Effect of different hole scavengers on the photoelectrochemical properties and photocatalytic hydrogen evolution performance of pristine and Pt-decorated TiO2 nanotubes

Role of Nb2O5 Crystal Phases on the Photocatalytic Conversion of Lignin Model Molecules and Selectivity for Value-Added Products

The photocatalytic conversion in aqueous media of phenol and guaiacol as a lignin model compound using Nb2O5 with different crystal phases was studied. Nb2O5 particles were synthesized using hydrothermal methods, where it was observed that changes in the solvent control their morphology and crystal phase. Different photocatalytic behavior of Nb2O5 was observed with the selected model compounds, indicating that its selection directly impacts the resulting conversion and selectivity rates as well as the reaction pathway, highlighting the relevance of model molecule selection. Photocatalytic conversion of phenol showed conversion rate (C%) up to 25 % after 2 h irradiation and high selectivity (S%) to pyrogallol (up to 50 %). Orthorhombic Nb2O5 spheres favored conversion through free hydroxyl radicals while monoclinic rods did not convert phenol. Guaiacol photocatalytic oxidation showed high conversion rate but lower selectivity. Orthorhombic and monoclinic Nb2O5 favored the formation of resorcinol with S % ~0.43 % (C % ~33 %) and ~13 % (C % ~27 %) respectively. The mixture of both phases enhanced the guaiacol conversion rate to ~55 % with ~17 % of selectivity to salicylaldehyde. The use of radical scavengers provided information to elucidate the reaction pathway for these model compounds, showing that different reaction pathways may be obtained for the same photocatalyst if the model compound is changed.

"Core/Shell" Nanocomposites as Photocatalysts for the Degradation of the Water Pollutants Malachite Green and Rhodamine B

"Core/shell" composites are based on a ferrite core coated by two layers with different properties, one of them is an isolator, SiO2, and the other is a semiconductor, TiO2. These composites are attracting interest because of their structure, photocatalytic activity, and magnetic properties. Nanocomposites of the "core/shell" МFe2O4/SiO2/TiO2 (М = Zn(II), Co(II)) type are synthesized with a core of MFe2O4 produced by two different methods, namely the sol-gel method (SG) using propylene oxide as a gelling agent and the hydrothermal method (HT). SiO2 and TiO2 layer coating is performed by means of tetraethylorthosilicate, TEOS, Ti(IV) tetrabutoxide, and Ti(OBu)4, respectively. A combination of different experimental techniques is required to prove the structure and phase composition, such as XRD, UV-Vis, TEM with EDS, photoluminescence, and XPS. By Rietveld analysis of the XRD data unit cell parameters, the crystallite size and weight fraction of the polymorphs anatase and rutile of the shell TiO2 and of the ferrite core are determined. The magnetic properties of the samples, and their activity for the photodegradation of the synthetic industrial dyes Malachite Green and Rhodamine B are measured in model water solutions under UV light irradiation and simulated solar irradiation. The influence of the water matrix on the photocatalytic activity is determined using artificial seawater in addition to ultrapure water. The rate constants of the photocatalytic process are obtained along with the reaction mechanism, established using radical scavengers where the role of the radicals is elucidated.

NiAlFe LTH /MoS2 p-n junction heterostructure composite as an effective visible-light-driven photocatalyst for enhanced degradation of organic dye under high alkaline conditions

Designing of an effectual heterostructure photocatalyst for catalytic organic pollutant exclusion has been the subject of rigorous research intended to resolve the related environmental aggravation. Fabricating p-n junctions is an effective strategy to promote electron-hole separation of semiconductor photocatalysts as well as enhance the organic toxin degradation performance. In this study, a series of n-type NiAlFe-layered triple hydroxide (LTH) loaded with various ratios of p-type MoS2 was synthesized for forming a heterostructure LTH/MoS2 (LMs) by an in situ hydrothermal strategy. The photocatalysts were characterized by XRD, SEM&EDX, TEM, FT-IR, XPS, as well as UV-vis DRS. The photoactivity of photocatalysts was tested by the degradation of Indigo Carmine (IC) dye. The optimized catalyst (LM1) degrades 100% of indigo dye in high alkaline pH under UV light for 100 min. Besides, the degradation rate of LM1 is 15 times higher than that of pristine NiAlFe-LTH. The enhanced photoactivity is attributed to the synergistic effect between NiAlFe-LTH and MoS2 as well as the p-n junction formation.

Hole-scavenging in photo-driven N2 reduction catalyzed by a CdS-nitrogenase MoFe protein biohybrid system

The light-driven reduction of dinitrogen (N2) to ammonia (NH3) catalyzed by a cadmium sulfide (CdS) nanocrystal‑nitrogenase MoFe protein biohybrid is dependent on a range of different factors, including an appropriate hole-scavenging sacrificial electron donor (SED). Here, the impact of different SEDs on the overall rate of N2 reduction catalyzed by a CdS quantum dot (QD)-MoFe protein system was determined. The selection of SED was guided by several goals: (i) molecules with standard reduction potentials sufficient to reduce the oxidized CdS QD, (ii) molecules that do not absorb the excitation wavelength of the CdS QD, and (iii) molecules that could be readily reduced by sustainable processes. Earlier studies utilized buffer molecules or ascorbic acid as the SED. The effectiveness of ascorbic acid as SED was compared to dithionite (DT), triethanolamine (TEOA), and hydroquinone (HQ) across a range of concentrations in supporting N2 reduction to NH3 in a CdS QD-MoFe protein photocatalytic system. It was found that TEOA supported N2 reduction rates comparable to those observed for dithionite and ascorbic acid. HQ was found to support significantly higher rates of N2 reduction compared to the other SEDs at a concentration of 50 mM. A comparison of the rates of N2 reduction by the biohybrid complex to the standard reduction potential (Eo) of the SEDs reveals that Eo is not the only factor impacting the efficiency of hole-scavenging. These findings reveal the importance of the SED properties for improving the efficiency of hole-scavenging in the light-driven N2 reduction reaction catalyzed by a CdS QD-MoFe protein hybrid.

Efficient As(III) removal from water by ZrO2 modified covalent organic framework under visible light irradiation

Adsorption-oxidation is a promising technique to decontaminate As(III) polluted water. In present study, ZrO2-modified covalent organic framework (ZrO2-COF) was fabricated and used to remove arsenic from water under visible light irradiation. The results showed that ZrO2-COF (0.2 g/L) could efficiently capture As(III) (5 mg/L) from water and then oxidize the adsorbed As(III) into less toxic As(V) under visible light irradiation (60 min), achieving the complete decontamination of As(III) polluted water. Based on characterization results and theoretical calculations, we found that in ZrO2-COF composite, ZrO2 served as sites for adsorption of As(III)/the latter transformed As(V), while COF worked as photocatalytic center for As(III) oxidation. Effective As(III) removal could also be achieved by ZrO2-COF under visible light irradiation in complex water chemistry conditions including wide solution pH range (3-11), broad solution ion strength range (1-100 mM), the copresence of natural organic matter (0.1-1 mg/L humic acid) and various coexisting ions in solutions, as well as in real water samples. In addition, we found that ZrO2-COF had excellent reuse performance in 4 consecutive cycles. Our results showed that under visible light irradiation, ZrO2-COF composites could be a promising technique for efficient As(III) removal from water.

Highly efficient ZnO/WO3 nanocomposites towards photocatalytic gold recovery from industrial cyanide-based gold plating wastewater

Discharging the gold-contained wastewater is an economic loss. In this work, a set of ZnO/WO3 was facile synthesized by hydrothermal method in order to recover gold from the industrial cyanide-based gold plating wastewater by photocatalytic process. Effect of ZnO contents coupled with WO3 was first explored. Then, effects of operating condition including initial pH of wastewater, type of hole scavenger, concentration of the best hole scavenger and photocatalyst dose were explored. A series of experimental results demonstrated that the ZnO/WO3 nanocomposite with 5 wt% ZnO (Z5.0/WO3) depicted the highest photocatalytic activity for gold recovery due to the synergetic effect of oxygen vacancies, a well-constructed ZnO/WO3 heterostructure and an appropriate band position alignment with respect to the redox potentials of [Au(CN)2]- and hole scavengers. Via this ZnO/WO3 nanocomposite, approximately 99.5% of gold ions was recovered within 5 h using light intensity of 3.57 mW/cm2, catalyst dose of 2.0 g/L, ethanol concentration of 20 vol% and initial pH of wastewater of 11.2. In addition, high stability and reusability were observed with the best nanocomposite even at the 5th reuse. This work provides the guidance and pave the way for designing the ZnO/WO3 nanocomposite for precious metal recovery from a real industrial wastewater.

Photocatalytic Gold Recovery from Industrial Gold Plating Effluent by ZnO Nanoparticles: Optimum Condition and Possible Applications

The comparative study of photocatalytic gold recovery from cyanide-based gold plating solution was explored via commercial and hydrothermally synthesized ZnO nanoparticles (NPs). The effects of hydrothermal temperatures on the properties and photocatalytic activities of synthesized ZnO NPs were investigated. In addition, the effects of operating parameters including types of hole scavenger, concentrations of the best hole scavenger, the initial pH of wastewater, and photocatalyst dosages were examined. The obtained results demonstrated that the commercial ZnO NPs exhibited a higher photocatalytic activity for gold recovery than that of the synthesized ones owing to their good crystal quality and the presence of non-lattice zinc ions and appropriate non-lattice oxygen ions. Via the commercial ZnO NPs, the gold ions were almost completely recovered from the cyanide-based gold plating effluent within 7 h at an initial pH of 11.0 in the presence of 10 vol % C2H5OH and 1.0 g/L of photocatalyst loading with a pseudo-first-order rate constant of 0.2637 h-1. Finally, the resultant gold-decorated ZnO NPs exhibited a higher photocatalytic property for color reduction from industrial wastewater and antibacterial activity than that of fresh ZnO NPs. The results obtained in this study possess benefits and pave the way for waste remediation and management for the plating industries.

Light-driven reversible charge transfers from ITO nanocrystals

The combination of semiconductors and redox active molecules for light-driven energy storage systems has emerged as a powerful solution for the exploitation of solar batteries. On account of this, transparent conductive oxide (TCO) nanocrystals (NCs) demonstrated to be interesting materials, thanks to the photo-induced charge accumulation enabling light harvesting and storage. The charge transfer process after light absorption, at the base of the proper use of these semiconductors, is a key step, often resulting in non-reversible transformations of the chemicals involved. However, if considering the photocharging through TCO NCs not only as a charge provider for the system but potentially as part of the storage role, the reversible transformation of the redox compound represents a crucial aspect. In this paper, we explore the possible interaction of indium tin oxide (ITO) NCs and typical redox mediators commonly employed in catalytic applications with a twofold scope of enhancing or supporting the light-induced charge accumulation on the metal oxide NC side and controlling the reversibility of the whole process. The work presented focuses on the effect of the redox properties on the doped metal oxide response, both from the stability point of view and the photodoping performance, by monitoring the changes in the optical behavior of ITO/redox hybrid systems upon ultraviolet illumination.

Strategies for Improving the Photocatalytic Hydrogen Evolution Reaction of Carbon Nitride-Based Catalysts

Due to the depletion of fossil fuels and their-related environmental issues, sustainable, clean, and renewable energy is urgently needed to replace fossil fuel as the primary energy resource. Hydrogen is considered as one of the cleanest energies. Among the approaches to hydrogen production, photocatalysis is the most sustainable and renewable solar energy technique. Considering the low cost of fabrication, earth abundance, appropriate bandgap, and high performance, carbon nitride has attracted extensive attention as the catalyst for photocatalytic hydrogen production in the last two decades. In this review, the carbon nitride-based photocatalytic hydrogen production system, including the catalytic mechanism and the strategies for improving the photocatalytic performance is discussed. According to the photocatalytic processes, the strengthened mechanism of carbon nitride-based catalysts is particularly described in terms of boosting the excitation of electrons and holes, suppressing carriers recombination, and enhancing the utilization efficiency of photon-excited electron-hole. Finally, the current trends related to the screening design of superior photocatalytic hydrogen production systems are outlined, and the development direction of carbon nitride for hydrogen production is clarified.

Spontaneous Dewetting of Au-Thin Layers on Oxide- and Fluorine-Terminated Single Crystalline Anatase and Efficient Use in Photocatalytic H2 Production

In the present work, the spontaneous dewetting of thin Au layers on single crystalline anatase nanosheets into narrow-disperse Au nanoparticles is investigated. Patterns of the Au particles can be formed on the main facets of anatase that provide a high co-catalytic activity for photocatalytic generation of H2 . Dewetting is distinctly influenced by the respective facets (001) and (101), the deposit thickness, and secondary thermal dewetting, but most strongly by the surface termination of the nanosheets. Fluoride termination not only leads to an enhanced Au-phobic behavior but strongly affects the co-catalytic activity for photocatalytic generation of H2 . While fluoride termination with or without Au decoration is detrimental for hole transfer, the interplay of the Au co-catalyst and surface fluoride yields highly beneficial effect for electron transfer. This results in a three-times higher photocatalytic H2 production for the F-terminated surface. The findings suggest that dewetting of Au on surface fluorinated TiO2 is an effective way to modulate surface dewetting and achieve a strongly enhanced photocatalytic activity.

Improved Sensitivity and Selectivity of Glutathione Detection through Target-Driven Electron Donor Generation in Photoelectrochemical Electrodes

Glutathione (GSH) plays a vital role in many physiological processes, and its abnormal levels have been found to be associated with several diseases. In contrast to traditional methods using electron donor-containing electrolytes for photoelectrochemical (PEC) sensing, in this study, a target-driven electron donor generation in a PEC electrode was developed to detect GSH. Using well-aligned TiO2 nanotube arrays (TNTs) as the PEC substrate, mesoporous MIL-125(Ti) was grown in the TNTs through an in situ solvothermal method and subsequent two-step annealing treatment. The accommodation capacity of mesoporous MIL-125(Ti) allows a well loading of cystine and Pt nanoclusters (NCs). Taking advantage of the specific cleavage ability of disulfide bonds by GSH, cystine was converted to cysteine, which served as the electron donor for the PEC process. Benefiting from the confinement effect of mesoporous MIL-125(Ti), cysteine was effectively oxidized to cysteine sulfinic acid by the photogenerated holes. Importantly, the highly active Pt NCs decorated in the mesopores not only improved the charge transfer but also accelerated the above oxidation reaction. The synergistic effect of these factors enabled the efficient separation of the photogenerated electron-hole pairs, which induced a significant photocurrent increase and in turn led to the high-sensitivity detection of GSH. Consequently, the proposed PEC biosensor exhibited excellent performance in the detection of GSH in serum specimens. The target-driven electron donor generation designed in this study might open a new route for developing sensitive and selective PEC biosensors with application in complex biological environments.

Integrating K and P co-doped g-C3N4 with ZnFe2O4 and graphene oxide for S-scheme-based enhanced absorption coupled photocatalytic real wastewater treatment

Recently, there has been a significant increase in the interest of using photocatalysis for environmental clean-up applications. In this research, potassium, and phosphorus co-doped graphitic carbon nitride (KPCN) photocatalyst modified with graphene oxide (GO) and heterostructured with ZnFe₂O₄ was synthesized via the hydrothermal method (KPCN/GO/ZnFe₂O₄). The photoactivity of KPCN/GO/ZnFe₂O₄ photocatalyst was examined for the photocatalytic degradation of target pollutants such as methylene blue (MB) dye, rhodamine B (RhB) dye, and tetracycline (TC) antibiotic. Furthermore, the chemical oxygen demand (COD) removal efficiency for real wastewater was determined to explore the practical application of KPCN/GO/ZnFe₂O₄ photocatalyst. The degradation efficiencies of bare graphitic carbon nitride, KPCN, KPCN/GO, and KPCN/GO/ZnFe₂O₄ photocatalysts for tetracycline antibiotics were 30%, 42%, 57%, and 87% within 60 min, respectively. Moreover, KPCN/GO/ZnFe₂O₄ photocatalyst showed 71% COD removal efficiency within 240 min. The ^•OH and ^•O₂^- were the major reactive species in the photocatalytic process. Results showed that the degradation efficiencies of graphitic carbon nitride were greatly enhanced upon doping and further improved with the addition of GO and ZnFe₂O₄. Doping improved light harvesting, GO enhanced the adsorption ability and heterojunction with ZnFe₂O₄ enhanced the charge separation as well as the reusability of synthesized KPCN/GO/ZnFe₂O₄ photocatalyst.

Alkali functionalized carbon nitride with internal van der Waals heterostructures: Directional charge flow to enhance photocatalytic hydrogen production

Improving the charge separation and migration in graphitic carbon nitride (CN) is the critical issue to enhance its photocatalytic performance, but still remains very challenging. Herein, the alkali metals were introduced into the interlayer and intralayer of CN to tackle this challenge. The lithium sodium-modifying carbon nitride layer (LiNaCN2) and the adjacent CN layer formed a van der Waals heterostructures (VDWHs), while the potassium-intercalating served as interlayer charge transfer channels to induce the directional charge flow. Experiments and theoretical calculations indicated that such unique construction provided intrinsic driving force to obtain the electrons from LiNaCN2 to CN via directional potassium channels. In accordance with the theoretical prediction, a dramatically red-shift of the light absorption feature was achieved for interlayer potassium-intercalating and intralayer lithium sodium-modifying co-functionalized carbon nitride (LiNaCN-K-CN2) to show narrowed bandgap energy of 2.15 eV. This directional charge flow in CN resulted in the rapid transfer of charge carriers in both interlayer as well as intralayer of CN, which reduced the electronic localization as well as extended the π conjugative effect. Consequently, the LiNaCN-K-CN2 displayed stable and remarkable hydrogen production rate of about 2.46 mmol g^-1 h^-1 with apparent quantum yield (AQY) of about 13.68% at 435 nm, which was 22 folds higher than that of the pristine CN. This finding provides the feasible strategy to precisely tune the directions of charge transfer for high-performance CN-based photocatalysts.

Manipulating the surface states of BiVO4 through electrochemical reduction for enhanced PEC water oxidation

Bismuth vanadate (BiVO₄) is a prospective candidate for photoelectrochemical (PEC) water oxidation, but its commercial application is limited due to the serious surface charge recombination. In this work, we propose a novel and effective electrochemical reduction strategy combined with co-catalyst modification to manipulate the surface states of the BiVO₄ photoanode. Specifically, an ultrathin amorphous structure is formed on the surface of BiVO₄ after electrochemical reduction ascribed to the breaking of the surface metal-O bonds. Photoelectrochemical measurements and first-principles calculation show that the electrochemical reduction treatment can effectively reduce the surface energy, thereby passivating the recombined surface states (r-ss) and increasing the mobility of photogenerated holes. In addition, the FeOOH co-catalyst layer further increases the intermediate surface states (i-ss) of BiVO₄, stabilizes the surface structure and enhances its PEC performance. Benefiting from the superior charge transfer efficiency and the excellent water oxidation kinetics, the -0.8/BVO/Fe photoanode achieves 2.02 mA cm^-2 photocurrent at 1.23 V_RHE (2.4 times that of the original BiVO₄); meanwhile, the onset potential shifts 90 mV to the cathode. These results provide a new surface engineering tactic to modify the surface states of semiconductor photoanodes for high-efficiency PEC water oxidation.

Photosynthesis of Au/TiO2 nanoparticles for photocatalytic gold recovery from industrial gold-cyanide plating wastewater

A series of Au_x/TiO₂ nanoparticles (NPs) with different gold loadings (x = 0.1-1.0 wt%) was synthesized by the photodeposition and then employed as photocatalysts to recover precious component from the industrial gold-cyanide plating wastewater. Effects of Au loading, catalyst dosage and types of hole scavenger on the photocatalytic gold recovery were investigated under ultraviolet-visible (UV-Vis) light irradiation at room temperature. It was found that different Au loadings tuned the light absorption capacity of the synthesized photocatalysts and enhanced the photocatalytic activity in comparison with the bare TiO₂ NPs. The addition of CH₃OH, C₂H₅OH, C₃H₈O, and Na₂S₂O₃ as a hole scavenger significantly promoted the photocatalytic activity of the gold recovery, while the H₂O₂ did not. Among different hole scavengers employed in this work, the CH₃OH exhibited the highest capability to promote the photocatalytic gold recovery. In summary, the Au_0.5/TiO₂ NPs exhibited the best photocatalytic activity to completely recover gold ions within 30 min at the catalyst dosage of 0.5 g/L, light intensity of 3.20 mW/cm² in the presence of 20 vol% CH₃OH as hole scavenger. The photocatalytic activity slightly decreased after the 5th cycle of recovery process, indicating its high reusability.

Gold Nanorods/Metal-Organic Framework Hybrids: Photo-Enhanced Peroxidase-Like Activity and SERS Performance for Organic Dyestuff Degradation and Detection

Metal-organic frameworks (MOFs) are widely used to mimic enzymes for catalyzing chemical reactions; however, low enzyme activity limit their large-scale application. In this work, gold nanorods/metal-organic frameworks (Au NRs/Fe-MOF) hybrids were successfully synthesized for photo-enhanced peroxidase-like catalysis and surface-enhanced Raman spectroscopy (SERS). The enzyme-like activity of Au NRs/Fe-MOF hybrids was significantly enhanced under localized surface plasmon resonance (LSPR), because the hot electrons produced on Au NRs surface were transferred into Fe-MOF, activating the Fenton reaction by Fe³⁺/Fe²⁺ conversion and preventing the recombination of hot electrons and holes. This photo-enhanced enzyme-like catalytic performance was investigated by X-ray photoelectron spectrometry (XPS), electrochemical analysis, activation energy measurement, and in situ Raman spectroscopy. Afterward, Methylene Blue (MB) was chosen to demonstrate the photo-enhanced peroxidase-like performance of Au NRs/Fe-MOFs. The Au NRs/Fe-MOF caused chemical and electromagnetic enhancement of Raman signals and exhibited a great potential for the detection of toxic chemicals and biological molecules. The detection limit of MB concentration is 9.3 × 10^-12 M. In addition, the Au NRs/Fe-MOF hybrids also showed excellent stability and reproducibility for photo-enhanced peroxidase-like catalysis. These results show that nanohybrids have great potential in many fields, such as sensing, cancer therapy, and energy harvesting.

Photocatalytic reduction-based liquid microjunction surface sampling-mass spectrometry for rapid in situ analysis of aromatic amines originating from azo dyes in packaging papers

A rapid in situ analytical method was developed for the detection of generated carcinogenic aromatic amines from banned azo dyes utilizing a photocatalytic reduction-based liquid microjunction surface sampling (LMJSS)-mass spectrometry (MS) system. We utilized photocatalytic reduction under UV irradiation with TiO₂ as catalyst to have rapid and mild reduction of azo dyes. The reaction conditions were optimized to have complete photocatalytic reduction within 2-5 min in pure methanol at room temperature. TiO₂ was immobilized in the inner wall of the capillaries in the LMJSS system to achieve in situ sampling-online rapid reduction-MS detection for aromatic amines originating from azo dyes in packaging surface. The yields of in-tube photocatalytic reduction were near 100% by delivering the azo dye extracts through the capillary at 1 μL/min under UV irradiation. With this design, in situ analysis was completed within 2 min via direct MS detection and 7 min via liquid chromatography (LC)-MS detection. The detection limits for five aromatic amines originating from four different azo dyes were in the range of 1-17 mg/kg with relative standard deviations (RSDs) < 8.5%. In the application of the new method, four carcinogenic aromatic amines were detected and identified in three commercial packaging materials, and the quantitation results were comparable with those obtained by the conventional chemical reduction-LC-MS method (relative recovery, 81-121%). Moreover, due to the spatial resolution of the present method with a flow probe, MS imaging was achieved demonstrating clear azo dye patterns of a lab-made sample.

The Role of Common Alcoholic Sacrificial Agents in Photocatalysis: Is It Always Trivial?

Photocatalytic hydrogen production is proposed as a sustainable energy source. Simultaneous reduction and oxidation of water is a complex multistep reaction with high overpotential. Photocatalytic processes involving semiconductors transfer electrons from the valence band to the conduction band. Sacrificial substrates that react with the photochemically formed holes in the valence band are often used to study the mechanism of H₂ production, as they scavenge the holes and hinder charge carrier recombination (electron-hole pairs). Here, we show that the desired sacrificial agent is one forming a radical that is a fairly strong reducing agent, and whose oxidized form is not a good electron acceptor that might suppress the hydrogen evolution reaction (HER). In an acidic medium, methanol was found to fulfill both these requirements better than ethanol and propan-2-ol in the TiO₂ -(M⁰ -NPs) (M=Au or Pt) system, whereas in an alkaline medium, the alcohols exhibit a reverse order of activity. Moreover, we report that CH₂ (OH)₂ is by far the most efficient sacrificial agent in a nontrivial mechanism in acidic media. Our study provides general guidelines for choosing an appropriate sacrificial substrate and helps to explain the variance in the performance of alcohol scavenger-based photocatalytic systems.

Engineering Homochiral MOFs in TiO2 Nanotubes as Enantioselective Photoelectrochemical Electrode for Chiral Recognition

Enantioselective sensing of chiral molecules is an important issue for both biomedical research and the pharmaceutical industry. Here, an enantioselective photoelectrochemical (PEC) sensor was constructed by integrating TiO₂ nanotubes (NTs) with metal-organic frameworks (MOFs) for the identification of enantiomers. TiO₂ NTs prepared by electrochemical anodization can not only be used as the PEC platform but also as the metal-ion precursor to react with terephthalic acid (BDC) to form MIL-125(Ti) in situ. A postsynthetic exchange (PSE) method was used for exchanging the ligand of MIL-125 by 2-aminoterephthalic acid (BDC-NH₂) for further functionalization. Homochirality was then successfully introduced into achiral MIL-125-NH₂ by postsynthetic modification (PSM) with l-histidine (l-His). The resulting homochiral metal-organic frameworks (MOF)-in-NT architecture exhibits excellent discrimination ability for the chiral recognition of 3,4-dihydroxyphenylalanine (l/d-DOPA) enantiomers. Moreover, by adjusting the charge-carrier separation-induced photocurrent variation mechanism, the as-proposed homochiral PEC electrode exhibits a broad application potential for the discrimination of enantiomers. Because of the construction of binder-free monochiral MOF-in-NT structure directly on a Ti-metal substrate, the valuable feature is that the PEC sensing platform can be used directly, thereby providing a stable, simplified, and low-cost sensing device for the recognition of chiral enantiomers.

Effect of Potential and Chlorides on Photoelectrochemical Removal of Diethyl Phthalate from Water

Removal of persistent pollutants from water by photoelectrocatalysis has emerged as a promising powerful process. Applied potential plays a key role in the photocatalytic activity of the semi-conductor as well as the possible presence of chloride ions in the solution. This work aims to investigate these effects on the photoelectrocatalytic oxidation of diethyl phthalate (DEP) by using TiO2 nanotubular anodes under solar light irradiation. PEC tests were performed at constant potentials under different concentration of NaCl. The process is able to remove DEP following a pseudo-first order kinetics: values of kapp of 1.25 × 10−3 min−1 and 1.56 × 10−4 min−1 have been obtained at applied potentials of 1.8 and 0.2 V, respectively. Results showed that, depending on the applied potential, the presence of chloride ions in the solution affects the degradation rate resulting in a negative effect: the presence of 500 mM of Cl− reduces the value of kapp by 50 and 80% at 0.2 and 1.8 V respectively.

Organic acid mediated photoelectrochemical reduction of U(vi) to U(iv) in waste water: electrochemical parameters and spectroscopy

The photoelectrochemical reduction of U(vi) is recognized as an economical and effective way to eliminate radioactive pollution. In this study, we construct a α-Fe₂O₃/TiO₂ film electrode-based photoelectrochemical cell to remove U(vi) and recover uranium from aqueous solution. Citric acid and oxalic acid could act as hole scavengers, being favorable for the photocatalytic reduction of U(vi). In the presence of 0.5 mM citric acid and oxalic acid, the uranium removal capacity reached 70% and 58%, respectively, while 24% was achieved for the system in the absence of acid. The XRD, SEM, FT-IR and XPS results revealed that a proportion of U(iv) was also precipitated as surface associated metastudtite. These novel observations have significant implications for the behavior of uranium within engineered and natural environments.

In this study, we focus on organic acid mediated photoelectrochemical reduction of U(vi) to U(iv) in waste water by α-Fe₂O₃/TiO₂ film electrodes, which have significant implications for the migration and transformation of uranium.

Sol-Gel Processing of Water-Soluble Carbon Nitride Enables High-Performance Photoanodes*

In spite of the enormous promise that polymeric carbon nitride (PCN) materials hold for various applications, the fabrication of high-quality, binder-free PCN films and electrodes has been a largely elusive goal to date. Here, we tackle this challenge by devising, for the first time, a water-based sol-gel approach that enables facile preparation of thin films based on poly(heptazine imide) (PHI), a polymer belonging to the PCN family. The sol-gel process capitalizes on the use of a water-soluble PHI precursor that allows formation of a non-covalent hydrogel. The hydrogel can be deposited on conductive substrates, resulting in formation of mechanically stable polymeric thin layers. The resulting photoanodes exhibit unprecedented photoelectrochemical (PEC) performance in alcohol reforming and highly selective (∼100 %) conversions with very high photocurrents (>0.25 mA cm-2 under 2 sun) down to <0 V vs. RHE. This enables even effective PEC operation under zero-bias conditions and represents the very first example of a 'soft matter'-based PEC system capable of bias-free photoreforming. The robust binder-free films derived from sol-gel processing of water-soluble PCN thus constitute a new paradigm for high-performance 'soft matter' photoelectrocatalytic systems and pave the way for further applications in which high-quality PCN films are required.

High chlorine evolution performance of electrochemically reduced TiO2 nanotube array coated with a thin RuO2 layer by the self-synthetic method

Recently, reduced TiO2 nanotube arrays via electrochemical self-doping (r-TiO2) are emerging as a good alternative to conventional dimensionally stable anodes (DSAs) due to their comparable performance and low-cost. However, compared with conventional DSAs, they suffer from poor stability, low current efficiency, and high energy consumption. Therefore, this study aims to advance the electrochemical performances in the chlorine evolution of r-TiO2 with a thin RuO2 layer coating on the nanotube structure (RuO2@r-TiO2). The RuO2 thin layer was successfully coated on the surface of r-TiO2. This was accomplished with a self-synthesized layer of ruthenium precursor originating from a spontaneous redox reaction between Ti3+ and metal ions on the r-TiO2 surface and thermal treatment. The thickness of the thin RuO2 layer was approximately 30 nm on the nanotube surface of RuO2@r-TiO2 without severe pore blocking. In chlorine production, RuO2@r-TiO2 exhibited higher current efficiency (∼81.0%) and lower energy consumption (∼3.0 W h g-1) than the r-TiO2 (current efficiency of ∼64.7% of and energy consumption of ∼5.2 W h g-1). In addition, the stability (ca. 22 h) was around 20-fold enhancement in RuO2@r-TiO2 compared with r-TiO2 (ca. 1.2 h). The results suggest a new route to provide a thin layer coating on r-TiO2 and to synthesize a high performance oxidant-generating anode.

Photocatalytic hydrogen evolution from biomass conversion

Biomass has incredible potential as an alternative to fossil fuels for energy production that is sustainable for the future of humanity. Hydrogen evolution from photocatalytic biomass conversion not only produces valuable carbon-free energy in the form of molecular hydrogen but also provides an avenue of production for industrially relevant biomass products. This photocatalytic conversion can be realized with efficient, sustainable reaction materials (biomass) and inexhaustible sunlight as the only energy inputs. Reported herein is a general strategy and mechanism for photocatalytic hydrogen evolution from biomass and biomass-derived substrates (including ethanol, glycerol, formic acid, glucose, and polysaccharides). Recent advancements in the synthesis and fundamental physical/mechanistic studies of novel photocatalysts for hydrogen evolution from biomass conversion are summarized. Also summarized are recent advancements in hydrogen evolution efficiency regarding biomass and biomass-derived substrates. Special emphasis is given to methods that utilize unprocessed biomass as a substrate or synthetic photocatalyst material, as the development of such will incur greater benefits towards a sustainable route for the evolution of hydrogen and production of chemical feedstocks.

SrTiO3/Bi4Ti3O12 Nanoheterostructural Platelets Synthesized by Topotactic Epitaxy as Effective Noble-Metal-Free Photocatalysts for pH-Neutral Hydrogen Evolution

Low-temperature hydrothermal epitaxial growth and topochemical conversion (TC) reactions offer unexploited possibilities for the morphological engineering of heterostructural and non-equilibrium shape (photo)catalyst particles. The hydrothermal epitaxial growth of SrTiO3 on Bi4Ti3O12 platelets is studied as a new route for the formation of novel nanoheterostructural SrTiO3/Bi4Ti3O12 platelets at an intermediate stage or (100)-oriented mesocrystalline SrTiO3 nanoplatelets at the completed stage of the TC reaction. The Bi4Ti3O12 platelets act as a source of Ti(OH)62- species and, at the same time, as a substrate for the epitaxial growth of SrTiO3. The dissolution of the Bi4Ti3O12 platelets proceeds faster from the lateral direction, whereas the epitaxial growth of SrTiO3 occurs on both bismuth-oxide-terminated basal surface planes of the Bi4Ti3O12 platelets. In the progress of the TC reaction, the Bi4Ti3O12 platelet is replaced from the lateral ends toward the interior by SrTiO3, while Bi4Ti3O12 is preserved in the core of the heterostructural platelet. Without any support from noble-metal doping or cocatalysts, the SrTiO3/Bi4Ti3O12 platelets show stable and 15 times higher photocatalytic H2 production (1265 μmol·g-1·h-1; solar-to-hydrogen (STH) efficiency = 0.19%) than commercial SrTiO3 nanopowders (81 μmol·g-1·h-1; STH = 0.012%) in pH-neutral water/methanol solutions. A plausible Z scheme is proposed to describe the charge-transfer mechanism during the photocatalysis.

Improved photocatalytic degradation of ketoprofen by Pt/MIL-125(Ti)/Ag with synergetic effect of Pt-MOF and MOF-Ag double interfaces: Mechanism and degradation pathway

It is a central issue to improve the separation efficiency of photogenerated charge carriers and the utilization of visible light in the field of photocatalysis. Herein, taking MIL-125(Ti) as a host material, the Pt/MIL-125(Ti) was first prepared by solvothermal method to build the interface of Schottky junction. Ag was then introduced onto the surface of Pt/MIL-125(Ti) to form the interface with the surface plasmon resonance effect. These double interfaces in the composite play a synergistic role on the photodagradation. The morphology, crystallinity and photochemical properties of the material were tested. By comparison, Pt/MIL-125(Ti)/Ag (4 wt% Ag) exhibited the best performance in the photodegradation of ketoprofen (KP, 10 mg/L) and the degradation process conformed to the pseudo-first-order kinetics. The photodegradation rate is 0.0253 min^-1, which was higher than MIL-125(Ti) (0.0009 min^-1). The TOC removal efficiency of KP reached approximately 51.5%. The electron paramagnetic resonance (EPR) and free radical capture tests verified that h⁺ and ·OH played the prominent roles during the reaction system. The degradation process, possible pathways and reaction mechanism were proposed. The design of the double interfaces between semiconductor and noble metals is a novel strategy to enhance the photocatalytic performance.