Photocatalytic Water Splitting for Solar Hydrogen Production Using the Carbonate Effect and the Z‐Scheme Reaction

The development of innovative technologies for solar energy conversion and storage is important for solving the global warming problem and for establishing a sustainable society. The photocatalytic water‐splitting reaction using semiconductor powders has been intensively studied as a promising technology for direct and simple solar energy conversion. However, the evolution of H₂ and O₂ gases in a stoichiometric ratio (H₂/O₂ = 2) is very difficult owing to various issues, such as an unfavorable backward reaction and mismatched band potentials. Two important findings have widened the variety of photocatalysts available for stoichiometric water‐splitting, viz. the carbonate anion effect and the Z‐scheme photocatalytic reaction using a redox mediator. The bicarbonate anion has been found to act as a redox catalyst via preferential peroxide formation and subsequent decomposition to O₂. As the Z‐scheme reaction using a redox mediator mitigates band potential mismatches, it is widely applicable for various visible‐light‐active photocatalysts. This review describes the development of photocatalytic water‐splitting for solar hydrogen production using the carbonate anion effect and the Z‐scheme reaction. Moreover, recent developments in photocatalysis–electrolysis hybrid systems, an advanced Z‐scheme reaction concept, are also reviewed for practical and economical hydrogen production.     

Enhanced Photocatalytic Activity for H2 Evolution under Irradiation of UV–Vis Light by Au-Modified Nitrogen-Doped TiO2

Background Purpose

Photocatalytic water splitting for hydrogen evolution is a potential way to solve many energy and environmental issues. Developing visible-light-active photocatalysts to efficiently utilize sunlight and finding proper ways to improve photocatalytic activity for H₂ evolution have always been hot topics for research. This study attempts to expand the use of sunlight and to enhance the photocatalytic activity of TiO₂ by N doping and Au loading.

Methods

Au/N-doped TiO₂ photocatalysts were synthesized and successfully used for photocatalytic water splitting for H₂ evolution under irradiation of UV and UV–vis light, respectively. The samples were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV–vis diffuse reflectance spectroscopy (DRS), photoluminescence spectroscopy (PL), and photoelectrochemical characterizations.

Results

DRS displayed an extension of light absorption into the visible region by doping of N and depositing with Au, respectively. PL analysis indicated electron-hole recombination due to N doping and an efficient inhibition of electron-hole recombination due to the loaded Au particles. Under the irradiation of UV light, the photocatalytic hydrogen production rate of the as-synthesized samples followed the order Au/TiO₂ > Au/N-doped TiO₂ > TiO₂ > N-doped TiO₂. While under irradiation of UV–vis light, the N-TiO₂ and Au/N-TiO₂ samples show higher H₂ evolution than their corresponding nitrogen-free samples (TiO₂ and Au/TiO₂). This inconsistent result could be attributed to the doping of N and the surface plasmonic resonance (SPR) effect of Au particles extending the visible light absorption. The photoelectrochemical characterizations further indicated the enhancement of the visible light response of Au/N-doped TiO₂.

Conclusion

Comparative studies have shown that a combination of nitrogen doping and Au loading enhanced the visible light response of TiO₂ and increased the utilization of solar energy, greatly boosting the photocatalytic activity for hydrogen production under UV–vis light.     

Layered Double Hydroxide Nanostructured Photocatalysts for Renewable Energy Production

An enormous research effort is currently being directed towards the development of efficient visible‐light‐driven photocatalysts for renewable energy applications including water splitting, CO₂ reduction and alcohol photoreforming. Layered double hydroxide (LDH)‐based photocatalysts have emerged as one of the most promising candidates to replace TiO₂‐based photocatalysts for these reactions_, owing to their unique layered structure, compositional flexibility, controllable particle size, low manufacturing cost and ease of synthesis. By introducing defects into LDH materials through the control of their size to the nanoscale, the atomic structure, surface defect concentration, and electronic and optical characteristics of LDH materials can be strategically engineered for particular applications. Furthermore, through the use of advanced characterization techniques such as X‐ray absorption fine structure, positron annihilation spectrometry, X‐ray photoelectron spectroscopy, electron spin resonance, density‐functional theory calculations, and photocatalytic tests, structure‐activity relationships can be established and used in the rational design of high‐performance LDH‐based photocatalysts for efficient solar energy capture. LDHs thus represent a versatile platform for semiconductor photocatalyst development with application potential across the energy sector.     

CdS/Graphene Nanocomposite Photocatalysts

Heterogeneous photocatalysis using semiconductors and renewable solar energy has been regarded as one of the most promising processes to alleviate, and even solve, both the world crises of energy supply and environmental pollution. In the past few years, many encouraging achievements have been made in the research area of graphene‐based semiconductor photocatalysts. Among them, CdS/graphene nanocomposites have attracted extensive attention as an important kind of photocatalyst in chemical and material science, due to its superior photocatalytic activity and photostability under visible‐light irradiation. The aim here is to address the enhancement mechanism of the photocatalytic performance of CdS/graphene composite photocatalysts, and systematically summarize recent progress regarding the design and synthesis of CdS/graphene nanocomposites. These nanocomposites are promising for a great diversity of applications in visible‐light photocatalytic fields, including artificial photosynthetic systems (photocatalytic hydrogen production and CO₂ reduction), environmental remediation, and organic photosynthesis. Special attention is given to the photocatalytic hydrogen production and pollutant photodegradation over CdS/graphene nanocomposite photocatalysts. Furthermore, perspectives on CdS/graphene‐based materials are discussed, including the various remaining challenges for large‐scale applications, identifying prospective areas for related research in this field.     

Seasonal variation in the properties of titania photocatalysts produced from Ti-salt flocculated bioresource sludge

Ti-salt flocculation of biologically treated sewage effluent (BTSE) was carried out on monthly basis during one year to trace the seasonal variation in the properties of BTSE, Ti-salt flocculated BTSE and titania photocatalysts. Titania photocatalysts were produced from incineration of Ti-salt flocculated sludge at 600 °C. The physio-chemical properties of BTSE, Ti-salt flocculated BTSE and titania photocatalysts were investigated. The photocatalytic activity of titania was examined using different substrates of rhodamine B and humic acid under UV light irradiation. Results indicated that the flocculation performance of Ti-salt was not affected by the seasonal variation of BTSE. BTSE characteristics resulted in marginal effect in titania characterisation and photocatalytic activity. Titania photocatalysts produced from Ti-salt flocculated sludge in different seasons showed constant anatase phase, high BET surface area and high photocatalytic activity.     

Amphiphilic molecular gels from ω-aminoalkylated l-glutamic acid derivatives with unique chiroptical properties

Self-assembling amphiphiles with unique chiroptical properties were derived from l-glutamic acid through ω-aminoalkylation and double long-chain alkylation. These amphiphiles can disperse in various solvents ranging from water to n-hexane. TEM and SEM observations indicate that the improvement in dispersity is induced by the formation of tubular and/or fibrillar aggregates with nanosized diameters, which makes these amphiphiles similar to aqueous lipid membrane systems. Spectroscopic observations, such as UV–visible and CD spectroscopies indicate that the aggregates are constructed on the basis of S- and R-chirally ordered structures through interamide interactions in water and organic media, respectively, and that these chiroptical properties can be controlled thermotropically and lyotropically. It is also reported that the chiral assemblies provide specific binding sites for achiral molecules and then induce chirality for the bonded molecules. Further, the applicability of the amphiphiles to template polymerization is discussed.     

Antibacterial performance of nanoscaled visible-light responsive platinum-containing titania photocatalyst in vitro and in vivo

Background

Traditional antibacterial photocatalysts are primarily induced by ultraviolet light to elicit antibacterial reactive oxygen species. New generation visible-light responsive photocatalysts were discovered, offering greater opportunity to use photocatalysts as disinfectants in our living environment. Recently, we found that visible-light responsive platinum-containing titania (TiO₂–Pt) exerted high performance antibacterial property against soil-borne pathogens even in soil highly contaminated water. However, its physical and photocatalytic properties, and the application in vivo have not been well-characterized.

Methods

Transmission electron microscopy, energy dispersive spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, ultraviolet–visible absorption spectrum and the removal rate of nitrogen oxides were therefore analyzed. The antibacterial performance under in vitro and in vivo conditions was evaluated.

Results

The apparent quantum efficiency for visible light illuminated TiO₂–Pt is relatively higher than several other titania photocatalysts. The killing effect achieved approximately 2 log reductions of pathogenic bacteria in vitro. Illumination of injected TiO₂–Pt successfully ameliorated the subcutaneous infection in mice.

Conclusions

This is the first demonstration of in vivo antibacterial use of TiO₂–Pt nanoparticles. When compared to nanoparticles of some other visible-light responsive photocatalysts, TiO₂–Pt nanoparticles induced less adverse effects such as exacerbated platelet clearance and hepatic cytotoxicity in vivo.

General significance

These findings suggest that the TiO₂–Pt may have potential application on the development of an antibacterial material in both in vitro and in vivo settings.     

Metal Halide Perovskites for Solar‐to‐Chemical Fuel Conversion

This review article presents and discusses the recent progress made in the stabilization, protection, improvement, and design of halide perovskite‐based photocatalysts, photoelectrodes, and devices for solar‐to‐chemical fuel conversion. With the target of water splitting, hydrogen iodide splitting, and CO₂ reduction reactions, the strategies established for halide perovskites used in photocatalytic particle‐suspension systems, photoelectrode thin‐film systems, and photovoltaic‐(photo)electrocatalysis tandem systems are organized and introduced. Moreover, recent achievements in discovering new and stable halide perovskite materials, developing protective and functional shells and layers, designing proper reaction solution systems, and tandem device configurations are emphasized and discussed. Perspectives on the future design of halide perovskite materials and devices for solar‐to‐chemical fuel conversion are provided. This review may serve as a guide for researchers interested in utilizing halide perovskite materials for solar‐to‐chemical fuel conversion.     

Visible light inactivation of bacteria and fungi by modified titanium dioxide.

Visible light induced photocatalytic inactivation of bacteria (Escherichia coli, Staphylococcus aureus, Enterococcus faecalis) and fungi (Candida albicans, Aspergillus niger) was tested. Carbon-doped titanium dioxide and TiO2 modified with platinum(IV) chloride complexes were used as suspension or immobilised at the surface of plastic plates. A biocidal effect was observed under visible light irradiation in the case of E. coli in the presence of both photocatalysts. The platinum(IV) modified titania exhibited a higher inactivation effect, also in the absence of light. The mechanism of visible light induced photoinactivation is briefly discussed. The observed detrimental effect of photocatalysts on various microorganism groups decreases in the order: E. coli > S. aureus approximately E. faecalis>C. albicans approximately A. niger. This sequence results most probably from differences in cell wall or cell membrane structures in these microorganisms and is not related to the ability of catalase production.     

Bismuth Tantalum Oxyhalogen: A Promising Candidate Photocatalyst for Solar Water Splitting

As wide range of light absorption and suitable redox potentials are prerequisites for photocatalytic water splitting, exploring new semiconductor‐based materials with proper band structures for water splitting still calls for longstanding efforts. In this work, a series of photocatalysts, bismuth tantalum oxyhalide, Bi₄TaO₈X (X = Cl, Br), with valence band and conduction band positions at ≈?0.70 and ≈1.80 eV versus the reversible hydrogen electrode (RHE), respectively, are found to be capable for both water oxidation and reduction under visible light irradiation. Using flux synthetic methods, Bi₄TaO₈X (X = Cl, Br) with microplatelet morphology can be successfully prepared. The photocatalyst based on these materials shows an apparent quantum efficiency as high as 20% at 420 nm for water oxidation. In addition, a Z‐scheme system coupling Bi₄TaO₈Br with Ru/SrTiO₃:Rh is successfully achieved for overall water splitting with a stoichiometric ratio of H₂ and O₂ evolutions. This work demonstrates a new series of semiconductors Bi₄TaO₈X (X = Cl, Br) with the promising application in the field of solar energy utilization.     

Atomic‐Level Reactive Sites for Semiconductor‐Based Photocatalytic CO2 Reduction

Photocatalytic CO₂ reduction is an effective means to generate renewable energy. It involves redox reactions, reduction of CO₂ and oxidation of water, that leads to the production of solar fuel. Significant research effort has therefore been made to develop inexpensive and practically sustainable semiconductor‐based photocatalysts. The exploration of atomic‐level active sites on the surface of semiconductors can result in an improved understanding of the mechanism of CO₂ photoreduction. This can be applied to the design and synthesis of efficient photocatalysts. In this review, atomic‐level reactive sites are classified into four types: vacancies, single atoms, surface functional groups, and frustrated Lewis pairs (FLPs). These different photocatalytic reactive sites are shown to have varied affinity to reactants, intermediates, and products. This changes pathways for CO₂ reduction and significantly impacts catalytic activity and selectivity. The design of a photocatalyst from an atomic‐level perspective can therefore be used to maximize atomic utilization efficiency and lead to a high selectivity. The prospects for fabrication of effective photocatalysts based on an in‐depth understanding are highlighted.     

The use of nanoscale visible light-responsive photocatalyst TiO2-Pt for the elimination of soil-borne pathogens

Exposure to the soil-borne pathogens Burkholderia pseudomallei and Burkholderia cenocepacia can lead to severe infections and even mortality. These pathogens exhibit a high resistance to antibiotic treatments. In addition, no licensed vaccine is currently available. A nanoscale platinum-containing titania photocatalyst (TiO(2)-Pt) has been shown to have a superior visible light-responsive photocatalytic ability to degrade chemical contaminants like nitrogen oxides. The antibacterial activity of the catalyst and its potential use in soil pathogen control were evaluated. Using the plating method, we found that TiO(2)-Pt exerts superior antibacterial performance against Escherichia coli compared to other commercially available and laboratory prepared ultraviolet/visible light-responsive titania photocatalysts. TiO(2)-Pt-mediated photocatalysis also affectively eliminates the soil-borne bacteria B. pseudomallei and B. cenocepacia. An air pouch infection mouse model further revealed that TiO(2)-Pt-mediated photocatalysis could reduce the pathogenicity of both strains of bacteria. Unexpectedly, water containing up to 10% w/v dissolved soil particles did not reduce the antibacterial potency of TiO(2)-Pt, suggesting that the TiO(2)-Pt photocatalyst is suitable for use in soil-contaminated environments. The TiO(2)-Pt photocatalyst exerted superior antibacterial activity against a broad spectrum of human pathogens, including B. pseudomallei and B. cenocepacia. Soil particles (<10% w/v) did not significantly reduce the antibacterial activity of TiO(2)-Pt in water. These findings suggest that the TiO(2)-Pt photocatalyst may have potential applications in the development of bactericides for soil-borne pathogens.     

In Situ Phase‐Induced Spatial Charge Separation in Core–Shell Oxynitride Nanocube Heterojunctions Realizing Robust Solar Water Splitting

Efficient spatial charge separation is critical for solar energy conversion over solid photocatalysts. The development of efficient visible‐light photocatalysts has been of immense interest, but with limited success. Here, multiband core–shell oxynitride nanocube heterojunctions composed of a tantalum nitride (Ta₃N₅) core and nitrogen‐doped sodium tantalate (NaTaON) shell have been constructed via an in situ phase‐induced etching chemical strategy. The photocatalytic water splitting performance of sub‐20‐nm Ta₃N₅@NaTaON junctions exhibits an extraordinarily high photocatalytic activity toward oxygen and hydrogen evolution. Most importantly, the combined experimental results and theoretical calculations reveal that the strong interfacial Ta? O? N bonding connection as a touchstone among Ta₃N₅@NaTaON junctions provides a continuous charge transport pathway rather than a random charge accumulation. The prolonged photoexcited charge carrier lifetime and suitable band matching between the Ta₃N₅ core and NaTaON shell facilitate the separation of photoinduced electron–hole pairs, accounting for the highly efficient photocatalytic performance. This work establishes the use of (oxy)nitride heterojunctions as viable photocatalysts for the conversion of solar energy into fuels.     

Contributions of Fe(III) to UV–Vis absorbance in river water: a case study on the Connecticut River and argument for the systematic tandem measurement of Fe(III) and CDOM

Biogeochemistry - Dissolved organic matter (DOM) impacts the structure and function of aquatic ecosystems. DOM absorbs light in the UV and visible (UV–Vis) wavelengths, thus impacting light...     

Decolorization and detoxification of sulphonated azo dye Red HE7B by <Emphasis Type="Italic">Bacillus</Emphasis> sp. VUS

Bacillus sp. VUS decolorized Red HE7B dye (100%) within 18 h in static anoxic conditions. A significant increase in activities of lignin peroxidase, laccase, NADH-DCIP and azo reductase was observed up to complete decolourization of RHE7B. The biodegradation was monitored by UV–Visible spectroscopy (UV–VIS), Fourier Transform Infrared (FTIR) spectroscopy and High Performance Liquid Chromatography (HPLC). The final products 4-methyl-3-(1-sulfo-ethyl)-5-([1,3,5] triazin-2-ylamino)-benzenesulfonic acid; 3-(1-sulfo-ethyl)-5-([1,3,5] triazin-2-ylamino)-benzenesulfonic acid and 3-(1,2-dihydro-[1,3,5] triazin-2-ylamino)-5-sulfomethyl-benzenesulfonic acid were characterized by gas chromatography–mass spectrometry (GC–MS). The phytotoxicity study revealed the non-toxic nature of the generated products with respect to Sorghum bicolor and Triticum aestivum. The metabolites produced after degradation increased the chlorophyll content of crop seedlings. The Ames test revealed the non-mutagenicity and non-carcinogenicity of the degraded products.     

Single‐Crystalline Nanomesh Tantalum Nitride Photocatalyst with Improved Hydrogen‐Evolving Performance

Tantalum nitride (Ta₃N₅) with a suitable bandgap (≈2 eV) is regarded as one of the most promising photocatalysts for efficient solar energy harvesting and conversion. However, Ta₃N₅ suffers from low hydrogen production activity due to the low carrier mobility and fast carrier recombination. Thus, the design of Ta₃N₅ nanostructures to facilitate charge carrier transport and improve photocatalytic performance remains a challenge. This study reports a new type of ultrathin (≈2 nm) Ta₃N₅ nanomesh with high specific surface area (284.6 m² g^?1) and excellent crystallinity by an innovative bottom‐up graphene oxide templated strategy. The resulting Ta₃N₅ nanomeshes demonstrate drastically improved electron transport ability and prolonged lifetime of charge carriers, due to the nature of high surface area and excellent crystallinity. As a result, when used as photocatalysts, the Ta₃N₅ nanomeshes exhibit a greater than tenfold improvement in solar hydrogen production compared to bulk counterparts. This work provides an effective and generic strategy for designing 2D ultrathin nanomesh structures for nonlayered materials with improved catalytic activity.     

Application of alkali-activated materials for water and wastewater treatment: a review

Alkali-activation (or geopolymer) technology has gained a great deal of interest for its potential applications in water and wastewater treatment during the last decade. Alkali-activated materials can be prepared via a relatively simple and low-energy process, most commonly by treating aluminosilicate precursors with concentrated alkali hydroxide and/or silicate solutions at (near) ambient conditions. The resulting materials are, in general, amorphous, have good physical and chemical stability, ion-exchange properties, and a porous structure. Several of the precursors are industrial by-products or other readily available low-cost materials, which further enhances the environmental and economic feasibility. The application areas of alkali-activated materials in water and wastewater treatment are adsorbents/ion-exchangers, photocatalysts, high-pressure membranes, filter media, anti-microbial materials, pH buffers, carrier media in bioreactors, and solidification/stabilization of water treatment residues. The purpose of this review is to present a comprehensive evaluation of the rapidly growing prospects of alkali-activation technology in water and wastewater treatment.

     

Ultraviolet light influences habitat preferences in a fish under predation risk

Environmental light conditions are of general importance in predator–prey interactions. In aquatic systems, prey individuals experience different levels of predation risk depending on the properties of the visual environment, such as structural complexity or water transparency. To reduce the threat of predation, prey should move to habitats providing better protection against visual predators. We studied the role of UV wavelengths in habitat choice behaviour under predation risk in a fish, the three-spined stickleback (Gasterosteus aculeatus) that uses UV signals in different contexts of intraspecific communication. In a laboratory experiment sticklebacks were exposed to a predatory threat and given the choice between two escape habitats, one providing full-spectrum conditions including UV light (UV+) and one without UV wavelengths (UV−). Fish from two rearing treatments were tested, one group had been raised under natural lighting conditions (UV+), the other group under UV-deficient lighting conditions (UV−). Sticklebacks from the UV+ group preferred the UV− habitat as a refuge which suggests that predator avoidance behaviour is UV-related in this species with UV− conditions presumably being advantageous for prey fish. However, individuals from the UV− treatment group were equally attracted to both presented light habitats. It is possible that these fish could not discriminate between the two light habitats due to physiological limitations caused by their rearing conditions. Further control trials with neutral-density filters revealed that the UV− habitat preference of UV+ fish in the main experiment was rather not influenced by a difference in achromatic brightness between the UV+ and UV− habitat.     

A Meta-Analysis of Chinese Herbal Medicine in Treatment of Managed Withdrawal from Heroin

Chinese herbal medicine has shown promise for heroin detoxification. This review extends a prior meta-analysis of Chinese herbal medicine for heroin detoxification, with particular attention to the time course of symptoms. Both English and Chinese databases were searched for randomized trials comparing Chinese herbal medicine to either α2-adrenergic agonists or opioid agonists for heroin detoxification. The methodological quality of each study was assessed with Jadad’s scale (1–2 = low; 3–5 = high). Meta-analysis was performed with fixed- or random-effect models in RevMan software; outcome measures assessed were withdrawal-symptoms score, anxiety, and adverse effects of treatment. Twenty-one studies (2,949 participants) were included. For withdrawal-symptoms score relieving during the 10-day observation, Chinese herbal medicine was superior to α2-adrenergic agonists in relieving opioid-withdrawal symptoms during 4–10 days (except D8) and no difference was found within the first 3 days. Compared with opioid agonists, Chinese herbal medicine was inferior during the first 3 days, but the difference became non-significant during days 4–9. Chinese herbal medicine has better effect on anxiety relieving at late stage of intervention than α2-adrenergic agonists, and no difference with opioid agonists. The incidence of some adverse effects (fatigue, dizziness) was significantly lower for Chinese herbal medicine than for α2-adrenergic agonists (sufficient data for comparison with opioid agonists were not available). Findings were robust to file-drawer effects. Our meta-analysis suggests that Chinese herbal medicine is an effective and safety treatment for heroin detoxification. And more work is needed to determine the specific effects of specific forms of Chinese herbal medicine.