Photoelectrocatalytic Efficiency Research Articles

Photoelectrochemically converting CO2 over Z-scheme heterojunction CoPc/K7HNb6O19 with high Faraday efficiency

Photoelectrochemically converting CO2 into the high value-added chemicals and fuels is a hopeful sustainable energy conversion and storage road to alleviate the energy crisis and global warming. In this study, CoN-x Z-scheme heterojunctions were prepared by coupling CoPc with K7HNb6O19 rich in oxygen vacancy with a low-temperature calcination, and used for photoelectrocatalytic (PEC) reduction of CO2 to CO. The prepared heterojunctions demonstrate Faraday efficiency of >90 % and TOF values over a wide voltage range (−0.65 ∼ −1 V vs. RHE). The superior PEC CO2 reduction performance is mainly attributed to the designed Z-scheme heterojunction, which endows a built-in electric field to the interface of the two semiconductors, benefiting the separation of electrons and holes greatly. Additionally, the oxygen vacancy from K7HNb6O19 provides an alternative pathway for CO2 adsorption and activation, and simultaneously the incorporated K7HNb6O19 inhibits the aggregation of cobalt phthalocyanine molecules, which is beneficial for the stability of PEC catalysts. Density-functional-theory (DFT) calculation also elucidates a favorable CO2 activation energy over CoPc coupling oxygen-vacancy-rich K7HNb6O19 heterojunctions. This work may provide a viable pathway for the rational designing of efficient PEC systems to reduce CO2 to valuable fuels.

Progress in Promising Semiconductor Materials for Efficient Photoelectrocatalytic Hydrogen Production.

Photoelectrocatalytic (PEC) water decomposition provides a promising method for converting solar energy into green hydrogen energy. Indeed, significant advances and improvements have been made in various fundamental aspects for cutting-edge applications, such as water splitting and hydrogen production. However, the fairly low PEC efficiency of water decomposition by a semiconductor photoelectrode and photocorrosion seriously restrict the practical application of photoelectrochemistry. In this review, the mechanisms of PEC water decomposition are first introduced to provide a solid understanding of the PEC process and ensure that this review is accessible to a wide range of readers. Afterwards, notable achievements to date are outlined, and unique approaches involving promising semiconductor materials for efficient PEC hydrogen production, including metal oxide, sulfide, and graphite-phase carbon nitride, are described. Finally, four strategies which can effectively improve the hydrogen production rate-morphological control, doping, heterojunction, and surface modification-are discussed.

Unlocking a Type-II CoO@BiVO4 Heterostructure for Wastewater Purification.

Developing a semiconductor-based heterostructure photoanode is crucial in improving the photoelectrocatalytic (PEC) efficiency for degrading refractory organic pollutants. Nevertheless, the PEC performance of the photoanodes is usually restricted by electron/hole pair recombination, oxygen evolution, and slow electron transfer. Herein, a novel CoO@BiVO4 nanowire array film (Ti/CoO@BiVO4) with n-type semiconductor characteristics was prepared via a straightforward hydrothermal method. The optimized Ti/CoO@BiVO4 electrode exhibited excellent PEC decolorization efficiency of active brilliant blue KN-R (∼92.8%) and long-term stability, outperforming recent reports. The insight reason for enhancing the PEC degradation efficiency of the Ti/CoO@BiVO4 electrodes can be attributed to the large electrochemical active area, low charge transfer resistance, and negative flat band potential. The formation of a type-II heterostructure was investigated between CoO and BiVO4 further to promote the generation and separation efficiency of electron/hole pairs, indicating that the optimized Ti/CoO@BiVO4 electrode has the potential for the water PEC degradation ability and superior service life.

Photoelectrocatalytic Processes of TiO2 Film: The Dominating Factors for the Degradation of Methyl Orange and the Understanding of Mechanism.

Various thicknesses of TiO2 films were prepared by the sol-gel method and spin-coating process. These prepared TiO2 films exhibit thickness-dependent photoelectrochemical performance. The 1.09-μm-thickTiO2 film with 20 spin-coating layers (TiO2-20) exhibits the highest short circuit current of 0.21 mAcm-2 and open circuit voltage of 0.58 V among all samples and exhibits a low PEC reaction energy barrier and fast kinetic process. Photoelectrocatalytic (PEC) degradation of methyl orange (MO) by TiO2 films was carried out under UV light. The roles of bias, film thickness, pH value, and ion properties were systematically studied because they are the four most important factors dominating the PEC performance of TiO2 films. The optimized values of bias, film thickness, and pH are 1.0 V, 1.09 μm, and 12, respectively, which were obtained according to the data of the PEC degradation of MO. The effect of ion properties on the PEC efficiency of TiO2-20 was also analyzed by using halide as targeted ions. The "activated" halide ions significantly promoted the PEC efficiency and the order was determined as Br > Cl > F. The PEC efficiency increased with increasing Cl content, up until the optimized value of 30 × 10-3 M. Finally, a complete degradation of MO by TiO2-20 was achieved in 1.5 h, with total optimization of the four factors: 1.0 V bias, 1.09-μm-thick, pH 12, and 30 × 10-3 M Cl ion content. The roles of reactive oxygen species and electric charge of photoelectrodes were also explored based on photoelectrochemical characterizations and membrane-separated reactors. Hydrogen peroxide, superoxide radical, and hydroxyl radical were found responsible for the decolorization of MO.

Construction of oxygen vacancy-rich MoO3-x and NiFe-LDH p-n heterojunction for enhanced carrier separation to promote photoelectrochemical oxidation of pollutants and hydrogen precipitation

In this work, a novel portable photoanode consisting of MoO3-x/NiFe@NF with excellent photoelectrochemical (PEC) performance and multifunctional applications was synthesized using a simple hydrothermal and electrodeposition method. In a visible-light-driven PEC system, the photoanode was able to simultaneously promote the degradation of tetracycline and the generation of hydrogen. The degradation efficiency of MoO3-x/NiFe@NF for TC (20 mg/L) was up to 99.9 %. The production of hydrogen (H2) at the cathode could be at most 32.07 μmol cm−2 within 100 min. In addition, the MoO3-x/NiFe@NF photoanode exhibited a positive bactericidal effect against Gram-negative bacteria (E. coli) which could be completely removed within 20 min under the synergistic effect of visible light and voltage. The improved photoelectrocatalytic efficiency is attributed to the built-in electric field resulting from the formation of a p-n heterojunction between MoO3-x and NiFe-LDH. Under the excitation of visible light, the built-in electric field is able to promote the separation and transmission of the photogenerated carriers. A possible mechanism of MoO3-x/NiFe@NF p-n heterojunction was proposed and discussed based on DRS, Mott-Schottky, UPS characterization and DFT theoretical calculations. Furthermore, the electrode's reusability and stability were evaluated through cycling experiments. This work provides a novel p-n heterojunction synthesis method with significant potential for wastewater treatment and future applications in ship ballast water treatment.

Heterostructured YFeO3/CeO2 as efficient photoelectrode for photoelectrochemical degradation of organic pollutants under visible light irradiation

A YFeO3/CeO2 catalyst is developed using a sol-gel procedure to overcome the recombination of photogenerated electron–hole pairs, which limits its degradation efficiency. The YFeO3/CeO2 catalyst is characterized by two distinct orthorhombic YFeO3 and CeO2 phases, with the size of YFeO3 crystal smaller than that of its parent phase. YFeO3/CeO2 with a higher CeO2 content (YC-30) reveals not only a porous morphology and hydrophilicity, but also a reduced energy bandgap (1.74 eV) that is lower than that of either single YFeO3 (1.90 eV) or CeO2 (2.86 eV). In the photocatalysis assessment of the YC-30 electrode, a high photocurrent density of up to 156.3 μA cm−2 at 1.4 V (vs. Ag/AgCl) under simulated sun irradiation is achieved. Furthermore, the YC-30 electrode achieves a favorable photoelectrocatalytic efficiency of 75.2 % under visible light irradiation for Reactive Black 5 dye degradation, which is better than those of the YFeO3 (31.9 %) and CeO2 (46.9 %) electrodes. The improved degradation performance of the YC-30 catalyst can be attributed to its smaller crystallite size, porous structure, and a certain amount of YFeO3/CeO2 heterojunction, which is conducive to the separation of electron–hole pairs owing to electron diffusion driven by the difference in electron density between YFeO3 and CeO2.

Hybrid TiO2/Carbon quantum dots heterojunction photoanodes for solar photoelectrocatalytic wastewater treatment

The modification of titanium dioxide (TiO2) is a strategy to maximize the utilization of sunlight. Carbon quantum dots (CQDs) are carbon nanomaterials with outstanding optical and electronic properties that are suitable for that purpose. In this work, three types of hybrid TiO2/CQD photoelectrodes were synthesized following different methods: 1) deposition of a CQD layer on top of TiO2 (labelled as TiO2-CQD); 2) deposition of a TiO2 layer on top of CQDs (labelled as CQD-TiO2) and; 3) deposition of a mixed CQD + TiO2 layer (labelled as CQD + TiO2). The photoelectrodes were investigated for the photoelectrocatalytic degradation of phenol as model pollutant under simulated solar light and TiO2-CQD showed the highest apparent reaction rate constant of kapp = 0.0117 min−1 with 40% of TOC removal in 6 h of treatment. CQDs were found to enhance photon absorption in the visible region of the electromagnetic spectrum and in turn phenol degradation by promoting the separation of photogenerated charge carriers through electron transfer via the Ti–O–C bonds formed at the TiO2-CQD interface. Finally, the performance of the TiO2-CQD photoanode was evaluated for the treatment of real wastewater from the membrane fabrication sector, confirming its photoelectrocatalytic efficiency under solar radiation with 93% of TOC removal in 8 h of treatment and kapp = 0.0058 min−1.

Tuning the Surface Properties of CuO Films Using the Precursor Aging Approach for Enhanced Photoelectrocatalytic Reactions

AbstractThe surface properties of semiconductors have a significant influence on their photoelectrocatalytic efficiency. This research presents a precursor aging method for tuning the surface properties of CuO films, for enhanced catalytic response. A precursor solution made using copper acetate, polyethylene glycol (PEG) 400, and diethanolamine, and aged for 1, 40, 80, 120, 150, 180, and 250 days is used in each case to fabricate CuO photocathodes via the dip‐coating method. The films fabricated using the 1‐day‐old precursor reveal compact and homogeneous nanoparticles. The films eventually get tuned to yield highly porous and rougher surfaces after aging the precursors for 180–250 days. The film's bandgap decreases by 9% after 180–250 days of precursor aging. Photocathodes prepared using the 180‐day‐old precursor produce the optimum photocurrent density of 1.6 mA cm−2 at 0.35 V versus reversible hydrogen electrode (RHE), representing a 196% increase relative to the films fabricated using the 1‐day‐old solution. They also produce an anodic onset potential shift of 260 mV. This improved photoelectrocatalytic response is due to the porous morphology of the films, which produces a larger surface area that enhances light absorption, increases active sites for catalytic reactions, and reduces charge transfer resistance at the photocathode‐electrolyte interface.

Photoelectrocatalytic Oxidation of Sulfamethazine on TiO2 Electrodes

The photoelectrocatalytic degradation and mineralization of sulfamethazine (SMT), a sulfonamide drug, were explored in aqueous solution. Working electrodes with TiO2 coatings on Ti substrates (TiO2/Ti) were used, which were produced by the dip coating method. TiO2 film electrodes were analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD) following annealing at 500 °C for 1.5 h. To photoelectrochemically characterize them, photocurrents vs. applied potential curves were used. The photoelectrocatalytic efficiency (PEC) of the TiO2/Ti electrodes regarding the oxidation of SMT has been assessed with reference to degradation and mineralization under different experimental conditions. The selected drug molecule was effectively degraded following the Langmuir–Hinshelwood (L-H) kinetic model. The degradation efficiency was shown to increase with increasing applied potential bias up to +1.5 V vs. Ag/AgCl. It was found to be more favorable in acidic environments compared to alkaline ones. A decrease in the destruction rate constant was recorded when the pH was increased from 3 to 5.6 (natural pH) and 9. The decomposition rate was shown to first increase and subsequently reach a saturation value at high concentrations of SMT, indicating that the degradation also depends on other parameters (e.g., the rate of the charge or the mass transfer on the electrode double layer). The results of the photoelectrocatalytic experiments were compared to those of electrochemical (EC) and photocatalytic (PC) degradation of SMT. A significant enhancement was recorded in the case of the PEC degradation, leading at +1.5 V to an increase of the apparent rate constants of degradation, k, and mineralization, kTOC, of 153 and 298%, respectively, compared to the simple photocatalytic process.

Achieving High-Efficient Photoelectrocatalytic Degradation of 4-Chlorophenol via Functional Reformation of Titanium-Oxo Clusters

Rational design of crystalline catalysts with superior light absorption and charge transfer for efficient photoelectrocatalytic (PEC) reaction coupled with energy recovery remains a great challenge. In this work, we elaborately construct three stable titanium-oxo clusters (TOCs, Ti10Ac6, Ti10Fc8, and Ti12Fc2Ac4) modified with a monofunctionalized ligand (9-anthracenecarboxylic acid (Ac) or ferrocenecarboxylic acid (Fc)) and bifunctionalized ligands (Ac and Fc). They have tunable light-harvesting and charge transfer capacities and thus can serve as outstanding crystalline catalysts to achieve efficient PEC overall reaction, that is, the integration of anodic organic pollutant 4-chlorophenol (4-CP) degradation and cathodic wastewater-to-H2 conversion. These TOCs can all exhibit very high PEC activity and degradation efficiency of 4-CP. Especially, Ti12Fc2Ac4 decorated with bifunctionalized ligands exhibits better PEC degradation efficiency (over 99%) and H2 generation than Ti10Ac6 and Ti10Fc8 modified with a monofunctionalized ligand. The study of the 4-CP degradation pathway and mechanism revealed that such better PEC performance of Ti12Fc2Ac4 is probably due to its stronger interactions with the 4-CP molecule and better •OH radical production. This work not only presents the effective combination of organic pollutant degradation and simultaneously H2 evolution reaction using crystalline coordination clusters as both anodic and cathodic catalyst but also develops a new PEC application for crystalline coordination compounds.

Efficient Bias-Free Degradation of Sulfamethazine by TiO2 Nanoneedle Arrays Photoanode and Co3O4 Photocathode System under LED-Light Irradiation

Solving high electrical-energy input for pollutants degradation is one of the core requirements for the practical application of photoelectrocatalytic (PEC) technology. Herein, we developed a self-driven dual-photoelectrode PEC system (TiO2 NNs-Co3O4) composed of a TiO2 nanoneedle arrays (TiO2 NNs) photoanode and Co3O4 photocathode for the first time. Under light-emitting-diode (LED) illumination, the bias-free TiO2 NNs-Co3O4 PEC system exhibited excellent PEC performance, with an internal bias as high as 0.19 V, achieving near complete degradation (99.62%) of sulfamethazine (SMT) with a pseudo-first-order rate constant of 0.042 min−1. The influences of solution pH, typical inorganic anions, natural organic matter, and initial SMT concentration on the PEC performance were investigated. Moreover, the main reactive oxygen species (h+, •OH, •O2−) in the dual-photoelectrode PEC system for SMT decomposition were elaborated. The practical application feasibility for efficient water purification of this unbiased PEC system was evaluated. It was proved that the TiO2 NNs photoanode provided a negative bias while the Co3O4 photocathode provided a positive bias for the photoanode, which made this system operate without external bias. This work elucidated the cooperative mechanism of photoelectrodes, providing guidance to develop a sustainable, efficient, and energy-saving PEC system for wastewater treatment.

CQDs improved the photoelectrocatalytic performance of plasma assembled WO3/TiO2-NRs for bisphenol A degradation.

Carbon quantum dots (CQDs) have been supported on WO3/TiO2-NRs using a hydrothermal method and a novel CQDs/WO3/TiO2-NRs composite formed via dielectric barrier discharge. The composite electrodes were characterized using morphology, structural, optical and electrochemical analysis. The CQDs were successfully prepared on the composite electrode with the highest photocurrent density reaching 2.51mA·cm-2 under UV-visible light irradiation (100mW·cm-2) and an applied voltage of 0.6V vs. Ag/AgCl. The CQDs/WO3/TiO2-NRs electrode exhibited a good degradation effect toward bisphenol A (BPA) (75.66 %) combined with the production of hydrogen (0.89mmol) in Na2SO4 system after 2h of the photoelectrocatalytic (PEC) reaction and the BPA degradation rate reached 100 % after 7min of reaction in both simulated and real seawater. The CQDs/WO3/TiO2-NRs exhibited excellent stability and efficient PEC performance in which the CQDs acted as electron reservoirs to capture and promote charge separation. Our analysis of intermediates of BPA degradation indicated the possible degradation pathways that mainly formed BPA polymers in the Na2SO4 system or chlorinated compounds in the high chloride salt system.

2D and 3D Nanomaterials for Photoelectrocatalytic Removal of Organic Pollutants from Water

AbstractThe presence of organics in water poses a serious risk to the environment, as well as to human health. It is important to develop effective water treatment techniques that are affordable, durable, and sustainable. The removal and degradation of organic matter in water is widely explored using 2D and 3D nanomaterials, which are advantageous in providing high surface to volume ratios and favorably photoelectro activity as compared to traditional materials. The robustness of nano semiconductors has made it highly sustainable in water treatment. This review concentrates on nanostructured materials, which are demonstrating the removal of pollutants from water using various 2D and 3D nanostructured materials as photoelectrocatalytic electrodes. The 2D and 3D nanostructured materials are divided into five types according to physical morphology: nanofilms/nanosheets, nanopore structure/nanonetwork, nanoflowers, nanotubes, and other forms. Apart from this, the preparation method, characterization technology, and photoelectrocatalytic efficiency of organic pollutants degradation are reviewed. Finally, the different factors influencing degradation of organic pollutants are discussed.

Multipurpose properties the Z-scheme dimanganese copper oxide/cadmium sulfide nanocomposites for photo- or photoelectro-catalytic, antibacterial applications, and thiamine detection process

Cadmium sulfide improved with dimanganese copper oxide (Mn2CuO4/CdO) was prepared and applied this semiconductor nanocomposites in advanced oxidative processes via the photoelectrocatalytic efficiency for cephalexin degradation. The Z-scheme Mn2CuO4/CdO nanocomposites was characterized by FESEM, XPS, UV–vis, EIS, and XRD. The photocurrent response were conducted in to evaluate the photo generated in the degradation reaction. Mn2CuO4/CdO was obtained to displays better photoelectrocatalytic response compared to bare CdO; this result shows the role of other semiconductor oxide material as photoanode construction. The degradation of cephalexin in photoelectrocatalytic response is higher than the photocatalytic process. A possible catalysis mechanism was depicted using free radicals scavengers. It was observed that hydroxyl radical and holes contribute as the main role in antibiotic degradation. The Mn2CuO4/CdO was used as great antibacterial agent versus Gram+/−. The Mn2CuO4/CdO probe was used to determine of vitamin B1 in peroxidase activity. The limit of detection was 41.21 nM.

Electrically conductive TiO2/CB/PVDF membranes for synchronous cross-flow filtration and solar photoelectrocatalysis

Combining photocatalysis (PC) and membrane filtration (MF) has emerged as an attractive technology for water purification, however, the water purification efficiency and membrane fouling are still challenging. Herein, we report a novel photoelectrocatalytic (PEC) membrane mediated by a ternary polyvinylidene fluoride (PVDF)-carbon black (CB)–TiO2 composite conductive membrane synthesized by a phase inversion method assisted by the mixed surfactants of polyvinylpyrrolidone (PVP) and sodium dodecyl sulfate (SDS). The resultant electrically conductive TiO2/CB/PVDF membrane features a homogeneous surface with obvious pore size of 20–150 nm, a thickness ∼116 μm, and an average resistivity as low as ∼3.165 Ω∙m. The cooperation of PVP and SDS surfactants dramatically improves the organic-inorganic interactions and thus eventually enhances the porosity, stability of porous structure, mechanical stability, and conductivity and electrochemical properties of the hybrid membrane. Upon the solvent evaperation of the wellblended casting solution and the phase inversion, TiO2/CB preferentially exist on the surface of PVDF membrane, enabling the efficient PEC degradation of organic pollutants. The synergistic coupling of TiO2 and CB in PVDF membrane results in efficient PEC properties with bi-functional membrane antifouling and enhanced water purification in azo dyes decolorization under the stationary mode and in our lab-made continuous cross-flow PEC system, superior to those by photocatalysis and electrocatalysis. The developed synchronous MF and PEC system mediated by the conductive TiO2/CB/PVDF membrane proves to a feasible route to improving the self-cleaning properties of the polymer membrane while simultaneously increasing the water decontaminating efficiency.

α-Fe2O3/Cu2O composites as catalysts for photoelectrocatalytic degradation of benzotriazoles

Given the difficulties of degrading benzotriazole (BTA), this study used a one-pot hydrothermal method to prepare α-Fe2O3/Cu2O (FC) composites for photoelectrocatalytic (PEC) degradation of BTA. The characterization of FC structure showed that Cu2O in cubic crystals was loaded with circular sheets of Fe2O3. Owing to this structure, FC showed efficient PEC degradation of BTA when exposed to ultraviolet light. The experimental results demonstrated that FC efficiently degraded BTA. When the PEC degradation continued for 60 min, 100% degradation of BTA was achieved because FC enhanced the photoelectron-hole separation and the separation and transfer of articulated carriers. High performance liquid chromatography–mass spectrometry showed that intermediates formed during the PEC degradation of BTA. Finally, various pathways for degradation of BTA were postulated. This FC-based PEC system provides a harmless and effective method for degradation of BTA.

Enhanced Photoelectrocatalytic Activity of TiO2 Nanowire Arrays via Copolymerized G-C3N4 Hybridization

Photoelectrocatalytic (PEC) oxidation is an advanced technology that combines photocatalytic oxidation (PC) and electrolytic oxidation (EC). PEC activity can be greatly enhanced by the PC and EC synergy effect. In this work, novel copolymerized g-C3N4 (denoted as CNx)/TiO2 core-shell nanowire arrays were prepared by chemical vapor deposition. CNx were deposited on the surface of TiO2 nanowire arrays using organic monomer 4,5-dicyanidazole and dicyandiamide as copolymerization precursor. TiO2 nanowire arrays provide a direct and fast electron transfer path, while CNx is a visible light responsive material. After CNx deposition, the light response range of TiO2 is broadened to 600 nm. The deposition of CNx shell effectively improves the PC efficiency and PEC efficiency of TiO2. Under visible light irradiation and 1 V bias potential, the rate constant k of PEC degradation of CNx/TiO2 core-shell nanowire arrays is 0.0069 min−1, which is 72% higher than that of pure TiO2 nanowires. The built-in electric field formed in the interface between TiO2 core and CNx shell would effectively promote photogenerated charge separation and PEC activity.

Atomically-thin Schottky-like photo-electrocatalytic cross-flow membrane reactors for ultrafast remediation of persistent organic pollutants

The remediation of persistent organic pollutants in surface and ground water represents a major environmental challenge worldwide. Conventional physico-chemical techniques do not efficiently remove such persistent organic pollutants and new remediation techniques are therefore required. Photo-electro catalytic membranes represent an emerging solution that can combine photocatalytic and electrocatalytic degradation of contaminants along with molecular sieving. Herein, macro-porous photo-electro catalytic membranes were prepared using conductive and porous stainless steel metal membranes decorated with nano coatings of semiconductor photocatalytic metal oxides (TiO2 and ZnO) via atomic layer deposition, producing highly conformal and stable coatings. The metal - semiconductor junction between the stainless steel membranes and photocatalysts provides Schottky - like characteristics to the coated membranes. The PEC membranes showed induced hydrophilicity from the nano-coatings and enhanced electro-chemical properties due to the Schottky junction. A high electron transfer rate was also induced in the coated membranes as the photocurrent efficiency increased by 4 times. The photo-electrocatalytic efficiency of the TiO2 and ZnO coated membranes were demonstrated in batch and cross flow filtration reactors for the degradation of persistent organic pollutant solution, offering increased degradation kinetic factors by 2.9 and 2.3 compared to photocatalysis and electrocatalysis, respectively. The recombination of photo-induced electron and hole pairs is mitigated during the photo-electrocatalytic process, resulting in an enhanced catalytic performance. The strategy offers outstanding perspectives to design stimuli-responsive membrane materials able to sieve and degrade simultaneously toxic contaminants towards greater process integration and self-cleaning operations.

Enhancing solar-driven photoelectrocatalytic efficiency of Au nanoparticles with defect-rich hydrogenated TiO2 toward ethanol oxidation

It is significant to promote the efficiency of solar fuel cells via improving the utilization of solar in photoelectrocatalysis. In this work, hydrogenated TiO2 (H-TiO2) nanoparticles with Ti3+ and/or oxygen vacancy (VO) defects were obtained and have extended light absorption to visible light region. As a photochemical component, H-TiO2 was further coupled with Au nanoparticles to assemble photoelectrocatalyst to apply in photo-assisted electrooxidation of ethanol. Due to the existence of defects in H-TiO2, the peak current density on Au/H-TiO2 is improved by 5.4% with the assistance of light irradiation, and even up to 18.3% under simulated sunlight illumination, which is much higher than that on Au/W-TiO2 (W-TiO2 loaded Au) under the light irradiation. Meanwhile, it is demonstrated that the charge transfer is improved on Au/H-TiO2 with Vo due to the existence of Vo, improving the separation of excited electrons and holes. Density function theory (DFT) based theoretical calculations illustrate that the energy barrier of reaction intermediates on Au-TiO2 with Vo defects is lower than that without Vo, indicating that Au-TiO2 with Vo is more energetically favorable. This work highlights the possibility of using defective semiconductor nanomaterials to combine photocatalysis and electrocatalysis in solar fuel cells and to further improve the efficiency of fuel cells under solar full spectrum.

Indirect charge transfer of holes via surface states in ZnO nanowires for photoelectrocatalytic applications

In this work, ZnO nanowires with high aspect ratio were obtained by fast and simple electrochemical anodization. Morphological, structural and photoelectrochemical characteristics of the synthesized ZnO nanowires were evaluated by using different techniques: field emission scanning electron microscopy, atomic force microscopy, high resolution transmission electron microscopy, Raman spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, UV–VIS spectroscopy, Mott-Schottky analysis and photoelectrochemical impedance spectroscopy. The synthesized ZnO nanowires presented high roughness and high crystallinity. Besides, surface defects were identified in the sample. The value of the donor density (ND) was in the order of 1019 cm−3 in the dark and 1020 cm−3 under illumination. In addition, the ZnO nanowires presented good photosensibility, with a photocurrent density response 85 times higher than a ZnO compact layer, and lower resistance to charge transfer. The charge transfer processes taking place at the ZnO/electrolyte interface were studied, since these processes strongly influence the photoelectrocatalytic efficiency of the material. According to the results, the charge transfer of holes in the synthesized ZnO nanowires occurs indirectly via surface states. In this regard, surface states may be an important feature for photoelectrocatalytic applications since they could provide lower onset voltages and higher anodic current densities.