-less conditions). The results evidence how the latter conditions allow productivity of about one order of magnitude higher and how to change the type of products formed. Under electrolyte-less conditions, the formation of >C2 products such as acetone and isopropanol is observed, but not in liquid-phase cell operations on the same electrodes. The relative order of productivity in CO2 electrocatalytic reduction in the series of electrodes investigated is also different between the two types of cells. The implications of these results in terms of possible differences in the reaction mechanism are commented on, as well as in terms of the design of photoelectrocatalytic (PEC) solar cells.

Comparison of the Photocatalytic and Photoelectrocatalytic Methyl Orange Color Removal over TiO(2-x-y)NxFy

Photocatalysis and photoelectrocatalysis are environmentally friendly methodologies for remediation pollutants present in waste waters, these methodologies employ semiconductor materials capable to generate electron hole pairs when they are illuminated [1]. Photogenerated charge carriers together with oxidizing transient species, mainly the hydroxyl radical (OH*), can produce deep changes in the chemical structure of the contaminants [2]. One of the most commonly used semiconductor for these purposes is the TiO2, however it is only active under UV light, therefore, many approaches have been conducted towards TiO2 modification with different metallic and nonmetallic elements in order to reduce the gap between the bands of the oxide, allowing to use visible light for this process [3,4]. Photocatalytic (PC) and photoelectrocatalytic (PEC) methyl orange (MO) color removal may involve two mechanisms simultaneously [5]. The first path is the oxidative radicals’ formation such OH* which carry the pollutant oxidation in the solution [6], and the second way refers to the direct interaction of photogenerated holes with the dye adsorbed on the semiconductor surface [7,8]. The dominant mechanism will depend on the kind of processes applied (PC or PEC), and the modification of the TiO2 that can induce defects such oxygen vacancies in the oxide lattice, or alter the surface properties of the oxide [9].TiO(2-x-y)-Nx-Fy powders and thin films were prepared by combining sol gel method and hydrothermal treatment. The obtained materials were characterized by X-ray diffraction (XRD), Raman and infrared (FTIR) spectroscopies, diffuse reflectance (UV-VIS) spectrophotometry and Zeta potential measurements. Moreover, the supported films on AISI 304 stainless steel foils were characterized by scanning electron microscopy (SEM) and contact angle measurements. Photocatalytic and photoelectrocatalytic test were carried in a 5 ppm MO solution at a pH of 6.8.The results showed that the PC Methyl orange degradation rate over fluoride modified photocatalysts (TiO2-yFy and TiO(2-x-y)NxFy) was comparatively lower than over pristine TiO2 and TiO2-xNx. However, the trend reversed when the MO degradation was carried in a photoelectrocatalytic cell (see Fig. 1). The material characterization showed that fluoride modification alter not only the crystallinity (XRD) and structure (UV-Vis and Raman) of the TiO2, but also its surface properties (Isoelectric point and wettability), favoring the generation of radical ions in the solution. When the degradation is carried in suspension (photocatalysis) there is a high surface area available for the oxidation process favoring the direct charge transfer between the dye and the semiconductor. On the other hand, when the process is carried over a thin film (photoelectrocatalysis) there is a low contact area causing limitation in the interfacial charge transfer between the pollutant and the oxide, then the generation of radical species that travel to the bulk of the solution to carried the MO discoloration, gain relevance in the process [10]. References I. Sirés, E. Brillas, Environment International 40, 212, 2012. Yu, J. G.; Yu, J. C.; Cheng, B.; Hark, S. K.; Lu, K. J. Solid State Chem., 2003, 174: 372. V. I. Shapovalov. Glass Physics and Chemistry, 2010 , Vol. 36, No. 2, pp. 121–157. D. Wu, M. Long, W. Cai, Ch. Chen, Y. Wu, J. Alloy Compd. 502, 289, 2010. R. Asahi, T. Morikawa, K. Ohwaki, Aoki, Y. Taga, Science, 293, 269, 2001. C. Minero; G. Mariella; V. Maurino; D. Vione; and E. Pelizzetti. Langmuir 2000, 16, 8964-8972. J. Zhu; Z. Deng, F; Chen, J; Zhang, H. Chen; M. Anpo; J. Huang; L. Zhang. Appl. Catal. B: Environ. 62, 2006, 329–335. L. Gomathi Devi; R. Kavitha. Electrochimica Acta 129, 2014, 137–141. A.E. Giannakas; E. Seristatidou; Y. Deligiannakis; I. Konstantinou. Appl. Cat. B: Environ. 132– 133, 2013, 460– 468. E. Peralta; G. Roa; J.A. Hernandez-Servin; R. Romero; P. Baldera; R. Natividad. Electrochimica Acta 129, 2014, 137–141.

Enhancement of the photoelectrocatalytic activity of TiO2/ACF for ethylene removal by Ag nanoparticles synthesized by γ-ray radiolysis

Silver nanoparticles (AgNPs), obtained by reduction from AgNO3 solution and stabilized in polyvinyl pyrrolidone (PVP), have been synthesized by γ-ray radiolysis. The application of photoelectrocatalytic (PEC) oxidation in ethylene gas treatment using a TiO2 photocatalyst combined with as-prepared AgNPs on activated carbon fiber (ACF) ([Ag+TiO2]/ACF) as a photoelectrode in a Nafion-based photoelectrocatalytic cell is presented. The effects on the PEC degradation of C2H4 of the preparation parameters of the AgNPs, such as the radiation dose and the amount of PVP added, have been systematically investigated. Emphasis is placed on comparison of the C2H4 decomposition rate constant with [Ag+TiO2/ACF] as a photoelectrode in a photoelectrocatalytic cell with that achieved with [TiO2/ACF]. The factors responsible for the enhancing effect of AgNPs on the PEC activity of the [Ag+TiO2/ACF] photoelectrode are discussed.

Photoelectrocatalytic degradation of carbamazepine using Ti/TiO2 nanostructured electrodes deposited by means of a pulsed laser deposition process

The objective of the present work is to evaluate the potential of photoelectrocatalytic oxidation (PECO) process using Ti/TiO2 for the degradation of carbamazepine (CBZ). Ti/TiO2 prepared by pulsed laser deposition (PLD) has been used as a photo-catalyst in a photoelectrocatalytic cell. The PLD TiO2 coatings were found to be of anatase structure consisting of nanocrystallites of approximately 15nm in diameter. Factorial and central and extreme composite design methodologies were successively employed to define the optimal operating conditions for CBZ degradation. Several factors such as current intensity, treatment time, pollutant concentration and cathode material were investigated. Using a 24 factorial matrix, the best performance for CBZ degradation (53.5%) was obtained at a current intensity of 0.1 A during 120min of treatment time and when the vitreous carbon (VC) was used at the cathode in the presence of 10mgL−1 of CBZ. Treatment time and pollutant concentration were found to be very meaningful for CBZ removal. The PECO process applied under optimal conditions (at current intensity of 0.3A during 120min in the presence of 10mgL−1 of CBZ with VC at the cathode) is able to oxidize around 73.5% ±2.8% of CBZ and to ensure 21.2%±7.7% of mineralization. During PECO process, CBZ was mainly transformed to acridine and anthranilic acid. Microtox biotests (Vibrio fisheri) showed that the treated – effluent was not toxic. The pseudo-second order kinetic model (k2=6×10−4Lmg−1min−1) described very well the oxidation of CBZ.

Roles of Cocatalysts in Photocatalysis and Photoelectrocatalysis

Since the 1970s, splitting water using solar energy has been a focus of great attention as a possible means for converting solar energy to chemical energy in the form of clean and renewable hydrogen fuel. Approaches to solar water splitting include photocatalytic water splitting with homogeneous or heterogeneous photocatalysts, photoelectrochemical or photoelectrocatalytic (PEC) water splitting with a PEC cell, and electrolysis of water with photovoltaic cells coupled to electrocatalysts. Though many materials are capable of photocatalytically producing hydrogen and/or oxygen, the overall energy conversion efficiency is still low and far from practical application. This is mainly due to the fact that the three crucial steps for the water splitting reaction: solar light harvesting, charge separation and transportation, and the catalytic reduction and oxidation reactions, are not efficient enough or simultaneously. Water splitting is a thermodynamically uphill reaction, requiring transfer of multiple electrons, making it one of the most challenging reactions in chemistry. This Account describes the important roles of cocatalysts in photocatalytic and PEC water splitting reactions. For semiconductor-based photocatalytic and PEC systems, we show that loading proper cocatalysts, especially dual cocatalysts for reduction and oxidation, on semiconductors (as light harvesters) can significantly enhance the activities of photocatalytic and PEC water splitting reactions. Loading oxidation and/or reduction cocatalysts on semiconductors can facilitate oxidation and reduction reactions by providing the active sites/reaction sites while suppressing the charge recombination and reverse reactions. In a PEC water splitting system, the water oxidation and reduction reactions occur at opposite electrodes, so cocatalysts loaded on the electrode materials mainly act as active sites/reaction sites spatially separated as natural photosynthesis does. In both cases, the nature of the loaded cocatalysts and their interaction with the semiconductor through the interface/junction are important. The cocatalyst can provide trapping sites for the photogenerated charges and promote the charge separation, thus enhancing the quantum efficiency; the cocatalysts could improve the photostability of the catalysts by timely consuming of the photogenerated charges, particularly the holes; most importantly, the cocatalysts catalyze the reactions by lowering the activation energy. Our research shows that loading suitable dual cocatalysts on semiconductors can significantly increase the photocatalytic activities of hydrogen and oxygen evolution reactions, and even make the overall water splitting reaction possible. All of these findings suggest that dual cocatalysts are necessary for developing highly efficient photocatalysts for water splitting reactions.

Photoelectrocatalytic bleaching of p-nitrosodimethylaniline using Ti/TiO2 nanostructured electrodes deposited by means of a pulsed laser deposition process

This study investigated the potential use of oxidation in a photoelectrocatalytic cell for bleaching p-nitrosodimethylaniline. The Ti/TiO2 used as photo-anode was prepared by a pulsed laser deposition method. The TiO2 coatings were found to have rutile and anatase structures consisting of approximately 10 and 15 nm in diameter, respectively. A relatively high degradation rate of p-nitrosodimethylaniline was recorded using the photoelectrocatalytic cell, compared to those measured during conventional electrochemical oxidation, direct photolysis and photocatalysis processes. The influence of different parameters such as crystallographic structure of Ti/TiO2, type of cathode, potential applied, electrolysis time, UV irradiation and initial pH were investigated. The photoelectrocatalytic cell using Ti/TiO2 (anatase structure) as photo-anode and vitreous carbon as cathode operated at a current intensity of 0.1 A for 120 min with 254 nm of UV irradiation was found to have the best conditions to remove high amounts of p-nitrosodimethylaniline (22.6 × 10−3 mM h−1).

Fabrication of TiO2/CdS/TiO2 Nanotube/Ti Mesh Electrode and Application in Photoelectro-catalytic Cell System for Degradation of Methylene Blue under Visible Light Illumination

A TiO2/CdS/TiO2 nanotube/Ti mesh composite structure photoelectrocatalyst was fabricated by decorating the anodized TiO2 nanotubes (TNT) on Ti mesh with CdS nanoparticles via successive ionic layer adsorption and reaction and subsequently coated with a TiO2 protection layer via a vacuum dip-coating process. For the photoelectrocatalysts severed as the photoanodes, the transient photocurrent (i(ph)) and linear sweep voltammetry (LSV) were investigated in a three-electrode system and the photoelectrocatalytic performance was evaluated in a photoelectrocatalytic cell (PEC) system for degradation of methylene blue under visible light illumination. The results show that the TiO2 nanotube arrays are decorated with relatively uniform CdS nanoparticles on the tube wall. Owing to the protection of TiO2 layer, the TiO2/CdS/TiO2 nanotube/Ti mesh electrode shows a stable and superior photoelectrocatalytic performance. The superimposition of mesh electrode can remarkably increase the photoelectrocatalytic efficiency for the degradation of methylene blue.

Sedimentation field flow fractionation and flow field flow fractionation as tools for studying the aging effects of WO 3 colloids for photoelectrochemical uses

WO 3 colloidal suspensions obtained through a simple sol–gel procedure were subjected to a controlled temperature aging process whose time evolution in terms of particle mass and size distribution was followed by sedimentation field flow fractionation (SdFFF) and flow field flow fractionation (FlFFF). The experiments performed at a temperature of 60 °C showed that in a few hours the initially transparent sol of WO 3 particles, whose size was less than 25 nm, undergoes a progressive size increase allowing nanoparticles to reach a maximum equivalent spherical size of about 130 nm after 5 h. The observed shift in particle size distribution maxima (SdFFF), the broadening of the curves (FlFFF) and the SEM–TEM observations suggest a mixed mechanism of growth-aggregation of initial nanocrystals to form larger particles. The photoelectrochemical properties of thin WO 3 films obtained from the aged suspensions at regular intervals, were tested in a biased photoelectrocatalytic cell with 1 M H 2SO 4 under solar simulated irradiation. The current–voltage polarization curves recorded in the potential range 0–1.8 V (vs. SCE) showed a diminution of the maximum photocurrent from 3.7 mA cm −2 to 2.8 mA cm −2 with aging times of 1 h and 5 h, respectively. This loss of performance was mainly attributed to the reduction of the electroactive surface area of the sintered particles as suggested by the satisfactory linear correlation between the integrated photocurrent and the cyclic voltammetry cathodic wave area of the W(VI) → W(V) process measured in the dark.