Showing papers in "Applied Catalysis B-environmental in 2016"
TL;DR: In this paper, the authors provide a state-of-the-art review on the development in heterogeneous catalysts including single metal, mixed metal, and nonmetal carbon catalysts for organic contaminants removal, with particular focus on peroxymonosulfate (PMS) activation.
Abstract: Sulfate radical-based advanced oxidation processes (SR-AOPs) employing heterogeneous catalysts to generate sulfate radical (SO4 −) from peroxymonosulfate (PMS) and persulfate (PS) have been extensively employed for organic contaminant removal in water. This article aims to provide a state–of–the–art review on the recent development in heterogeneous catalysts including single metal, mixed metal, and nonmetal carbon catalysts for organic contaminants removal, with particular focus on PMS activation. The hybrid heterogeneous catalyst/PMS systems integrated with other advanced oxidation technologies is also discussed. Several strategies for the identification of principal reactive radicals in SO4 −–oxidation systems are evaluated, namely (i) use of chemical probe or spin trapping agent coupled with analytical tools, and (ii) competitive kinetic approach using selective radical scavengers. The main challenges and mitigation strategies pertinent to the SR-AOPs are identified, which include (i) possible formation of oxyanions and disinfection byproducts, and (ii) dealing with sulfate produced and residual PMS. Potential future applications and research direction of SR-AOPs are proposed. These include (i) novel reactor design for heterogeneous catalytic system based on batch or continuous flow (e.g. completely mixed or plug flow) reactor configuration with catalyst recovery, and (ii) catalytic ceramic membrane incorporating SR-AOPs.
1,802 citations
TL;DR: In this paper, the state-of-the-art progress on various heterogeneous cobalt-based catalysts for sulfate radical-based advanced oxidation processes (SR-AOPs) is reviewed.
Abstract: Recently sulfate radical-based advanced oxidation processes (SR-AOPs) attract increasing attention due to their capability and adaptability in decontamination. The couple of cobalt and peroxymonosulfate (PMS) is an efficient way to produce reactive sulfate radicals. This article reviews the state-of-the-art progress on various heterogeneous cobalt-based catalysts for PMS activation, including cobalt oxides, cobalt-ferrite and supported cobalt by diverse substrates. We summarize the intrinsic properties of these catalysts and their fundamental behaviors in PMS activation, as well as synthetic approaches. In addition, influencing factors and synergistic techniques of Co/PMS systems in organic degradation and possible environmental applications are also discussed. Finally, we propose perspectives on challenges related to cobalt-based catalysts, heterogeneous Co/PMS systems and their potential applications in practical environmental cleanup.
1,553 citations
TL;DR: In this article, a review of the fundamental aspects of photocatalysis as a pollution remediation strategy is presented, followed by an introduction to graphitic carbon nitride as a photocatalyst, preparation strategies and its properties, and a comprehensive and critical discussion of the various most recent developments towards enhancing the visible light photocatalytic properties of g-C 3 N 4 for pollution alleviation.
Abstract: Engineering photocatalytic materials for renewable energy generation and environmental decontamination has always been a very exciting prospect to counter the global energy demands and pollution challenges. Graphitic carbon nitride (g-C 3 N 4 ), a polymeric, metal-free semiconductor with a mild band gap (2.7 eV) has become hot-spot in various scientific exploits such as environmental pollution mitigation, energy generation and storage, organic synthesis, sensors, etc. These applications exploit the interesting properties of g-C 3 N 4 such as good visible light absorption, graphene-like structure, good thermal and chemical stability and photocatalytic properties. In this review we begin with an overview of the fundamental aspects of photocatalysis as a pollution remediation strategy. This is followed by an introduction to graphitic carbon nitride as a photocatalyst, preparation strategies and its properties. Subsequently, a comprehensive and critical discussion of the various most recent developments towards enhancing the visible light photocatalytic properties of g-C 3 N 4 for pollution alleviation, selected results and important photocatalytic degradation mechanisms, is given. Summary remarks and future perspective conclude the review.
920 citations
TL;DR: In this article, a direct solid-state Z-scheme V2O5/g-C3N4 heterojunctions were synthesized via a facile in-situ growth strategy for the first time.
Abstract: The constructing of direct solid-state Z-scheme heterojunction photocatalytic system has received much attention in environmental purification and hydrogen generation from water. In this study, a novel direct solid-state Z-scheme V2O5/g-C3N4 heterojunctions were synthesized via a facile in-situ growth strategy for the first time. The photocatalytic performance was evaluated by the degradation of rhodamine B (RhB) and tetracycline (TC) under visible light irradiation (λ > 420 nm). Results show that the as-synthesized heterojunctions can significantly enhance photocatalytic activity in comparison with pure g-C3N4 and V2O5. The optimum photocatalytic efficiency of VC1.0% sample for the degradation RhB was about 7.3 and 13.0 times higher than that of individual g-C3N4 and V2O5, respectively. In addition, the VC1.0% sample as well as can efficiently degrade methyl orange (MO) and methylene blue (MB) under visible light. By further experimental study, the possible for the enhancing photocatalytic mechanism was found to be a direct solid-state Z-scheme heterojunction system based on the active species trapping and electron spin resonance (ESR) experiments, which not only can improve the photogenerated electron–hole pair’s separation but also exhibit a strong oxidation and reduction ability for efficiency degradation of organic pollutants. This work will be useful for the design of other direct solid-state Z-scheme photocatalytic systems for application in energy conversion and environmental remediation.
597 citations
TL;DR: Wang et al. as discussed by the authors developed a p-n junction BiOI@Bi12O17Cl2 heterostructure via facilely in situ depositing biOI nanosheets on the surface of Bi 12O 17Cl2 plates.
Abstract: Synthesis of reactive exposing facets and p-n junction are of great importance for semiconductor photocatalysis. Herein, we develop a p-n junction BiOI@Bi12O17Cl2 heterostructure via facilely in situ depositing BiOI nanosheets on the surface of Bi12O17Cl2 plates. Owing to the charge inducement, the BiOI nanosheets are all vertically assembled onto the Bi12O17Cl2 large plates to form a unique front-lateral surfaces coupling heterostructure, which enables high exposure of {001} reactive exposing facets of BiOI. The photocatalytic properties are systematically evaluated by degrading multiform industrial contaminants and antibiotic, like 2,4-dichlorophenol (2,4-DCP), rhodamine B (RhB), phenol, bisphenol A (BPA), and tetracycline hydrochloride. It reveals that the BiOI@Bi12O17Cl2 heterostructure not only shows dramatically strengthened photocatalytic activity, but also unfold powerful and nonselective photooxidation ability under visible-light illumination. The photoelectrochemical characterizations demonstrated that the drastically promoted separation and transfer of charge carriers that derived from the benefits of BiOI {001} active facets and BiOI@Bi12O17Cl2 p-n junction are in charge of the high photo-activity. Detailed radicals detection and quantification experiments further corroborate our conclusions. The study may give us some new hints on designing novel heterostructured photoelectronic materials with integrating p-n junction and active exposing facets.
554 citations
TL;DR: In this paper, a possible enhanced photocatalytic mechanism of g-C 3 N 4 /TiO 2 heterojunction photocatalyst was proposed, which is attributed to high surface area and heterostructure formation.
Abstract: Mesoporous photocatalytic materials with macroporous structures have attracted more and more attention because of their textural mesopores and intrinsic interconnected pore networks, which are able to efficiently transport guest species to framework binding sites. In this work, macro/mesoporous g-C 3 N 4 /TiO 2 heterojunction photocatalysts were fabricated without templates or additives by a facile calcination method using tetrabutyl titanate and melamine as the feedstocks. Photocatalytic experiments of the as-prepared samples were measured by the photocatalytic oxidation degradation of RhB solution at room temperature under visible light irradiation. The results indicated that the melamine content in the precursors had an important influence on photocatalytic activity of the as-prepared samples. At the optimal loading content, the apparent reaction rate constant ( k ) was 47.8 × 10 −3 min −1 for RhB degradation, exceeding that of pure TiO 2 (6.6 × 10 −3 min −1 ) and g-C 3 N 4 (15.2 × 10 −3 min −1 ) by factor of 7.2 and 3.1 respectively. The improved photocatalytic activity was attributed to high surface area and heterostructure formation of g-C 3 N 4 /TiO 2 composites. The trapping experiment results showed that O 2 − and h + were main active species in the decomposition of RhB. A possible enhanced photocatalytic mechanism of g-C 3 N 4 /TiO 2 heterojunction photocatalysts was proposed.
539 citations
TL;DR: In this article, the peroxymonosulfate (PMS) activation was investigated on different carbocatalysts with varying carbon-conjugation structures and functional groups, and density functional theory (DFT) calculations were employed to explore the adsorption and O O O bond activation on the different carbon configurations.
Abstract: Metal-free activation of superoxides provides an efficient and environmentally benign strategy for heterogeneous catalytic oxidation. In this study, nanocarbons with varying carbon-conjugation structures and functional groups were investigated for peroxymonosulfate (PMS) activation. It was discovered that radical and nonradical oxidations could occur on different carbocatalysts depending on the carbon structure. Radical oxidation occurs exclusively on MWCNTs and CMK-3, similar to a metal oxide, MnO2. Both radical and nonradical oxidations are very pronounced in nanodiamond (AND-900)/PMS whilst nonradical oxidation is dominated in reduced graphene oxide (rGO-900)/PMS. Density functional theory (DFT) calculations were employed to explore the PMS adsorption and O O bond activation on the different carbon configurations for an in-depth probe of the activation mechanism. The intact sp2-conjugated π system in MWCNTs and electron-rich ketonic groups (as Lewis basic sites) in CMK-3 can stimulate PMS dissociation to generate SO4 − and OH, similar to metal-based catalysts. However, the defective edges at the boundary of carbon network are able to facilitate the organic degradation without generation of the reactive radicals, which is well supported by both experiments and the DFT calculation. The emerging nonradical oxidation induced by the carbocatalysis is superior to the radical oxidation on most metal oxides for effective degradation of various organics. The influences of solution pH, various anions (H2PO42−, HCO3− and Cl−) and background organic matters (humic acid) on the nonradical oxidation were further evaluated. The nonradical oxidation on carbocatalysts can be utilized as a green and effective oxidation strategy for aqueous environmental remediation and nonaqueous phase oxidation.
536 citations
TL;DR: In this paper, a core-shell In2S3@MIL-125(Ti) photocatalytic adsorbent was successfully prepared by a facile solvothermal method.
Abstract: Metal-organic frameworks (MOFs) have been attracted considerable attention in the field of energy generation and environmental remediation. In this article, a novel core–shell In2S3@MIL-125(Ti) (MLS) photocatalytic adsorbent was successfully prepared by a facile solvothermal method. The as-obtained materials were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, N2 adsorption–desorption isotherm, X-ray photoelectron spectroscopy, UV–vis diffuse reflection spectroscopy and zeta potentials. It is indicated that the hybrids consisted of MIL-125(Ti) as the core and three-dimensional In2S3 sheets network as the shell has high surface area, mesoporous structure, and improved electronegativity and visible-light absorption. The MLS exhibited excellent adsorption performance for the removal of tetracycline (TC) from water. The adsorption process is sensitive to the solution pH, ionic strength and initial TC concentration. The Langmuir isotherm and pseudo-second-order mode could well describe the adsorption process and adsorption kinetics. The adsorption mechanism is mainly responsible for surface complexation, π–π interactions, hydrogen bonding and electrostatic interactions. Further, in TC degradation experiments under visible light exposure in presence of core–shell MLS, the optimal additive content of MIL-125(Ti) in synthesis process was 0.1 g, and the corresponding photodegradation efficiency for TC was 63.3%, which was higher than that of pure In2S3 and pure MIL-125(Ti). The improved photocatalytic performance was mainly ascribed to the opened porous structure, effective transfer of photo-generated carriers, Ti3+–Ti4+ intervalence electron transfer and the synergistic effect between MIL-125(Ti) and In2S3. The degradation by-products of TC molecules were monitored by three-dimensional excitation-emission matrix fluorescence spectroscopy. Parts of TC molecules were mineralized into CO2 and H2O. The core–shell MLS composites also revealed good performance for the removal of TC from real wastewater including medical wastewater, municipal wastewater and river water. Therefore, the novel hybrids may be used as promising photocatalytic adsorbent for wastewater purification.
510 citations
TL;DR: In this paper, a mini-review presented photocatalytic reduction of Cr(VI) into Cr(III) in some MOFs or their derivatives/composites.
Abstract: This mini-review presented photocatalytic reduction of Cr(VI) into Cr(III) in some MOFs or their derivatives/composites. The reported examples are collected and analyzed; and the reaction mechanism, the influence of various factors on the photocatalytic performance, the involved challenges, and the outlooks of MOFs as photocatalyst to carry out Cr(VI) reduction are discussed. It can be found that the optical properties of MOFs can be flexible modulated via incorporation into NH 2 group, conductor photocatalysts like ZnO and metal sulfides nanoparticles, noble metal nanoparticles and graphene oxide (GO). It is clear that MOFs have a bright prospective in the fields of photocatalytic reduction of Cr(VI), or even dual functional photcatalysts to carry out Cr(VI) reduction and organic pollutants degradation under visible light irradiation.
494 citations
TL;DR: In this article, the preparation, characterization, and application of selected perovskite oxides such as SrTiO 3, KTaO 3, NaTaO3, KNbO3, and NaNbO 3 in photocatalysis are described.
Abstract: Perovskite-type oxides are a class of compounds with the general formula ABO 3 . They are a very important family of materials and exhibit properties suitable for numerous applications. In this review, the preparation, characterization, and application of selected perovskite oxides such as SrTiO 3 , KTaO 3 , NaTaO 3 , KNbO 3 , and NaNbO 3 in photocatalysis are described. In addition, various strategies for enhancing their photocatalytic activities are discussed, including doping with metals and nonmetals (Cr, Ni, Mn, Pb, Bi, N, Br, S, C, and F), modification with noble metal (Au, Ag, Pt, Pd, Ph, Ru) nanoparticles, and doping with rare earth elements (La). Moreover, the review summarizes the influence of different morphologies and surface properties on the photoactivity of the materials.
464 citations
TL;DR: In this paper, the authors demonstrated that photocatalytic H2O2 production at g-C3N4 could be improved by as much as 14 times in the absence of an organic scavenger through a carbon vacancy-based strategy.
Abstract: Hydrogen peroxide (H2O2) is of great significance in biological and environmental processes as well as in chemical industry. Even though anthraquinone autoxidation (AO) process has been the major artificial way to produce H2O2, its energy cost and non-green nature have been motivating people to develop more efficient, economic and green technologies as alternatives. Here we demonstrated that photocatalytic H2O2 production at g-C3N4 could be improved by as much as 14 times in the absence of organic scavenger through a carbon vacancy-based strategy. Both the experimental and theoretical calculation results indicated that the creation of carbon vacancies could reduce the symmetry of g-C3N4 and produce the effect of electron delocalization. This will allow g-C3N4 to possess more excitable electrons and a narrower band gap. On the other hand, carbon vacancies provided more sites to adsorb molecular oxygen and thereby help electrons transfer from g-C3N4 to the surface adsorbed O2. More interestingly, the presence of carbon vacancies changed the H2O2 generation pathway from a two-step single-electron indirect reduction to an one-step two-electron direct reduction. This study could not only develop a novel strategy to improve the H2O2 production activity of semiconductors, but also shed light on the deep understanding of the role played by surface defect structure on photocatalytic activity of semiconductor photocatalysts.
TL;DR: In this paper, the degradation of methylene blue (MB) over Ag 3 PO 4 @g-C 3 N 4 composites was investigated to evaluate their photocatalytic performance.
Abstract: Ag 3 PO 4 @g-C 3 N 4 core@shell composites were prepared via an ultrasonication/chemisorption method. The degradation of methylene blue (MB) over Ag 3 PO 4 @g-C 3 N 4 composites was investigated to evaluate their photocatalytic performance. The Ag 3 PO 4 @g-C 3 N 4 sample presented the best photocatalytic activity, degrading 97% MB after irradiation for 30 min. Superior stability was also observed in the cyclic runs. The composite has excellent photocatalytic activity and photo-stability and the optimal content of g-C 3 N 4 in the composites is 7.0 wt.%. The efficient photo-generated charge separation originated from a strong interaction in the intimately contact interface, which was confirmed by the results of photocurrent and EIS measurements. Based on the experimental results, a photocatalytic mechanism for organics degradation over Ag 3 PO 4 @g-C 3 N 4 photocatalysts was proposed.
TL;DR: In this article, the electronic structures, optical properties and effective masses of charge carriers of N-, C- and S-doped ZnO were investigated by first-principle density functional theory calculation.
Abstract: In general, N-, C- and S-doped ZnO exhibit much higher phototcatalytic activity than the pure ZnO. However, the essential factors and underlying mechanism regarding the enhancement of photocatalytic activity are still unclear. In this work, the electronic structures, optical properties and effective masses of charge carriers are investigated by first-principle density functional theory calculation. Due to the nature of p-type doping, N and C doping can generate vacant states above the Fermi level and shift the conduction band into lower energy region, resulting in narrowing of band gap. Thus, N- and C-doped ZnO demonstrate much stronger light absorption in both visible and ultraviolet region. In contrast, because of the absence of vacant states, only limited enhancement of light absorption is observed for S-doped ZnO whose improved photocatalytic performance can only be attributed to the direct reduction of band gap. The calculation of the effective masses show that ZnO typically possess light electrons and heavy holes, confirming its intrinsic character of n-type semiconductor, while N, C and S doping can generally render electrons lighter and holes heavier, resulting in slower recombination rate of photogenerated electron–hole pairs. Noticeably, C doping can discourage such recombination to the greatest extent and separate electron–hole pairs most efficiently compared with N and S doping, serving as a potentially promising pathway to increase the quantum efficiency of ZnO-based photocatalysts. This work will provide some new insights into the understanding of doping effect over the enhancement of photocatalytic activity of N-, C- and S-doped ZnO.
TL;DR: In this paper, a facile synthesis of bromine doped graphitic carbon nitride (g-C 3 N 4 ) photocatalysts for hydrogen evolution with visible light irradiation is presented.
Abstract: Hydrogen production by semiconductor photocatalysis using abundant sunlight and water is an ideal method to address the globe energy and environment issues. Here, we present a facile synthesis of bromine doped graphitic carbon nitride (g-C 3 N 4 ) photocatalysts for hydrogen evolution with visible light irradiation. Bromine modification is shown to enhance the optical, conductive and photocatalytic properties of g-C 3 N 4 , while still keeping the poly-tri-s(triazine) core structure as the main building blocks of the materials. This modification method can be generally applicable to several precursors of g-C 3 N 4 , including urea, dicyandiamide, ammonium thiocyanide, and thiourea. The optimal sample CNU-Br 0.1 shows more than two times higher H 2 evolution rates than pure CNU sample under visible light irradiation, with high stability during the prolonged photocatalytic operation. Results also found that the photocatalytic O 2 evolution activity of CNU-Br 0.1 was promoted when the sample was subjected to surface kinetic promotion by loading with cobalt oxide as a cocatalyst. This study affords us a feasible modification pathway to rationally design and synthesize g-C 3 N 4 based photocatalysts for a variety of advanced applications, including CO 2 photofixation, organic photosynthesis and environmental remediation.
TL;DR: In this article, a two-dimensional (2D) porous ultrathin oxygen-doped g-C3N4 nanosheets (PUOCNs) were prepared to study morphology, microstructure, crystal structure, chemical states and photocatalytic performance of PUOCNs.
Abstract: The novel two-dimensional (2D) porous ultrathin oxygen-doped g-C3N4 nanosheets (PUOCNs) were prepared. The comprehensive characterization methods were used to study morphology, microstructure, crystal structure, chemical states and photocatalytic performance of PUOCNs. 2D porous ultrathin structure and the introduction of oxygen are beneficial to the enhancement of the photocatalytic activity of PUOCNs. The average H2 evolution rate of PUOCNs is ∼189.3 μmol h−1, which is ∼5.2 times higher than that of the bulk g-C3N4 and the degradation efficiency of organic dye methyl orange (MO) is almost 71 times higher than that of the bulk g-C3N4. The enhanced photocatalytic activity is due to the more adsorption sites and more active sites, the enhanced redox ability and improved electron transport ability, which leads to less recombination and a more efficient separation of photogenerated electron and hole pairs. Through XPS VB and ESR analysis, the photocatalytic mechanism was also researched in detail.
TL;DR: In this paper, a metal-free activation of persulfate (PS) on annealed nanodiamonds (ANDs) was investigated, which demonstrated superior performances in decomposition of various pollutants to conventional metal-based catalysis.
Abstract: Production of radicals by metal-free catalysis is expected to offer a promising oxidative reaction for remediation of emerging contaminants. In this study, novel metal-free activation of persulfate (PS) on annealed nanodiamonds (ANDs) was investigated, which demonstrated superior performances in decomposition of various pollutants to conventional metal-based catalysis. Comprehensive investigations on the effects of reaction parameters, such as solution pH, reaction temperature, initial phenol concentration, catalyst loading, PS usage, the presence of chlorine ions and humic acid, on phenol degradation were carried out. In addition, nanodiamond (ND) material optimization and reusability were also studied. Electron paramagnetic resonance (EPR) and selective organic degradation unraveled that the PS/AND system may produce both hydroxyl radicals ( OH) and sulfate radicals (SO4 −), initialized from oxidizing water molecules on the nanodiamond surface. The carbocatalysts served as an excellent electron tunnel to facilitate the charge transfer from water or hydroxide ions to PS, and the oxidized intermediates may play a crucial role in PS activation. Electrochemical analyses in PS oxidant solution and oxygen reduction reaction (ORR) were carried out to understand O O bond activation by the metal-free catalysis. This study provides an environmentally benign and highly efficient oxidative reaction system with reactive radicals along with insights into the metal-free PS activation process.
TL;DR: In this paper, a facile strategy to synthesize porous FexCo3−xO4 nanocages by heating Prussian blue analogues FeyCo1−y[Co(CN)6]0.67 nH2O nanospheres with tunable size and morphology was reported.
Abstract: Here we report a facile strategy to synthesize porous FexCo3−xO4 nanocages by heating Prussian blue analogues FeyCo1−y[Co(CN)6]0.67 nH2O nanospheres with tunable size and morphology. The iron doping amount had significant influence on the final morphology and the most uniform nanocages were obtained from x = 0.8. The catalytic performance of the nanocages was thoroughly evaluated by activation of peroxymonosulfate (PMS) for removal of bisphenol A (BPA) in water. The influence of different process parameter on the BPA degradation efficiency was examined and the catalytic stability was tested. The BPA degradation pathway was proposed based on GC–MS and LC–MS results. The involved radicals were identified through radical scavenging experiments and electron paramagnetic resonance spectroscopy. Mossbauer and XPS techniques were applied to illustrate the catalytic mechanism and B-site CoII on the surface of FexCo3−xO4 nanocages was determined as the main factor for PMS activation. Results indicate that porous FexCo3−xO4 nanocages are available to serve as alternative environmentally friendly catalysts for pollutants removal by activation of PMS.
TL;DR: In this paper, a p-n heterojunction photocatalyst n-BiVO4@p-MoS2 with core-shell structure was successfully fabricated for the first time through a facile in-situ hydrothermal method.
Abstract: The novel p–n heterojunction photocatalyst n-BiVO4@p-MoS2 with core–shell structure was successfully fabricated for the first time through a facile in-situ hydrothermal method, in which MoS2 shell thickness was easily tuned by varying the concentration of MoS2 precursor in the solution. The photocatalytic performances of samples were systematically investigated via the photocatalytic reduction of Cr6+ and oxidation of crystal violet (CV) under visible-light irradiation. The BiVO4@MoS2 samples exhibited excellent photocatalytic performance, among which, the BiVO4@MoS2 (10 wt%) sample with MoS2 shell 300 nm thickness, showed the highest photoreduction and photooxidation activities. The enhanced photocatalytic activities could be attributed to the suppression of charge recombination, the high specific surface area and strong adsorption ability toward the pollutant molecule, and the enhanced or tunable light absorption of BiVO4@MoS2. Especially, the core–shell structure geometry also increases the contact area between BiVO4 and MoS2, which facilitates the charge transfer at the BiVO4/MoS2 interface. The photocatalytic mechanism of BiVO4@MoS2 for reduction of Cr6+ and oxidation of CV was discussed in detail. Moreover, 12 photocatalytic degradation intermediates and products of CV were also identified by the gas chromatography–mass spectrometer (GC–MS).
TL;DR: In this article, a novel one-step ionic liquid induced strategy has been reported for the controlled synthesis of carbon quantum dots (CQDs)/BiOX (X = Br, Cl) hybrid nanosheets with tunable CQDs loading contents.
Abstract: A novel one-step ionic liquid induced strategy has been reported for the controlled synthesis of carbon quantum dots (CQDs)/BiOX (X = Br, Cl) hybrid nanosheets with tunable CQDs loading contents. Such synthetic process allows the CQDs well dispersed on the surface of BiOX nanosheets. Three different types of pollutants, such as phenol rhodamine B (RhB), antibacterial agent ciprofloxacin (CIP) and endocrine disrupting chemical bisphenol A (BPA) were chosen to evaluate the photocatalytic activity of CQDs/BiOX composite nanosheets. They show very interesting CQDs loading content and X composition-dependent photocatalytic activity with 3 wt% CQDs/BiOBr nanosheets showing the highest photocatalytic activity (much better than pure BiOBr nanosheets) for the degradation of RhB, CIP and BPA under visible light irradiation. The results reveal that there are three factors in promoting the photocatalysis of 3 wt% CQD/BiOBr nanosheets: high visible light absorbance, high separation efficiency of photo-induced electrons and holes and lower resistance. The active species trap experiments and electron spin resonance (ESR) reveal that during the photocatalytic process, hole and O2 − become the dominant species for the degradation of pollutants. The unique strategy demonstrated here can be extended to other hybrid nanosheet systems, making it possible to design and tune advanced photocatalysts for other important chemical and catalytic reactions.
TL;DR: In this article, the authors reported the controlled preparation of anatase TiO2 nanosheet photocatalyst by selectively depositing Co3O4 nanoparticles (NPs) as water oxidation cocatalyst (WOC) and Pt NPs as water reduction cocatalysts (WRC) on {001} and {101} facets, respectively, using a two-step photodeposition method.
Abstract: The photocatalytic hydrogen production using solar energy through water splitting has received great attention due to the increasingly serious energy crisis. Herein, we report the controlled preparation of anatase TiO2 nanosheet photocatalyst by selectively depositing Co3O4 nanoparticles (NPs) as water oxidation cocatalyst (WOC) and Pt NPs as water reduction cocatalyst (WRC) on {001} and {101} facets, respectively, using a two-step photodeposition method. The prepared TiO2-Co3O4-Pt composite photocatalyst exhibits a greatly enhanced photocatalytic H2-production activity at the optimal weight percentage of Co3O4 and Pt (both 1.0 wt%), exceeding that of TiO2 nanosheet deposited with single Co3O4 or Pt cocatalyst by 9.4 and 1.8 times, respectively. The enhanced H2-production activity is due to the synergetic effect of surface heterojunction between {001} and {101} facets and selective deposition of Co3O4 and Pt dual-cocatalysts at {001} and{101} facets, respectively. The former is beneficial for the transfer and separation of charge carriers, the latter can reduce the recombination rate of photogenerated electrons and holes and also catalyze the redox reactions. This work will provide a new route for the rational design and fabrication of highly efficient photocatalysts with dual-cocatalysts through selective surface deposition.
TL;DR: In this article, a highly efficient visible-light-driven acidified g-C3N4 (ACNS)/g-C 3N4 isotype heterojunction photocatalysts were synthesized by ultrasonic dispersion assisted electrostatic self-assembly strategy for the first time.
Abstract: A highly efficient visible-light-driven acidified g-C3N4 (ACNS)/g-C3N4 isotype heterojunction photocatalysts were synthesized by ultrasonic dispersion assisted electrostatic self-assembly strategy for the first time. The photocatalytic oxidation ability of the novel photocatalysts were evaluated using methyl orange (MO) as a target pollutant. The obtained ACNS/g-C3N4 photocatalysts were characterized by X-ray powder diffraction (XRD), Fourier transform infrared spectrometry (FTIR), UV–vis diffuse reflection spectroscopy (DRS), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) methods. The photocatalysts exhibited a significantly enhanced photocatalytic performance in degrading MO under visible light illumination (λ > 420 nm) compared with the pristine ACNS and g-C3N4 solely. The optimal ACNS content for the photocatalytic activity of the heterostructured composites was determined. The 30 wt.% ACNS/g-C3N4 exhibited the highest photocatalytic activity, which showed a reaction rate constant as high as 0.0216 min−1, 4.3 times higher than that of bare g-C3N4. The mechanism of the photocatalysts was investigated by determination of reactive species in the photocatalytic reactions and photoluminescence technique. The quenching effects of different scavengers displayed that the reactive h+ and O2− played major role in the reaction systems. The synergic effect between the ACNS and g-C3N4 was found to lead to an improved photo-generated carrier separation and hence the photocatalytic activities of the composite photocatalysts were increased significantly.
TL;DR: In this article, the photocatalytic activity of the ZnO/rGO composite was investigated under solar light and under visible light irradiation using the Orange II dye in aqueous solution.
Abstract: Small-sized ZnO rods with an average length of ca. 180 nm and a diameter of ca. 16 nm were successfully associated to reduced graphene oxide (rGO) via a solvothermal reaction conducted in ethanol. A set of characterization including TEM, SEM, XRD, BET, Raman spectroscopy and UV–vis absorption confirm that the ZnO/rGO composite is composed of highly dispersed ZnO rods bound to rGO nanosheets. The photocatalytic activity of the ZnO/rGO composite was investigated under solar light and under visible light irradiation using the Orange II dye in aqueous solution. Results indicate that the ZnO/rGO composite containing 10 wt% rGO used under solar light irradiation exhibit the highest photocatalytic activity and that the kinetic of reduction is of pseudo-fist-order. The photocatalyst is only weakly sensitive to pH changes and to the presence of inorganic salts or to glucose in the reaction medium. In addition, the reusability of the ZnO/rGO composite was studied and the results demonstrate that the photocatalyst can be reused up to fifteen times with nearly negligible loss of activity. The high photocatalytic performances can be attributed to the high specific surface of ZnO rods, to the enhanced visible light absorption of the ZnO/rGO composite and to the strong decrease of charge carrier recombinations originating from the association of ZnO rods with rGO.
TL;DR: In this article, the effect of MnO2 structure on ozone decomposition has been investigated, and three tunnel-structure polymorphs, i.e., α-, β- and γ-MnO2 were prepared and characterized by BET, TEM, XRD, H2-TPR, O2TPD, NH3-TPD and TGA-MS and XPS.
Abstract: Ozone is a ubiquitous pollutant and manganese dioxide (MnO2) has been widely used for ozone decomposition. However, the effect of MnO2 structure on ozone decomposition has never been investigated. Three tunnel-structure polymorphs, i.e., α-, β- and γ-MnO2 were prepared and characterized by BET, TEM, XRD, H2-TPR, O2-TPD, NH3-TPD, TGA-MS and XPS. The activity of three MnO2 polymorphs for ozone decomposition followed the order of α- > γ- > β-MnO2. The α-MnO2 owned the largest specific surface area and lowest average oxidation state of Mn. Furthermore, the adsorbed oxygen species on the surface of α-MnO2 were more easily reduced. In-situ Raman spectroscopy results showed that peroxide species formed during ozone decomposition, and over α-MnO2 they were more easily decomposed by increasing reaction temperature. It was found that the catalytic activity of MnO2 strongly depended on the density of oxygen vacancies. Accordingly, the ozone decomposition mechanism based on the involvement and recycling of oxygen vacancy (VO) is proposed. The decomposition of peroxide species is a rate-limiting step. These findings are helpful for designing more effective catalyst for ozone removal.
TL;DR: In this paper, a highdispersed Ag/Fe3O4/g-C3N4 composite photocatalyst is firstly synthesized by means of the selective photo-deposition technique.
Abstract: A high-dispersed Ag/Fe3O4/g-C3N4 composite photocatalyst is firstly synthesized by means of the selective photo-deposition technique. It exhibits the obvious improvement of photocatalytic activity and stability for degrading tetracycline besides retaining the recycled magnetic property. The enhanced photocatalytic activity originates from synergetic effect of Ag, Fe3O4 and g-C3N4 that improves light absorption capacity and separation efficiency of charge carriers. This work provides a promising approach to develop visible-light-response and recycled photocatalysts applied to antibiotic wastewater treatment.
TL;DR: In this paper, a visible light response photocatalyst was synthesized via a simple reduction-oxidation method, and the structure, morphology and optical properties of the catalyst were well characterized.
Abstract: In this study, a visible (Vis) light response photocatalyst was synthesized via a simple reduction–oxidation method. The structure, morphology and optical properties of the catalyst were well characterized. The absorption capability of ZnFe 2 O 4 in visible-light region was demonstrated by the high rate of Orange II decolorization under Vis/ZnFe 2 O 4 /H 2 O 2 process. Guided by studies to explore the effects of radical scavengers and to quantify the yield of hydroxyl radical ( OH) production, OH on the surface of the catalyst was found to be the dominating reactive species for the Orange II removal. Moreover, ZnFe 2 O 4 maintained high activity, crystallinity and extremely low iron and zinc leaching during repeated experiments. The intermediate products were identified by GC–MS and a possible pathway is accordingly proposed to elucidate the mechanism of Orange II degradation by OH. In addition, high extent of mineralization was obtained as the chemical oxygen demand (COD) and total organic carbon (TOC) removal efficiencies were 86.6% and 60.4%, respectively, within 60 min reaction. The toxicity tests with activated sludge indicated that the toxicity of the solution increased during the first 30 min but then decreased significantly as the oxidation proceeded.
TL;DR: In this paper, an Ag/g-C3N4 composite photocatalyst, which was synthesized by thermal polymerization of melamine precursor combined with the photo-assisted reduction method, was applied as an efficient visible-light-driven photatalyst for inactivating Escherichia coli (E. coli).
Abstract: Ag/g-C3N4 composite photocatalyst, which was synthesized by thermal polymerization of melamine precursor combined with the photo-assisted reduction method, was applied as an efficient visible-light-driven photocatalyst for inactivating Escherichia coli (E. coli). The composite photocatalysts exhibited significantly enhanced photocatalytic disinfection efficiency than pure g-C3N4 powders. The mechanism of enhanced disinfection activity was systematically investigated by UV–visible diffuse reflectance spectra, photoluminescence spectra, and photo-electrochemical methods including photogenerated current densities, electrochemical impedance spectroscopy (EIS) spectra and Mott–Schottky plots. The enhanced photocatalytic bactericidal effect was attributed to the hybrid effect from Ag and g-C3N4, which resulted in enhanced adsorption of visible light, reduced recombination of free charges, rapid separation and transportation of photogenerated electrons–holes. The disinfection mechanism was studied by employing chemical scavengers and ESR technology, indicating the important role of h+ and e−. Considering the bulk availability and excellent disinfection activity of Ag/g-C3N4, it is a promising solar-driven photocatalyst for cleaning microbial contaminated water in practice.
TL;DR: In this article, a facile, environmentally friendly route is demonstrated for the synthesis of Pt-Au dendrimer-like nanoparticles on the surface of polydopamine (PDA)-wrapped reduced graphene oxide (RGO), in which Pt alloy nanoparticles are synthesized by the reduction of H2PtCl6 and HAuCl4 with ascorbic acid.
Abstract: A facile, environmentally friendly route is demonstrated for the synthesis of Pt–Au dendrimer-like nanoparticles on the surface of polydopamine (PDA)-wrapped reduced graphene oxide (RGO), in which Pt–Au alloy nanoparticles are synthesized by the reduction of H2PtCl6 and HAuCl4 with ascorbic acid. The effects of support material and chemical composition on the catalytic activity for the reduction of 4-nitrophenol (4-NP) are investigated in detail. Pt nanoparticles supported on PDA/RGO (Pt-PDA/RGO) exhibit significantly higher catalytic activity as compared to those exhibited by Pt nanoparticles deposited on pristine graphene sheets (Pt-RGO) and commercial Pt/C catalyst. Furthermore, the chemical composition seriously affects the catalytic ability of the catalysts. With Pt-to-Au molar ratios of 3/1 and 1/1, significantly enhanced catalytic activities are observed, outperforming the support decorated with each single constituent. The high activity of Pt-Au-PDA/RGO can be explained by electronic effect involving in two types of electron transfers: (1) from the PDA coating to both Au and Pt atoms; (2) from Au to Pt atoms. Moreover, the Pt3Au1-PDA/RGO composite keeps a stable conversion efficiency of around 100% over six successive reduction reaction cycles. Through an experimental device of “filtering and catalyzing,” the Pt3Au1- PDA/RGO sample exhibits superior efficiency for the purification of water containing 4-NP. Within 8 s, the water becomes colorless.
TL;DR: In this article, the authors developed a one-step hydrothermal process to synthesize TiO 2 /C 3 N 4 heterojunctions with (001) facet exposed to solve the recombination problem of photogenerated charge carriers.
Abstract: Z-scheme overall water splitting based on semiconductors has been extensively investigated for hydrogen fuel production from renewable resources. The Z-scheme involves two different kinds of semiconductor photocatalysts and the closely related oxidation-reduction processes. However, the two separated photocatalytic processes require good production and separation of carriers, which greatly limit the efficiency of overall water splitting. Herein, to solve the recombination problem of photogenerated charge carriers, we developed a one-step hydrothermal process to synthesize TiO 2 /C 3 N 4 heterojunctions. The layered g-C 3 N 4 provides the template and guidance for the anatase TiO 2 heterogeneous nucleation with (001) facet exposed. Meanwhile, the bulk g-C 3 N 4 turned into thicker nanosheets through the self-exfoliation with the growth of TiO 2 . The heterojunctions with TiO 2 (001) facet exposed ensure the efficient separation of photogenerated carriers, which accordingly enhances the photocatalytic hydrogen evolution. WO 3 and BiVO 4 are chosen as photocatalysts for the half reaction of oxygen evolution. With suitable redox mediators i.e. I − /IO 3 − or Fe 2+ /Fe 3+ , the overall water splitting to H 2 and O 2 could be achieved. If β-Ni(OH) 2 , was loaded on the oxygen evolution photocatalysts, the efficiency of Z-scheme water splitting was greatly enhanced. Under the monochromatic light irradiation of LEDs, the apparent quantum efficiency (AQE) values for the Z-scheme of TiO 2 /C 3 N 4 , β-Ni(OH) 2 /WO 3 (PtOx) and I − /IO 3 − system were 4.94% and 4.01% under 365 and 405 nm, respectively. The heterojunction strategy for enhancing Z-scheme overall water splitting may give us some hints on the design of composite systems for overall water splitting.
TL;DR: In this article, a 3D network structure for removing Cr(VI) from aqueous solutions using a TiO 2 -graphene hydrogel with three-dimensional (3D) network structure was proposed.
Abstract: A novel method for removing Cr(VI) from aqueous solutions used a TiO 2 -graphene hydrogel with three-dimensional (3D) network structure. Graphene is capable of non-porous surface adsorption and π-π interaction adsorption. The combination between graphene and TiO 2 nanospheres promoted photo-induced charge transport and separation, thereby facilitating photocatalytic reduction of Cr(VI). For TiO 2 -rGH, the synergy of adsorption and photocatalysis played an important role in removing Cr(VI) from aqueous solutions. The TiO 2 -rGH material exhibited superb adsorption-photocatalysis performance, removing 100% Cr(VI) from a solution containing (5 mg/L) within 30 min under UV irradiation. Under continuous flow conditions, the adsorption-photocatalytic activity of TiO 2 -rGH was well maintained and the Cr(VI)-removing percentage was kept at 100% for a prolonged period until the breakthrough point was reached.
TL;DR: In this paper, carbon dots (C-dots) modified graphitic carbon nitride (g-C 3 N 4 ) hybrid was successfully prepared by a novel strategy using carbon dots and dicyandiamide as starting materials, and the photocatalyst was characterized by scanning electron microscope (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), FT-IR, UV-Vis diffuse reflectance spectrum (DRS), powder photoluminescence (PL) and surface photovoltage spectrum (SPS).
Abstract: As a promising metal-free photocatalyst, graphitic carbon nitride (g-C 3 N 4 ) has attracted increasing attention. However, from the viewpoint of practical application, the quantum efficiency of g-C 3 N 4 needs to be further improved. In this article, carbon dots (C-dots) modified g-C 3 N 4 hybrid was successfully prepared by a novel strategy using C-dots and dicyandiamide as starting materials. The photocatalyst was characterized by scanning electron microscope (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), FT-IR, UV–Vis diffuse reflectance spectrum (DRS), X-ray photoelectron spectroscopy (XPS), powder photoluminescence (PL) and surface photovoltage spectrum (SPS). Both the photocatalytic activity of C-dots modified g-C 3 N 4 was evaluated by degradation of Rhodamine B under UV irradiation and photocatalytic hydrogen production under visible irradiation. The experimental results show that C-dots modification causes the lattice distortion of g-C 3 N 4 . With increase in the loading amount of C-dots, the photocatalytic activity of g-C 3 N 4 increase first and then decrease. g-C 3 N 4 modified with 0.25 wt.% C-dots shows the highest photocatalytic activity, which is 3 times higher than pristine g-C 3 N 4 . C-dots act as electron-sinks, which prevent the recombination of photo-generated electron-hole pairs, enhancing the photocatalytic activity of g-C 3 N 4 . However, too much C-dots become recombination centers, which is detrimental to the photocatalytic activity of g-C 3 N 4 .