Characterization of Photoelectrocatalytic Processes at Nanoporous TiO2 Film Electrodes: Photocatalytic Oxidation of Glucose

Mechanistic insights into 2,4-D photoelectrocatalytic removal from water with TiO2 nanotubes under dark and solar light irradiation

Removal of recalcitrant pollutants from water is a major challenge, to which the photoelectrocatalytic processes may be a solution. Applied potential plays a key role in the photocatalytic activity of the semiconductor. This paper investigated the effect of applied potential on the photoelectrocatalytic oxidation of 2,4-Dichlorophenoxyacetic acid (2,4-D) with TiO₂ nanotubular anodes under solar light irradiation. The process was investigated at constant potentials in different regions of the polarization curve: the ohmic region, the saturation region and in the region of the Schottky barrier breakdown. PEC tests were performed in aqueous solutions of 2,4-D, and in the presence of methanol or formic acid, as scavengers of OH^• radicals and holes. Results showed the main mechanism is oxidation by OH^• radicals from water oxidation, while runs with hole scavenger revealed a second mechanism of direct oxidation by holes photogenerated at the electrode surface, with high removal rates due to current doubling effect.

Influence of ruthenium doping on UV- and visible-light photoelectrocatalytic color removal from dye solutions using a TiO2 nanotube array photoanode

The photocatalytic activity of TiO₂ anodes was enhanced by synthesizing Ru-doped Ti|TiO₂ nanotube arrays. Such photoanodes were fabricated via Ti anodization followed by Ru impregnation and annealing. The X-ray diffractograms revealed that anatase was the main TiO₂ phase, while rutile was slightly present in all samples. Scanning electron microscopy evidenced a uniform morphology in all samples, with nanotube diameter ranging from 60 to 120 nm. The bias potential for the photoelectrochemical (PEC) treatment was selected from the electrochemical characterization of each electrode, made via linear sweep voltammetry. All the Ru-doped TiO₂ nanotube array photoanodes showed a peak photocurrent (PP) and a saturation photocurrent (SP) upon their illumination with UV or visible light. In contrast, the undoped TiO₂ nanotubes only showed the SP, which was higher than that reached with the Ru-doped photoanodes using UV light. An exception was the Ru(0.15 wt%)-doped TiO₂, whose SP was comparable under visible light. Using that anode, the activity enhancement during the PEC treatment of a Terasil Blue dye solution at E_bias(PP) was much higher than that attained at E_bias(SP). The percentage of color removal at 120 min with the Ru(0.15 wt%)-doped TiO₂ was 98% and 55% in PEC with UV and visible light, respectively, being much greater than 82% and 28% achieved in photocatalysis. The moderate visible-light photoactivity of the Ru-doped TiO₂ nanotube arrays suggests their convenience to work under solar PEC conditions, aiming at using a large portion of the solar spectrum.

Synthesis of MoS2/graphene hybrid supported Au and Ag nanoparticles with multi-functional catalytic properties

The detection and removal of nitroaromatic compounds is an important issue for environmental protection. In this study, a hybrid of molybdenum disulfide (MoS₂) and graphene (GR) was first synthesized using a facile hydrothermal method. Au and Ag nanoparticles were then deposited onto the surface of the MoS₂/GR hybrid with sodium citrate as the stabilizer and reductant. Compared to using pure MoS₂ as the support, the obtained Au (Ag)-MoS₂/GR composites showed improved activity for electrochemical detection and chemical reduction of 4-nitrophenol. The activity enhancement appears to be due to the addition of GR, which could improve the conductivity as well as provide more active sites. The successful synthesis of Au (Ag)-MoS₂/GR composites could provide new multi-function catalysts for environmental protection.

Photoelectrocatalytic inactivation of fecal coliform bacteria in urban wastewater using nanoparticulated films of TiO2 and TiO2/Ag

Photocatalysis has shown the ability to inactivate a wide range of harmful microorganisms with traditional use of chlorination. Photocatalysis combined with applied bias potential (photoelectrocatalysis) increases the efficiency of photocatalysis and decreases the charge recombination. This work examines the inactivation of fecal coliform bacteria present in real urban wastewater by photoelectrocatalysis using nanoparticulated films of TiO₂ and TiO₂/Ag (4%w/w) under UV light irradiation. The catalysts were prepared with different thicknesses by the sol-gel method and calcined at 400°C and 600°C. The urban wastewater samples were collected from the sedimentation tank effluent of the university sewage treatment facility. The rate of bacteria inactivation increases with increasing the applied potential and film thicknesses; also, the presence of silver on the catalyst surface annealed at 400°C shows better inactivation than that at 600°C. Finally, a structural cell damage of Escherichia coli (DH5α), inoculated in water, is observed during the photoelectrocatalytic process.

Heterogeneously Catalyzed Hydrothermal Processing of C5-C6 Sugars

Biomass has been long exploited as an anthropogenic energy source; however, the 21st century challenges of energy security and climate change are driving resurgence in its utilization both as a renewable alternative to fossil fuels and as a sustainable carbon feedstock for chemicals production. Deconstruction of cellulose and hemicellulose carbohydrate polymers into their constituent C₅ and C₆ sugars, and subsequent heterogeneously catalyzed transformations, offer the promise of unlocking diverse oxygenates such as furfural, 5-hydroxymethylfurfural, xylitol, sorbitol, mannitol, and gluconic acid as biorefinery platform chemicals. Here, we review recent advances in the design and development of catalysts and processes for C₅-C₆ sugar reforming into chemical intermediates and products, and highlight the challenges of aqueous phase operation and catalyst evaluation, in addition to process considerations such as solvent and reactor selection.

MoS2/reduced graphene oxide hybrid with CdS nanoparticles as a visible light-driven photocatalyst for the reduction of 4-nitrophenol

Photocatalytic reduction of nitroaromatic compounds to aromatic amines using visible light is an attractive process that utilizes sunlight as the energy source for the chemical conversions. Herewith we synthesized a composite material consisting of CdS nanoparticles grown on the surface of MoS2/reduced graphene oxide (rGO) hybrid as a novel photocatalyst for the reduction of 4-nitrophenol (4-NP). The CdS-MoS2/rGO composite is shown as a high-performance visible light-driven photocatalyst. Even without a noble-metal cocatalyst, the catalyst exhibited a great activity under visible light irradiation for the reduction of 4-NP to much less toxic 4-aminophenol (4-AP) with ammonium formate as the sacrificial agent. Composite CdS-0.03(MoS2/0.01rGO) was found to be the most effective photocatalyst for 4-NP reduction. The high photocatalytic performance is apparently resulted from the synergetic functions of MoS2 and graphene in the composite, i.e. the cocatalysts serve as both the active adsorption sites for 4-NP and electron collectors for the separation of electron-hole pairs generated by CdS nanoparticles. The laboratory results show that the CdS-MoS2/rGO composite is a low-cost and stable photocatalyst for effective reduction and detoxification of nitroaromatic compounds using solar energy.

Photoelectrochemical manifestation of intrinsic photoelectron transport properties of vertically aligned {001} faceted single crystal TiO2 nanosheet films

{001} faceted anatase TiO₂ nanosheet array films after calcination exhibited high photoelectrocatalytic activity toward water oxidation owing to superiorly intrinsic photoelectron transport properties of {001} faceted single crystal nanosheets.

Photoelectrochemical determination of intrinsic kinetics of photoelectrocatalysis processes at {001} faceted anatase TiO2 photoanodes

Intrinsic degradation kinetic constant of oxalic acid at a double layered anatase TiO₂ photoanode with dominantly exposed {001} facets.

Photoelectrochemical activity and its mechanism of mesoporous TiO2 nanotube arrays prepared with chemical etching method

Mesopores are etched in the tube walls of TiO₂ nanotubes, which can enhance the photoelectrochemical activities of TiO₂ nanotube arrays (NTAs).

Photoelectrochemical characterization of hydrogenated TiO2 nanotubes as photoanodes for sensing applications

In this work, hydrogenated TiO2 nanotubes (H-TNTs) electrodes were successfully fabricated via the anodization of a titanium sheet followed by a hydrogenation process. Oxygen vacancies were induced in the crystalline structure of TiO2 nanotubes (TNTs) as shallow donors that enhance the electronic conductivity of the TNTs. This improvement in the electronic conductivity and photoelectrocatalytic (PEC) performance was confirmed and evaluated by a photoelectrochemical characterization. Most importantly, the H-TNTs electrode was able to degrade potassium hydrogen phthalate (strong adsorbent) and glucose (weak adsorbent) indiscriminately. The corresponding photocurrents at the H-TNTs were 2-fold greater than that of the TNTs samples for the same concentrations of the organic compounds. This suggests that the H-TNTs electrode can be a promising sensor for the PEC determination of individual organic compounds or as an aggregative parameter of organic compounds (e.g., chemical oxygen demand).

Vapor-phase hydrothermal transformation of HTiOF3 intermediates into {001} faceted anatase single-crystalline nanosheets

For the first time, a facile, one-pot hydrofluoric acid vapor-phase hydrothermal (HF-VPH) method is demonstrated to directly grow single-crystalline anatase TiO(2) nanosheets with 98.2% of exposed {001} faceted surfaces on the Ti substrate via a distinctive two-stage formation mechanism. The first stage produces a new intermediate crystal (orthorhombic HTiOF(3) ) that is transformed into anatase TiO(2) nanosheets during the second stage. The findings reveal that the HF-VPH reaction environment is unique and differs remarkably from that of liquid-phase hydrothermal processes. The uniqueness of the HF-VPH conditions can be readily used to effectively control the nanostructure growth.

Photocatalytic synthesis of TiO(2) and reduced graphene oxide nanocomposite for lithium ion battery

In this work, we synthesized graphene oxide (GO) using the improved Hummers' oxidation method. TiO2 nanoparticles can be anchored on the GO sheets via the abundant oxygen-containing functional groups such as epoxy, hydroxyl, carbonyl, and carboxyl groups on the GO sheets. Using the TiO2 photocatalyst, the GO was photocatalytically reduced under UV illumination, leading to the production of TiO2-reduced graphene oxide (TiO2-RGO) nanocomposite. The as-prepared TiO2, TiO2-GO, and TiO2-RGO nanocomposite were used to fabricate lithium ion batteries (LIBs) as the active anode materials and their corresponding lithium ion insertion/extraction performance was evaluated. The resultant LIBs of the TiO2-RGO nanocomposite possesses more stable cyclic performance, larger reversible capacity, and better rate capability, compared with that of the pure TiO2 and TiO2-GO samples. The electrochemical and materials characterization suggest that the graphene network provides efficient pathways for electron transfer, and the TiO2 nanoparticles prevent the restacking of the graphene nanosheets, resulting in the improvement in both electric conductivity and specific capacity, respectively. This work suggests that the TiO2 based photocatalytic method could be a simple, low-cost, and efficient approach for large-scale production of anode materials for lithium ion batteries.

The electrochemistry of nanostructured titanium dioxide electrodes

Several of the multiple applications of titanium dioxide nanomaterials are directly related to the introduction or generation of charge carriers in the oxide. Thus, electrochemistry plays a central role in the understanding of the factors that must be controlled for the optimization of the material for each application. Herein, the main conceptual tools needed to address the study of the electrochemical properties of TiO(2) nanostructured electrodes are reviewed, as well as the electrochemical methods to prepare and modify them. Particular attention is paid to the dark electrochemical response of these nanomaterials and its direct connection with the TiO(2) electronic structure, interfacial area and grain boundary density. The physical bases for the generation of currents under illumination are also presented. Emphasis is placed on the fact that the kinetics of charge-carrier transfer to solution determines the sign and value of the photocurrent. Furthermore, methods for extracting kinetic information from open-circuit potential and photocurrent measurements are briefly presented. Some aspects of the combination of electrochemical and spectroscopic measurements are also dealt with. Finally, some of the applications of TiO(2) nanostructured samples derived from their electrochemical properties are concisely reviewed. Particular attention is paid to photocatalytic processes and, to a lesser extent, to photosynthetic reactions as well as to applications related to energy from the aspects of both saving (electrochromic layers) and accumulation (batteries). The use of TiO(2) nanomaterials in solar cells is not covered, as a number of reviews have been published addressing this issue.

Nanostructured TiO2 photocatalysts for the determination of organic pollutants

Owing to the inherent advantages of nanostructured TiO(2) photocatalysts, including high photocatalytic activity, strong oxidation power, low cost, environmental benignity and excellent stability, TiO(2) photocatalyts have recently attracted extensive attention from scientific researchers, technology developers and investors for use in sensing applications. The TiO(2) sensors can be used for lab-based analyses, on-line and on-site determination of organic pollutants in wastewater. This work reviews the application of TiO(2) nanomaterials in photocatalytic and photoelectrocatalytic monitoring of aggregative organic parameters such as total organic carbon (TOC) and chemical oxygen demand (COD), as well as individual organic compounds in aqueous solution.

High-performance nanoporous TiO2/La2O3 hybrid photoanode for dye-sensitized solar cells

An organic lanthanum solution was prepared and used for modifying the nanoporous TiO(2) photoanode for dye-sensitized solar cells (DSSCs). The preliminary characterization results demonstrate that La(2)O(3) was formed on the surface of the TiO(2) photoanodes. The X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses suggest that La(3+) was introduced into the TiO(2) nanocrystalline, while, the scanning electron microscopy (SEM) and tunnelling electron microscopy (TEM) characterizations suggest that a thin La(2)O(3) layer forms on surface of the TiO(2) nanostructure. The La(2)O(3) layer is able to alleviate the electron recombination as a passivation layer. Though the slight decrease in surface areas were induced by the surface modification, the dye loading were maintained, which can be attributed to the formation of strong co-ordination bonding between the dye molecules and the lanthanide. The bonding can also facilitate the electron transfer between the dye molecules and TiO(2) conduction band. Consequently, the open circuit potential and short circuit current were boosted significantly and the overall energy conversion efficiency of the DSSCs was remarkably improved from 6.84% for the control film to 9.67% for the La(3+)-modified film.

Anatase TiO₂ crystal facet growth: mechanistic role of hydrofluoric acid and photoelectrocatalytic activity

This work reports a facile hydrothermal approach to directly grow anatase TiO(2) crystals with exposed {001} facets on titanium foil substrate by controlling pH of HF solution. The mechanistic role of HF for control growth of the crystal facet of anatase TiO(2) crystals has been investigated. The results demonstrate that controlling solution pH controls the extent of surface fluorination of anatase TiO(2), hence the size, shape, morphology, and {001} faceted surface area of TiO(2) crystals. The theoretical calculations reveal that {001} faceted surface fluorination of anatase TiO(2) can merely occur via dissociative adsorption of HF molecules under acidic conditions while the adsorption of Na(+)F(-) is thermodynamically prohibited. This confirms that the presence of molecular form of HF but not F(-) is essential for preservation of exposed {001} facets of anatase TiO(2). Anatase TiO(2) crystals with exposed {001} facets can be directly fabricated on titanium foil by controlling the solution pH ≤ 5.8. When pH is increased to near neutral and beyond (e.g., pH ≥ 6.6), the insufficient concentration of HF ([HF] ≤ 0.04%) dramatically reduces the extent of surface fluorination, leading to the formation of anatase TiO(2) crystals with {101} facets and titanate nanorods/nanosheets. The anatase TiO(2) nanocrystals with exposed {001} facets exhibits a superior photoelectrocatalytic activity toward water oxidation. The findings of this work clarify the mechanistic role of HF for controlling the crystal facet growth, providing a facile means for massive production of desired nanostructures with high reactive facets on solid substrates for other metal oxides.

Fabrication of highly ordered TiO2 nanorod/nanotube adjacent arrays for photoelectrochemical applications

This work reports a facile approach to fabricate a perpendicularly aligned and highly ordered TiO(2) nanorod/nanotube (NR/NT) adjacent film by directly anodizing a modified titanium foil. The titanium foil substrate was modified with a layer of crystalline TiO(2) film via a hydrothermal process in 0.05 M (NH(4))(2)S(2)O(8). The resultant NR/NT architecture consists of a highly ordered nanorod top layer that directly adjoins to a highly ordered nanotube array bottom layer. The thickness of the top nanorod layer was approximately 90 nm with average nanorod diameter of 22 nm after 20 min of anodization. The thickness of the bottom nanotube array layer was found to be ca. 250 nm after 20 min of anodization, having an average outer and inner tubular diameters of 120 and 80 nm, respectively. A broad implication of the method is that a simple modification to the substrate surface can lead to new forms of nanostructures. For as-anodized NR/NT samples, XRD analysis reveals that the nanorods are of anatase TiO(2) crystalline form while the nanotubes are amorphous. Anatase TiO(2) crystalline form of NR/NT film with high crystallinity can be obtained by thermally treating the as-anodized sample at 450 degrees C for 2 h in air. The resultant NR/NT film was used as a photoanode for photoactivity evaluation. Comparing with a nanotube array photoanode prepared by direct anodization of unmodified titanium foil, the NR/NT photoanode exhibits a unique feature of selective photocatalytic oxidation toward organics, which makes it very attractive to photocatalytic degradation of organic pollutants, sensing, and other applications.

Photoelectrochemical characterization of a robust TiO2/BDD heterojunction electrode for sensing application in aqueous solutions

Titanium dioxide (TiO(2)) and boron-doped diamond (BDD) are two of the most popular functional materials in recent years. In this work, TiO(2) nanoparticles were immobilized onto the BDD electrodes by a dip-coating technique. Continuous and uniform mixed-phase (anatase and rutile) and pure-anatase TiO(2)/BDD electrodes were obtained after calcination processes at 700 and 450 degrees C, respectively. The particle sizes of both types of TiO(2) film range from 20 to 30 nm. In comparison with a TiO(2)/indium tin oxide (ITO) electrode, the TiO(2)/BDD electrode demonstrates a higher photoelectrocatalytic activity toward the oxidation of organic compounds, such as glucose and potassium hydrogen phthalate. Among all the tested TiO(2) electrodes, the mixed-phase TiO(2)/BDD electrode demonstrated the highest photoelectrocatalytic activity, which can be attributed to the formation of the p-n heterojunction between TiO(2) and BDD. The electrode was subsequently used to detect a wide spectrum of organic compounds in aqueous solution using a steady-state current method. An excellent linear relationship between the steady-state photocurrents and equivalent organic concentrations was attained. The steady-state oxidation photocurrents of the mixed-phase TiO(2)/BDD electrode were insensitive to pH in the range of pH 2-10. Furthermore, the electrodes exhibited excellent robustness under strong acidic conditions that the TiO(2)/ITO electrodes cannot stand. These characteristics bestow the mixed-phase TiO(2)/BDD electrode to be a versatile material for the sensing of organic compounds.

Facile formation of branched titanate nanotubes to grow a three-dimensional nanotubular network directly on a solid substrate

The hydrothermal formation of branched titanate nanotubes that grow a 3D nanotubular network directly onto a titanium substrate is reported. The resultant 3D nanotubular network exhibits a unique all-dimensional uniform porous structure. The inner and outer tubular diameters of branched titanate nanotubes were found to be approximately 6 and 12 nm, respectively. For the majority of the nanotubes, the wall is formed from three layers of titanate with an approximate 7.7 A interlayer space. In terms of individual nanotubes, these characteristics are quantitatively similar to those of previously reported nonbranched nanotubes. However, in terms of how nanotubes are arranged in the film, the all-dimensional uniform nanotubular network structure obtained here is distinctively different from those of previously reported structures. The 3D nanotubular network structure was formed by the jointing of branched nanotubes. In contrast, the previously reported nanotubes tend to grow vertically on the substrate, and the resultant tubular films are formed by interwoven nonbranched nanotubes. The branched titanate nanotubes can be readily formed on titanium substrates but not in solution suspension forms. A continuous seed formation-oriented crystal growth mechanism was proposed for the branched titanate nanotubular network formation. Such a network structure could be useful for applications such as photocatalysis, membrane separation, field emission, and photovoltaic devices.

A new coral structure TiO2/Ti film electrode applied to photoelectrocatalytic degradation of Reactive Brilliant Red

A novel structure TiO2/Ti film was prepared on a titanium matrix using anodic oxidation technique and applied to degrade Reactive Brilliant Red (RBR) dye in simulative textile effluents. The film was characterized by Field-Emission Scanning Electron Microscope (FE-SEM), Laser Micro-Raman Spectrometer (LMRS), UV-vis spectrophotometer (UVS) and Photoelectrocatalytic (PEC) experiment. The results show that the surface morphology of the film is coral structure, and the crystal structure of the film is anatase. The absorbency of the coral structure TiO2/Ti film is 87-93% in the UV light region, and 77-87% in the visible light region. PEC experiment indicates that the photocurrent density of the coral structure TiO2/Ti film electrode achieves 160 microA/cm(2). The color and Chemical Oxygen Demand (COD) removal efficiencies of RBR achieve 73% and 60% in 1h, respectively. These are 16% and 58% higher than those of nanotube TiO2/Ti film electrode. These were attributed to that these electrodes with different surface morphologies exhibit distinct surface areas and light absorption rate.

Photoelectrocatalytic degradation of tetracycline by highly effective TiO2 nanopore arrays electrode

The widely utilization of pharmaceutical and personal care products (PPCPs) in the pharmaceutical therapies and agricultural husbandry has led to the worldwide pollution in the environment. In this study, the photoelectrocatalytic (PEC) behaviors of typical PPCPs, tetracycline (TC), were performed via a highly effective TiO(2) nanopore arrays (TNPs) electrode, comparing with electrochemical (EC) and photocatalytic (PC) process. A significant photoelectrochemical synergetic effect in TC degradation was observed on the TNPs electrode and the rate constant for the PEC process of TNPs electrode was approximately 6.7 times as high as its PC process. The TC removal rate achieved approximately 80% within 3h PEC reaction by TNPs electrode, which is approximately 25% higher than that obtained for a conventional coated TiO(2) nanofilm electrode fabricated by sol-gel method. The possible mechanism involved in the PEC degradation of TC by TNPs electrode was discussed. Furthermore, the TNPs electrode also shows enhanced photocurrent response compared with that for the coated TiO(2) nanofilm electrode. Such kind of TiO(2) nanopores will have many potential applications in various areas as an outstanding photoelectrochemical material.

A portable photoelectrochemical probe for rapid determination of chemical oxygen demand in wastewaters

A photoelectrochemical probe for rapid determination of chemical oxygen demand (COD) is developed using a nanostructured mixed-phase TiO2 photoanode, namely PeCOD probe. A UV-LED light source and a USB mircroelectrochemical station are powered and controlled by a laptop computer, which makes the probe portable for onsite COD analyses. The photoelectrochemical measurement of COD was optimized in terms of light intensity, applied bias, and pH. Under the optimized conditions, the net steady state currents originated from the oxidation of organic compounds were found to be directly proportional to COD concentrations. A practical detection limit of 0.2 ppm COD and a linear range of 0-120 ppm COD were achieved. The analytical method using the portable PeCOD probe has the advantages of being rapid, low cost, robust, user-friendly, and environmental friendly. It has been successfully applied to determine the COD values of the synthetic samples consisting of potassium hydrogen phthalate, D-glucose, glutamic acid, glutaric acid, succinic acid, and malonic acid, and real samples from various industries, such as bakery, oil and grease manufacturer, poultry, hotel, fine food factory, and fresh food producer, commercial bread manufacturer. Excellent agreement between the proposed method and the conventional COD method (dichromate) was achieved.