Photoelectrocatalytic Systems Research Articles

Level Alignment as Descriptor for Semiconductor/Catalyst Systems in Water Splitting: The Case of Hematite/Cobalt Hexacyanoferrate Photoanodes.

The realization of artificial photosynthesis may depend on the efficient integration of photoactive semiconductors and catalysts to promote photoelectrochemical water splitting. Many efforts are currently devoted to the processing of multicomponent anodes and cathodes in the search for appropriate synergy between light absorbers and active catalysts. No single material appears to combine both features. Many experimental parameters are key to achieve the needed synergy between both systems, without clear protocols for success. Herein, we show how computational chemistry can shed some light on this cumbersome problem. DFT calculations are useful to predict adequate energy-level alignment for thermodynamically favored hole transfer. As proof of concept, we experimentally confirmed the limited performance enhancement in hematite photoanodes decorated with cobalt hexacyanoferrate as a competent water-oxidation catalyst. Computational methods describe the misalignment of their energy levels, which is the origin of this mismatch. Photoelectrochemical studies indicate that the catalyst exclusively shifts the hematite surface state to lower potentials, which therefore reduces the onset for water oxidation. Although kinetics will still depend on interface architecture, our simple theoretical approach may identify and predict plausible semiconductor/catalyst combinations, which will speed up experimental work towards promising photoelectrocatalytic systems.

Artificial Photosynthetic Systems for CO2 Reduction: Progress on Higher Efficiency with Cobalt Complexes as Catalysts.

The conversion of CO2 into fuels or value-added chemicals is currently a field of great research interest. Molecular cobalt catalysts have long been used as mediators of reductive transformations of CO2 . In this Minireview, the cobalt complex-based photocatalytic and photoelectrocatalytic systems for CO2 reduction are discussed and summarized, alongside progress on the design of new molecular cobalt catalysts and their performance in photocatalysis.

Unveiling the photoelectrocatalytic inactivation mechanism of Escherichia coli: Convincing evidence from responses of parent and anti-oxidation single gene knockout mutants

This study investigated photoelectrocatalytic (PEC) inactivation mechanism of bacteria using parental Escherichia coli (E. coli) BW25113 and its isogenic mutants deficient in catalase HPI (katG−, JW3914-1) and Mn-SOD (sodA−, JW3879-1). BW25113 in the mid-log phase was less susceptible to PEC inactivation than those in early-log and stationary phases, consistent with the peak activities of catalase and superoxide dismutase (SOD) at mid-log phase (30.6 and 13.0 Unit/ml/OD600). For different strains all in mid-log phase, PEC inactivation efficiency followed the order katG− > sodA− > BW25113, with the duration of 60, 60 and 90 min for complete inactivation of ∼2 × 107 CFU mL−1 bacteria, respectively. Correspondingly, catalase and SOD levels of BW25113 were also higher than the mutants by 5.9 and 11.7 Unit/mL/OD600, respectively. Reactive oxygen species (ROSs) concentrations in PEC systems revealed that the inactivation performance coincided with H2O2 levels, rather than OH. Moreover, pre-incubation with H2O2 elevated catalase activities and PEC inactivation resistance of BW25113 were positively correlated. The above results indicated that H2O2 was the dominant PEC generated bactericide, and anti-oxidative enzymes especially catalase contributed greatly to the bacterial PEC resistance capacity. Further tests revealed that PEC treatment raised the intracellular ROSs concentration by more than 3 times, due to the permeated H2O2 and its intracellular derivative, OH. However, oxidative stress response of E. coli, such as increased catalase or SOD were not observed, perhaps because the ROSs overwhelmed the bacterial protective capacity. The accumulated ROSs subsequently caused oxidative damages to E. coli cells, including membrane damage, K+ leakage, and protein oxidation. Compared with BW25113, the mutants experienced damages earlier and at higher levels, confirming the essential roles of catalase and SOD in the bacterial PEC resistance.

Synergetic effect enhanced photoelectrocatalysis.

We report synergetic effect enhanced photoelectrocatalysis, in which Fe(3+) and Br(-) are used as the acceptors of photogenerated charges on TiO2 nanoparticles. The kinetic rate of interfacial charge transfer is promoted from (4.0 ± 0.5) × 10(-4) cm s(-1) (TiO2/(O2, Br(-))) to (1.5 ± 0.5) × 10(-3) cm s(-1) (TiO2/(Fe(3+), Br(-))). The synergetic effect provides a valuable approach to the design of photoelectrocatalytic systems.

Unraveling the charge transfer/electron transport in mesoporous semiconductive TiO2 films by voltabsorptometry.

In this work, we demonstrate that chronoabsorptometry and more specifically cyclic voltabsorptometry are particularly well suited techniques for acquiring a comprehensive understanding of the dynamics of electron transfer/charge transport within a transparent mesoporous semiconductive metal oxide film loaded with a redox-active dye. This is illustrated with the quantitative analysis of the spectroelectrochemical responses of two distinct heme-based redox probes adsorbed in highly-ordered mesoporous TiO2 thin films (prepared from evaporation-induced self-assembly, EISA). On the basis of a finite linear diffusion-reaction model as well as the establishment of the analytical expressions governing the limiting cases, it was possible to quantitatively analyse, predict and interpret the unusual voltabsorptometric responses of the adsorbed redox species as a function of the potential applied to the semiconductive film (i.e., as a function of the transition from an insulating to a conductive state or vice versa). In particular, we were able to accurately determine the interfacial charge transfer rates between the adsorbed redox species and the porous semiconductor. Another important and unexpected finding, inferred from the voltabsorptograms, is an interfacial electron transfer process predominantly governed by the extended conduction band states of the EISA TiO2 film and not by the localized traps in the bandgap. This is a significant result that contrasts those previously observed for dye-sensitized solar cells formed of randomly sintered TiO2 nanoparticles, a behaviour that was ascribed to a particularly low density of localized surface states in EISA TiO2. The present methodology also provides a unique and straightforward access to an activation-driving force relationship according to the Marcus theory, thus opening new opportunities not only to investigate the driving-force effects on electron recombination dynamics in dye-sensitized solar cells but also to study the electron transfer/transport mechanisms in heterogeneous photoelectrocatalytic systems combining nanostructured semiconductor electrodes and heterogeneous redox-active catalysts.

Comparative study on the photoelectrocatalytic inactivation of Escherichia coli K-12 and its mutant Escherichia coli BW25113 using TiO2 nanotubes as a photoanode

Photoelectrocatalytic (PEC) and photocatalytic (PC) inactivation of ancestor Escherichia coli K-12 and its mutant E. coli BW25113 were systematically compared using a TiO2 nanotubular photoanode. The results showed that PEC inactivation was more effective to both bacterial strains than PC process, and E. coli BW25113 showed higher resistance than E. coli K-12 in both PEC and PC systems. The findings indicate that the two strains with different genes are varied in their susceptibilities and responses to the PEC and PC treatments. The h+ was found to be the major reactive species and predominantly responsible for PEC inactivation. Scanning electron microscopy images demonstrated that the cells were severely damaged and resulted in a leakage of the intracellular components during PEC inactivation process. For a given bacterial strain, no significant effect was found on the PEC inactivation efficiency as different electrolytes were employed. However, in the presence of NaCl or NaBr, PEC inactivation efficiencies of both strains were remarkably enhanced. This phenomenon can be attributed to the efficient formation of halide and dihalide radical anions during PEC process. The different efficiencies of the two bacterial strains under same conditions can be ascribed to their different abilities to resist the inactivation of bacterial strains with different genotypes.

Comparative studies of photocatalytic and photoelectrocatalytic inactivation of E. coli in presence of halides

The bactericidal performances of photocatalytic and photoelectrocatalytic systems in absence and presence of low concentrations of X− (X=Br, Cl) were quantitatively studied and meaningfully compared under identical experimental conditions. The photoelectrocatalytic system in presence of low concentration of Br− was found to be the most effective bactericidal system amongst all systems investigated, capable of inactivating 100% of 9.0x106CFU/mL Escherichia coli within 1.57s. The photocatalytically generated active oxygen species are important attributes to the bactericidal performance of all cases investigated except the case of photoelectrocatalytic inactivation in presence of Br− for which the bactericidal performance is dominated by the photoelectrocatalytically generated Br−/Br2−. The results revealed that in a photocatalysis or photoelectrocatalysis bactericidal process, X− acts as an electron mediator that does not change its chemical form before and after bactericidal process, and a high bactericidal performance photoelectrocatalytic system can be established with Br− concentration as low as 50μM. The findings of this work confirm that new forms of effective bactericides such as X−/X2− can be generated in situ via a photocatalysis or photoelectrocatalysis process for high performance bactericidal system.Keywords: Biohazards; Disinfection; Br−/Br2−; Photocatalysis; Photoelectrocatalysis

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.

Unraveling the Mechanism of Catalytic Reduction of O2 by Microperoxidase-11 Adsorbed within a Transparent 3D-Nanoporous ITO Film

Nanoporous films of indium tin oxide (ITO), with thicknesses ranging from 250 nm to 2 μm, were prepared by Glancing Angle Deposition (GLAD) and used as highly sensitive transparent 3D-electrodes for quantitatively interrogating, by time-resolved spectroelectrochemistry, the reactivity of microperoxidase-11 (MP-11) adsorbed within such films. The capacitive current densities of these 3D-electrodes as well as the amount of adsorbed MP-11 were shown to be linearly correlated to the GLAD ITO film thickness, indicating a homogeneous distribution of MP-11 across the film as well as homogeneous film porosity. Under saturating adsorption conditions, MP-11 film concentration as high as 60 mM was reached. This is equivalent to a stack of 110 monolayers of MP-11 per micrometer film thickness. This high MP-11 film loading combined with the excellent ITO film conductivity has allowed the simultaneous characterization of the heterogeneous one-electron transfer dynamics of the MP-11 Fe(III)/Fe(II) redox couple by cyclic voltammetry and cyclic voltabsorptometry, up to a scan rate of few volts per second with a satisfactory single-scan signal-to-noise ratio. The potency of the method to unravel complex redox coupled chemical reactions was also demonstrated with the catalytic reduction of oxygen by MP-11. In the presence of O(2), cross-correlation of electrochemical and spectroscopic data has allowed us to determine the key kinetics and thermodynamics parameters of the redox catalysis that otherwise could not be easily extracted using conventional protein film voltammetry. On the basis of numerical simulations of cyclic voltammograms and voltabsorptograms and within the framework of different plausible catalytic reaction schemes including appropriate approximations, it was shown possible to discriminate between different possible catalytic pathways and to identify the relevant catalytic cycle. In addition, from the best fits of simulations to the experimental voltammograms and voltabsorptograms, the partition coefficient of O(2) for the ITO film as well as the values of two kinetic rate constants could be extracted. It was finally concluded that the catalytic reduction of O(2) by MP-11 adsorbed within nanoporous ITO films occurs via a 2-electron mechanism with the formation of an intermediate Fe(III)-OOH adduct characterized by a decay rate of 11 s(-1). The spectroelectroanalytical strategy presented here opens new opportunities for characterizing complex redox-coupled chemical reactions not only with redox proteins, but also with redox biomimetic systems and catalysts. It might also be of great interest for the development and optimization of new spectroelectrochemical sensors and biosensors, or eventually new photoelectrocatalytic systems or biofuel cells.

A comparative study between photocatalytic and photoelectrocatalytic properties of Pt deposited TiO 2 thin films for glucose degradation

This study compares the photocatalytic (PC) and photoelectrocatalytic (PEC) oxidation of glucose by Pt–TiO 2 films under similar conditions. The overall oxidation efficiency of the PEC process was better than the PC process, for both TiO 2 and Pt–TiO 2 films. The Pt deposits enhanced the PEC oxidation efficiency of glucose at low cathodic (−0.1 V) and low anodic potential bias (+0.1 V). As the applied anodic bias was increased to >+0.5 V, the undoped-TiO 2 films outperformed their Pt–TiO 2 counterparts. This suggests that in the studied systems, at an anodic bias of >+0.5 V, more holes were available for the oxidation of glucose on the surface of TiO 2 compared to Pt–TiO 2 films. In the PEC systems, above a certain applied bias, Pt deposits may have a detrimental effect on the performance of a PEC system because the Pt deposits could merely block the surface of TiO 2 (particularly at the highest Pt loadings). Lower measured photocurrents from Pt–TiO 2 films compared to undoped-TiO 2 and a decrease in the photocurrent as the Pt loading increased corroborate this postulation.

Photoelectrocatalysis: principles, nanoemitter applications and routes to bio-inspired systems

An overview on processes that are relevant in light-induced fuel generation, such as water photoelectrolysis or carbon dioxide reduction, is given. Considered processes encompass the photophysics of light absorption, excitation energy transfer to catalytically active sites and interfacial reactions at the catalyst/solution phase boundary. The two major routes envisaged for realization of photoelectrocatalytic systems, e.g. bio-inspired single photon catalysis and multiple photon inorganic or hybrid tandem cells, are outlined. For development of efficient tandem cell structures that are based on non-oxidic semiconductors, stabilization strategies are presented. Physical surface passivation is described using the recently introduced nanoemitter concept which is also applicable in photovoltaic (solid state or electrochemical) solar cells and first results with p-Si and p-InP thin films are presented. Solar-to-hydrogen efficiencies reach 12.1% for homoepitaxial InP thin films covered with Rh nanoislands. In the pursuit to develop biologically inspired systems, enzyme adsorption onto electrochemically nanostructured silicon surfaces is presented and tapping mode atomic force microscopy images of heterodimeric enzymes are shown. An outlook towards future envisaged systems is given.

Effect of pH, charge separation and oxygen concentration in photoelectrocatalytic systems: active chlorine production and chlorate formation

In this study, photoanodic chlorine generation and chlorate formation were investigated using a nanoporous TiO 2 photo-electrode under an illumination intensity of 7.4 mW/cm 2 UV and biased at +1.0 V vs. SCE. Chlorine formation increased over time finally reaching a steady-state concentration of 42 mg/l in 45 min. On the other hand, chlorate formation was observed after only 5 min irradiation and increased over time. The effect of pH, oxygen concentration and charge separation on chlorine generation and chlorate formation was also investigated. Chlorine production was practically constant over a pH range of 4.0–8.0, but decreased slightly at higher pH values. Chlorate production could be detected at pH values ≥7.0. A significant effect of oxygen concentration on chlorate formation was observed. Chlorate was significantly reduced under nitrogen gas. Chlorine production in the separated photoanode compartment was about 10 fold higher than that in a combined photoanode-cathode system.

Photocatalytic and Photoelectrocatalytic Performance of 1% Pt Doped TiO2for the Detoxification of Water

The degradation of formic acid (HCOOH), FA (a surrogate contaminant) using titanium dioxide (TiO2) and 1% Pt doped TiO2 electrodes, prepared by sol—gel methods, was investigated in a photoelectrocatalytic (PEC) system in order to determine the effect of Pt doping on the oxidation potential of TiO2. Pt doping shifts the position of band edge and therefore the direct and indirect oxidation potentials of TiO2 in PEC systems. As a result, the degradation of formic acid via the generation of hydrogen peroxide production on 1%Pt—TiO2 electrodes was much better than that on non-doped electrodes. The degradation of HCOOH was also examined with respect to the faradaic efficiency of this process. It was found that the 1%Pt—TiO2 photoanode had a 30% higher efficiency than that of non-doped TiO2 photoanodes.