Photoelectrochemical behavior of nanostructured WO3 thin-film electrodes: The oxidation of formic acid.

Nanostructured tungsten trioxide thin-film electrodes are prepared on conducting glass substrates by either potentiostatic electrodeposition from aqueous solutions of peroxotungstic acid or direct deposition of WO3 slurries. Once treated thermally in air at 450 degrees C, the electrodes are found to be composed of monoclinic WO3 grains with a particle size around 30-40 nm. The photoelectrochemical behavior of these electrodes in 1 M HClO4 apparently reveals a low degree of electron-hole recombination. Upon addition of formic acid, the electrode showed the current multiplication phenomenon together with a shift of the photocurrent onset potential toward less positive values. Photoelectrochemical experiments devised on the basis of a kinetic model reported recently [I. Mora-Seró, T. Lana-Villarreal, J. Bisquert, A. Pitarch, R. Gómez, P. Salvador, J. Phys. Chem. B 2005, 109, 3371] showed that an interfacial mechanism of inelastic, direct hole transfer takes place in the photooxidation of formic acid. This behavior is attributed to the tendency of formic acid molecules to be specifically adsorbed on the WO3 nanoparticles, as evidenced by attenuated total reflection infrared spectroscopy.     

Electrochemical behavior of nitrogenated nanocrystalline diamond electrodes

Nitrogenated nanocrystalline diamond films with controlled conductivity are deposited from microwave plasma in CH₄-Ar-H₂-N₂ gas mixtures. They are characterized using atomic force microscopy, Raman spectroscopy, and electrophysical measurements. Their electrochemical properties are studied by cyclic voltammetry and electrochemical impedance spectroscopy. Kinetic parameters of reactions in [Fe(CN)₆]^3-/4- redox system are determined. The character of electrode behavior is controlled by the degree on nitrogenation. With the increasing of the nitrogen content in the reaction gas mixture (from 0 to 25%), the potential window somewhat narrows, the background current increases, the reversibility of reactions in the [Fe(CN)₆]^3-/4- redox system increases. By and large, the transition occurs from the electrochemical behavior of a “poor conductor” to that of a metal-like electrode.     

Comparison of the hole mobility in undoped and boron-doped polycrystalline CVD diamond films

The drift mobility of nonequilibrium holes injected in undoped polycrystalline diamond films was determined, by a transit-time technique, as ca. 10⁻³ cm²/(V s). This hole mobility is three orders of magnitude lower than the “equilibrium” mobility in boron-doped diamond films [0.1–1 cm²/(V s)], determined from the films' dc conductivity. This difference is explained by the effect of a nonequilibrium charge carrier trapping during the carrier transport in polycrystalline diamond. Received: 3 December 1997 / Accepted: 9 April 1998     

Electrooxidation of ethylenediaminetetraacetic acid at a polycrystalline boron-doped diamond anode

Electrooxidation of ethylenediaminetetraacetic acid at a thin-film boron-doped polycrystalline diamond anode is studied by cyclic voltammetry and amperometry. It is shown that diamond electrodes can be used in the analytical determination of ethylenediaminetetraacetic acid: they have low background current; the detection limit is also rather low.     

Direct electrochemical assay of glucose using boron-doped diamond electrodes

We report linear sweep and square wave voltammetric studies on glucose oxidation at boron-doped diamond (BDD) electrodes in an alkaline medium in efforts to evaluate the techniques for electrochemically assaying glucose. The bare BDD electrode showed good linear responses to glucose oxidation for a concentration range from 0.5 to 10 mM glucose, which well encompasses the physiological range of 3-8 mM. The BDD electrodes did not experience interferences from ascorbic acid or uric acid during glucose detection. This method, when applied to real blood samples, gave results similar to those obtained by a commercial glucose monitor.     

Cadmium detection via boron-doped diamond electrodes: surfactant inhibited stripping voltammetry

The deposition of cadmium on boron-doped diamond is investigated with square-wave anodic stripping voltammetry. The system was investigated in quiescent conditions, in the presence of an acoustic field and then in the presence of the neutral surfactant Triton^® X-100. The effect of optimised insonation was to increase the sensitivity from 0.63 (under silent conditions) to 3.78 μA μM⁻¹ and to reduce the limit of detection by an order of magnitude from 10⁻⁸ to 10⁻⁹ M. Measurements with or without insonation were found to deteriorate in the presence of the surfactant. Studies using AFM and chronoamperometry showed that this was due to inhibition of the deposition of the metal. Comparative data obtained for analogous measurements for copper on glassy carbon in the presence of industrial effluent, which also leads to signal deterioration under silent but not insonated conditions, showed that for this case also it was the nucleation of copper rather than the metal dissolution which was adversely affected.     

Recent electroanalytical applications of boron-doped diamond electrodes

This review overviews recent reports on the electroanalytical applications of boron-doped diamond (BDD) electrodes. Because BDD electrodes have excellent features for electroanalysis, such as wide potential window, low background current, electrochemical stability, and fouling resistance, they can be useful for sensitive and stable detection of various substances, including drugs, bio-related substances, metal ions, and organic pollutants. Many articles have reported high-sensitivity detection of real samples, demonstrating that this electrode material is practically applicable. Surface modification of the BDD electrodes using metal nanoparticles, nanocarbons, and polymers can increase the sensitivity of the electrochemical detection. Furthermore, research on the electroanalytical device equipped with BDD electrodes will be expanded by combining peripheral technologies related to the device fabrication.     

Detection of aniline at boron-doped diamond electrodes with cathodic stripping voltammetry

Boron-doped diamond (BDD) electrodes were used to investigate the possibility of detecting aniline by linear-sweep cathodic stripping voltammetry. It was found that the dimeric species (p-aminodiphenylamine and benzidine) formed by anodic oxidation of aniline during the accumulation period are involved in electrochemically reversible redox processes and, in acidic media, the shape of the stripping voltammetric response is suitable for aniline detection in the micromolar concentration range. The low background current of conductive diamond is an advantage compared to other electrode materials and allows a detection limit of 1 μM. Weak adsorption properties and the extreme electrochemical stability are additional advantages of BDD and it was found that, even after long-time measurements, the electrode surface can regain its initial activity by an anodic polarization in the potential region of water decomposition.     

The application of boron-doped diamond electrodes in amperometric biosensors

Boron-doped diamond (BDD) electrodes outperform conventional electrodes in terms of high stability, chemical inertness, wide potential window and low background current. Combining the superior properties of BDD electrodes with the merits of biosensors, such as specificity, sensitivity, and fast response, amperometric biosensors based on BDD electrodes have attracted the interests of many researchers. In this review, the latest advances of BDD electrodes with different surfaces including hydrogen-terminated, oxygen-terminated, metal nanoparticles-modified, amine-terminated, and carboxyl-terminated thin films, and microelectrodes, for the construction of various biosensors or the direct detection of biomolecules were demonstrated. The future trends of BDD electrodes in biosensing were also discussed.     

Application of boron-doped diamond electrodes for electrochemical oxidation of real wastewaters

Recently, the synthesis of boron-doped diamond electrodes on different substrates and shapes have reached a promising development stage. Now, these electrodes can also be effectively used to destroy toxic or biorefractory organics in real effluents, such as municipal wastewaters effluents, hospital wastewaters, groundwater, petrochemical effluent, wastewaters from agri–food activities, and so on. The results of this mini-review show that BDD is effectively even for such real effluents, allowing the removal of pollutants under several different conditions. Nevertheless, further efforts are necessary to reach a wider market; in particular, the next stages must face the optimization of cell design and the integration of the electrochemical system with other water treatment and renewable energy sources.     

Solar-Assisted eBiorefinery: Photoelectrochemical Pairing of Oxyfunctionalization and Hydrogenation Reactions

Inspired by natural photosynthesis, biocatalytic photoelectrochemical (PEC) platforms are gaining prominence for the conversion of solar energy into useful chemicals by combining redox biocatalysis and photoelectrocatalysis. Herein, we report a dual biocatalytic PEC platform consisting of a molybdenum (Mo)-doped BiVO₄ (Mo:BiVO₄) photoanode and an inverse opal ITO (IO-ITO) cathode that gives rise to the coupling of peroxygenase and ene-reductase-mediated catalysis, respectively. In the PEC cell, the photoexcited electrons generated from the Mo:BiVO₄ are transferred to the IO-ITO and regenerate reduced flavin mononucleotides to drive ene-reductase-catalyzed trans-hydrogenation of ketoisophrone to (R)-levodione. Meanwhile, the photoactivated Mo:BiVO₄ evolves H₂O₂ in situ via a two-electron water-oxidation process with the aid of an applied bias, which simultaneously supplies peroxygenases to drive selective hydroxylation of ethylbenzene into enantiopure (R)-1-phenyl-1-hydroxyethane. Thus, the deliberate integration of PEC systems with redox biocatalytic reactions can simultaneously produce valuable chemicals on both electrodes using solar-powered electrons and water.     

Preparation of boron-doped semiconducting diamond films using BF3 and the electrochemical behavior of the semiconducting diamond electrodes

Boron-doped semiconducting diamond films were prepared using BF₃ by microwave plasma assisted chemical vapor deposition. B-doping was confirmed by SIMS and Raman spectroscopic measurements and the B-doping levels were estimated. Electrochemical behaviors of boron-doped diamond thin-film electrodes prepared using B₂H₆ and BF₃ were studied by measuring cyclic voltammograms for anodic oxidation of 1,4-difluorobenzene in the liquid electrolyte, neat Et₄NF·4HF. The results of the direct thermal interaction of elemental fluorine with hydrogenated and oxidized diamond surfaces are also presented.     

Analysis of Impedance Spectra of Vacuum-Annealed Undoped Polycrystalline Diamond Electrodes

Impedance spectra for a new electrode material, undoped polycrystalline diamond films vacuum-annealed at 1775 to 1915 K, taken in indifferent electrolytes are analyzed. Regular changing of parameters of the equivalent circuit (constant-phase element, ohmic and faradaic resistances) with increasing anneal temperature is demonstrated. It is concluded that both the amount of a nondiamond conducting phase in the electrode bulk increased and its properties (first and foremost, the conductivity) changed with increasing anneal temperature.     

Photoelectrochemical oxidation behavior of organic substances on TiO2 thin-film electrodes

The photoelectrocatalytic oxidation characteristics of salicylic acid, formic acid and methanol on anodized nanoporous titanium dioxide (TiO₂) thin-films were investigated by using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization techniques. From dark to ultraviolet illumination, the open circuit potential (OCP) and film resistance of TiO₂ films decreased markedly. A general equivalent circuit model was proposed for the photoelectrochemical system anodic TiO₂ thin-film electrode/test solution. The photoelectrochemical oxidation process of the organic compounds showed similar impedance features at OCP and was controlled by the charge transfer step. According to the polarization curves of the base solution and organic solutions, the kinetic rate curves for the photoelectrocatalytic oxidation of pure organic species were obtained as a function of the potential bias. One photooxidation peak was first observed at a bias potential of ca. 0.26 V for these species with low concentrations.     

Microwave-enhanced electro-deposition and stripping of palladium at boron-doped diamond electrodes

In situ microwave activation has been applied to the electro-deposition and stripping of palladium metal (which is widely used as a catalyst) at cavitation resistant boron-doped diamond electrodes. Focused microwave radiation leading to heating, boiling, and cavitation is explored as an option to improve the speed and sensitivity of the analytical detection procedure. The deposition and anodic stripping of palladium by linear sweep voltammetry in 0.1 M KCl (pH 2) solution and at boron-doped diamond electrodes is shown to be strongly enhanced by microwave activation due to both (i) the increase in mass transport and (ii) the increase in the kinetic rate of deposition and stripping.The temperature at the electrode surface is calibrated with the reversible redox couple Fe(CN)₆⁴⁻/Fe(CN)₆³⁻ and found to be reach 380 K. In the presence of microwave radiation, the potential of onset of the deposition of palladium is strongly shifted positive from −0.4 to +0.1 V versus SCE. The optimum potential for deposition in the presence of microwaves is −0.4 V versus SCE and the anodic stripping peak current is shown to increase linearly with deposition time. Under these conditions, the stripping peak current varies linearly with the palladium concentration down to ca. 2 μM. At concentration lower than this a logarithmic variation of the stripping peak current with concentration is observed down to ca. 0.1 μM (for 5 min pre-concentration in presence of microwave radiation).     

Promoting CO2 electroreduction on boron-doped diamond electrodes: Challenges and trends

CO₂ electroreduction (eCO₂R) into fuel products is a promising technology to mitigate the effects of greenhouse gas emissions and store renewable energy. The main metal-based electrocatalysts widely employed in CO₂ reduction are characterized by high overpotentials, low stability, and unsatisfactory selectivity. As a result, a growing interest in the use of boron-doped diamond (BDD)-based electrocatalysts have been observed due to its excellent properties. This review sheds light on the techniques applied toward the eCO₂R on BDD surface and the effects of the operational conditions. Particular emphasis will be given on recent advances made in the quest for enhancing the performance of BDD in eCO₂R through its modification with defects insertion or functionalization with metal-based materials. The review will also present a brief overview of the challenges and directions of future research with respect to the development of different electrochemical systems for eCO₂R on BDD electrodes.     

Solar‐Assisted eBiorefinery: Photoelectrochemical Pairing of Oxyfunctionalization and Hydrogenation Reactions

Inspired by natural photosynthesis, biocatalytic photoelectrochemical (PEC) platforms are gaining prominence for the conversion of solar energy into useful chemicals by combining redox biocatalysis and photoelectrocatalysis. Herein, we report a dual biocatalytic PEC platform consisting of a molybdenum (Mo)‐doped BiVO₄ (Mo:BiVO₄) photoanode and an inverse opal ITO (IO‐ITO) cathode that gives rise to the coupling of peroxygenase and ene‐reductase‐mediated catalysis, respectively. In the PEC cell, the photoexcited electrons generated from the Mo:BiVO₄ are transferred to the IO‐ITO and regenerate reduced flavin mononucleotides to drive ene‐reductase‐catalyzed trans‐hydrogenation of ketoisophrone to (R)‐levodione. Meanwhile, the photoactivated Mo:BiVO₄ evolves H₂O₂ in situ via a two‐electron water‐oxidation process with the aid of an applied bias, which simultaneously supplies peroxygenases to drive selective hydroxylation of ethylbenzene into enantiopure (R)‐1‐phenyl‐1‐hydroxyethane. Thus, the deliberate integration of PEC systems with redox biocatalytic reactions can simultaneously produce valuable chemicals on both electrodes using solar‐powered electrons and water.     

Boron-doped diamond electrodes for the mineralization of organic pollutants in the real wastewater

The presence of recalcitrant organic compounds in wastewater poses a serious threat to the ecosystem and human health. Electrochemical advanced oxidation processes constitute a promising way for the mineralization of persistent organic compounds. They are commonly used for the transformation of organic pollutants into more biodegradable compounds or their complete removal from water. In this review, we present the recent advances in the use of boron-doped diamond (BDD) electrodes in the anodic oxidation process for the mineralization of real wastewater. First, the characteristic properties of BDD electrodes are discussed followed by the degradation mechanism. In addition, an overview on the application of BDD electrodes for mineralization of real wastewater is provided.     

Electro catalytic generation of reactive species at diamond electrodes and applications in microbial inactivation

Use of robust and safe water disinfection technologies which are inexpensive and energy-efficient are need of the hour to combat the problem of inadequate access of safe and clean drinking water. Energy and chemically intensive water treatment technologies warrant the need for a safe and environmentally sound treatment technology. Electrochemical disinfection or electrodisinfection (ED) is experiencing a great resurgence among the scientific communities owing to its novel use of electrode materials and electric current in an inexpensive and energy-efficient way for achieving the inactivation of microorganisms. Among the various electrodes used in the ED, boron-doped diamonds emerge as a sustainable alternate for their ability to electro generate strong potent oxidants which result in effective pathogen control in drinking water. ED for disinfecting waters occurs via generation of the reactive species which act in the bacterial inactivation mechanisms. In this mini-review, a critical discussion on the fundamentals and applications of promising electrochemical methods using boron-doped diamond anodes (namely electrochemical oxidation), evidencing their advantages for the remediation of drinking water infected with waterborne agents, is given.     

Theoretical Efficiency Limits of Photoelectrochemical CO2 Reduction: A Route-Dependent Thermodynamic Analysis

Solar-fuel formation via photoelectrochemical (PEC) routes using water and CO₂ as feedstock has attracted much attention. Most PEC CO₂ reduction studies have been focused on the development of novel photoactive materials; however, there is still a lack of understanding of the key limiting factors of this process. In this study, the theoretical limits of Solar-to-Fuel (STF) efficiencies of single- and dual-junction photo-absorbing materials are illustrated for single-step multi-electron CO₂ reduction into fuels including HCOO⁻, CO, CH₃OH and C₂H₅OH. It is also highlighted that STF efficiency depends on the route of two-step PEC CO₂ reduction process using CH₃OH as a model fuel. Finally, it is illustrated the beneficial role of alternative strategies such as dual-junction photo-absorbing electrodes, externally applied bias and subsequent reactor chambers on the maximum theoretical efficiencies of PEC CO₂ reduction.