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Hydrogen peroxide (H2O2) is a mild, environmentally friendly, and versatile oxidizer. The current main production process, the anthraquinone method, has high energy consumption and pollution. However, the photocatalytic production of H2O2 is considered to be a green and sustainable process. In this paper, the research progress of photocatalytic production of H2O2 was reviewed. Firstly, the basic principle of photocatalytic production of H2O2 and the three production pathways, namely, oxygen reduction, water oxidation, and dual-channel were introduced. Then, advanced photocatalysts for H2O2 production were introduced with emphasis. From the reactants and products, several new reaction systems and improvement strategies were introduced, including the construction of a spontaneous system without a sacrifice agent, a gas–liquid–solid three-phase system, and inhibition of H2O2 decomposition. The in-situ applications of photocatalytic H2O2 production in the environmental field, including in-situ disinfection and in-situ degradation of pollutants are discussed. Finally, the research progress and future research directions in this field are summarized and prospected. [ABSTRACT FROM AUTHOR]

Hydrogen peroxide (H2O2) has received increasing attention as an important green-oxidate and energy carrier, but the H2O2 production industry has been plagued by high energy consumption and high environment risk in current processing technology. Photocatalytic production of H2O2 from earth-abundant water and molecular oxygen (O2) is seemed to be a potential solution. Core–shell CdS@CdIn2S4 heterojunctions was prepared through a "sequential two-step hydrothermal growth" route. The photoexcited CdIn2S4 moiety transfers the conduction band electrons to CdS, leading to selective production of H2O2 via single-electron reduction of O2. In contrast, the valence band holes photoformed on the CdS moieties are transferred to CdIn2S4, leading to efficient oxidation of water. The electron–hole separation enhanced by the core–shell structure of CdS@CdIn2S4 heterojunctions and significantly suppresses the charge recombination, which resulting in greatly enhanced photocatalytic activity for H2O2 production. The resultant catalyst shows an ultra-high H2O2 production rate of 1408.4 μmol/L in non-sacrificial system, which is about 3.8 times higher than that of pure CdS. This inorganic photocatalysis therefore shows potential as an artificial photosynthesis for H2O2 production by powdered catalysts. [ABSTRACT FROM AUTHOR]

The development of high-efficiency economic photocatalyst for H2O2 production is of great significance for renewable energy technologies. Here, we use a successive in situ growth and phosphating method successfully prepared NiCoP/g-C3N4 composite photocatalyst. NiCoP was uniformly dispersed in the form of nanoparticles on the surface of g-C3N4 nanosheet as a cocatalyst. The obtained NiCoP/g-C3N4 composite showed excellent performance in photocatalytic production of H2O2, which was due to NiCoP, has a better electronic conductivity to efficiently transfer charge, improves charge separation efficiency and enhances visible light absorption. This study can provide an effective theoretical basis for the design of bimetallic phosphide modified photocatalysts for photocatalytic production of H2O2. [ABSTRACT FROM AUTHOR]

The utilization of TiO2-based photocatalysts for an efficient removal of dye pollutants is limited due to their low surface area, high recombination rate of the photo-generated electrons and holes and an insufficient absorption of sunlight with only 3–4% use of solar energy. Based on this background, we have successfully fabricated a carbon nitride quantum dots (CNQDs)-modified mesoporous TiO2–SiO2 (Me-TSCN) via a simple calcination method since the mesoporous structure is expected to induce a great increase in the specific surface area by carbon nitride quantum dots, being favorable for the adsorption of the reactant molecules and the mass transfer of the reaction products. Furthermore, such CNQD modification facilitates the separation of photo-generated electrons and holes. As a result, the composite's modification with structural design could significantly improve the photocatalytic activity of TiO2-based catalysts, achieving outstanding photocatalytic activity in the degradation of dye pollutants such as rhodamine B. [ABSTRACT FROM AUTHOR]

A composite catalyst combining CoP with g-C3N4 was explored for the photocatalytic production of H2O2 under visible-light irradiation. The composite catalyst was fabricated by growth of CoP nanoparticles on the surface of g-C3N4. The CoP nanoparticles were well dispersed on the surface of g-C3N4 and could interact with g-C3N4 to improve charge separation and electron transfer. The composite catalyst showed superior catalytic activity compared with pure CoP or g-C3N4 under visible-light irradiation. The optimal catalyst with 1.76 wt.-% CoP loading exhibited the best photocatalytic efficiency with an H2O2 production of 140 µm in 2 h, which is about 4.6 and 23.3 times those of pure g-C3N4 and pure CoP, respectively. A possible mechanism for the visible-light-driven photocatalytic production of H2O2 is proposed. The composite catalyst exhibited stable performance without obvious loss of catalytic activity after seven successive runs, and thus has good application prospects in sustainable H2O2 production. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ag/AgVO 3 was heterojuncted with C-rich g-C 3 N 4 via hydrothermal-calcining method. • The Z-scheme mechanism, instead of type II principle, drove the charge transfer. • The loading of AgVO 3 widened the light absorption edge. • The degradation pathway of SFZ was proposed and the toxicity was evaluated. • The effects of water parameters on SFZ degradation were systematically assessed. A novel Z-scheme Ag/AgVO 3 /carbon-rich g-C 3 N 4 heterojunction with excellent solar-light-driven photocatalytic activity was constructed via a facile hydrothermal-calcining method. The Ag/AgVO 3 /carbon-rich g-C 3 N 4 composites displayed superior performance for the photocatalytic degradation of sulfamethiadiazole (SFZ) under solar irradiation. The optimal composite with a 10 wt% Ag/AgVO 3 content showed the highest photocatalytic activity, its degradation rate constant (k) for SFZ degradation was ∼13 and 30 times than that of carbon-rich g-C 3 N 4 (CCN) and Ag/AgVO 3 , respectively. Furthermore, •O 2 – was identified as the most crucial reactive species in the Z-scheme photocatalysis system. The greatly improved photocatalytic activities are derived from the built-in electric field (BIEF) of CCN and efficient Z-scheme charge transfer with Ag nanoparticles as charge transmission-bridge. The possible photocatalytic degradation mechanism and pathway over Ag/AgVO 3 /carbon-rich g-C 3 N 4 were proposed based on LC-MS analysis and density functional theory (DFT) calculation, and the toxicity of intermediates was evaluated by Quantitative structure–activity relationship (QSAR) based prediction. In summary, this work provides new insight into constructing highly efficient Z-scheme photocatalyst, which is promising for implementation in surface water remediation. [ABSTRACT FROM AUTHOR]

[Display omitted] • Electrons rapidly transfer through Fe-O-Bi electronic channels at the interface. • The internal electric field promotes the separation and transfer of charge carriers. • DFT calculation reveals the effect of pollutant structure on photocatalysis rate. • Photolysis rate of FQs is positively correlated with ƒ0 of main active carbon atoms. • MIL-53(Fe)/Bi 4 O 5 I 2 achieves efficient LEV photolysis at different water matrix. Building a heterojunction is a fascinating option to guarantee sufficient carrier separation and transfer efficiency, but the mechanism of charge migration at the heterojunction interface has not been thoroughly studied. Herein, MIL-53(Fe)/Bi 4 O 5 I 2 photocatalyst with a Z-scheme heterojunction structure is constructed, which achieves efficient photocatalytic decontamination under solar light. Driven by the newly-built internal electric field (IEF), the formation of Fe-O-Bi electron migration channel allows for rapid separation and transfer of charge carriers at the heterojunction interface, confirmed by the material characterization and density functional theory (DFT) calculation. The narrower band gap and improved visible light response also contribute to the enhanced photocatalytic activity of composite materials. With levofloxacin as the target pollutant, the optimal MIL-53(Fe)/Bi 4 O 5 I 2 achieves complete removal of pollutant within 150 min, the photocatalysis rate of which is ca. 4.4 and 26.0 times that of pure Bi 4 O 5 I 2 and MIL-53(Fe), respectively. Simultaneously, the optimal composite material exhibits satisfactory photodegradation of seven fluoroquinolones, and the photocatalysis rates are as follows: lomefloxacin > ciprofloxacin > enrofloxacin > norfloxacin > pefloxacin > levofloxacin > marbofloxacin. DFT calculations reveal a positive relationship between degradation rate and Fukui index (ƒ0) of main carbon atoms in seven fluoroquinolones. This study sheds light on the existence of electron migration channels at Z-scheme heterojunction interface to ensure sufficient photoinduced carrier transfer, and reveals the influence of pollutant structure on photolysis rate. [ABSTRACT FROM AUTHOR]

Titanate nanowire has been explored as a precursor for the preparation of TiO 2 catalysts with controllable morphologies by a facile hydrothermal method with HF and urea as assistants. During the hydrothermal process, HF erodes the titanate nanowires to form TiF 6 2− ions, and urea generates OH − in situ to initiate the hydrolysis of TiF 6 2− . The morphology of TiO 2 products can be tuned by simply adjusting the experimental parameters, such as the amounts of HF and urea, reaction time, reaction temperature and the concentration of titanate precursor. The photocatalytic activity of TiO 2 catalysts with different morphologies were evaluated by the photocatalytic degradation of Rhodamine B (RhB) and methyl orange (MO). The experimental results have demonstrated that catalysts with different morphologies have different photocatalytic activities. Along with many advantages of the synthesis process, such as being surfactant-free and easy to operate, the synthesis method developed in this study provides a new inspiration for further investigation into the preparation of TiO 2 catalysts with advanced structures and high photocatalytic activity. [ABSTRACT FROM AUTHOR]

[Display omitted] • Heterocyclic polymerization was developed for g-C 3 N 4 modification. • Efficient g-C 3 N 4 was obtained via polymerization of various precursors with TAP. • Integration of TAP in the unit structure of g-C 3 N 4 resulted in tubular structure. • Copolymerization reduced π-electron defects and facilitated charge migration. A novel heterocyclic polymerized g-C 3 N 4 nanotube was prepared by thermal polycondensation using urea and 2,4,6-triaminopyrimidine (TAP). The TAP-polymerized g-C 3 N 4 composite with 0.5 wt% TAP content showed the highest photocatalytic activity for ciprofloxacin (CIP) degradation under solar irradiation. The optimal composite derived from urea achieved ∼ 10.8 and ∼ 7.5 time of CIP degradation activity when compared with TAP-polymerized g-C 3 N 4 derived from melamine and thiourea. The copolymerization of TAP molecules reduces the π electron defects in the g-C 3 N 4 conjugated system thus accelerates the migration of photogenerated carriers. The doping of TAP in g-C 3 N 4 reduces its band gap, which enhances the light absorption capacity and improves the utilization efficiency of visible light. The fluffy porous nanotube structure of the material endows it with unique surface morphology and excellent photoelectric characteristic. Electron spin resonance (ESR) and probe technology were used for qualitative or quantitative detection of free radicals in this work. Superoxide radicals ( O 2 · - ) played the major roles in the photodegradation of CIP. Recycling experiments displayed the high stability and activity of the modified materials, which is potentially applicable in practical engineering. Moreover, the photodegradation pathways and mechanisms of CIP were proposed and its toxicity evolution was assessed in this study. This work revealed the specific application of heterocyclic polymerization in modifying g-C 3 N 4 material and provided unique insights for its application in photocatalytic degradation of antibiotic. [ABSTRACT FROM AUTHOR]

Mixed-phase TiO 2 photocatalysts with different proportions of anatase and rutile have been successfully synthesized in an acidic hydrothermal system, using tartaric acid (C 4 H 6 O 6 ) as a phase content regulator and titanium trichloride (TiCl 3 ) as the titanium source. The obtained samples were characterized by X-ray diffraction spectroscopy (XRD), infrared spectroscopy (IR), scanning electron microscopy (SEM) and so on. The phase contents of anatase and rutile in the TiO 2 particles were successfully controlled by simply adjusting the molar ratio of C 4 H 6 O 6 to TiCl 3 . And the regulation degree could be further controlled by the concentration of sodium chloride (NaCl) in the reaction system. In addition, the effect of the reaction time, hydrothermal temperature and acidity on the phase structure of as-prepared products have also been investigated. A mechanism was proposed to interpret the evolution of the phases based on the experimental results. Finally, the photocatalytic activity of the prepared TiO 2 were evaluated by the degradation of Rhodamine B (RhB) and methyl orange (MO) in aqueous solutions. The mixed-phase TiO 2 exhibited higher activity than pure phase TiO 2 , and the catalyst containing 77% anatase and 23% rutile had the highest photocatalytic activity. The enhanced photocatalytic activity could be explained by the mixed-phase effect between anatase and rutile. [ABSTRACT FROM AUTHOR]

[Display omitted] • Co 3 O 4 /PDA was designed for PMS activation under NIR light for the first time. • Co 3 O 4 could activate PMS and PDA could accelerate Co2+/Co3+ redox cycle. • Both Co 3 O 4 and PDA could convert NIR light into heat to boost the PMS activation. • A higher LEV degradation rate was achieved in Co 3 O 4 /PDA + PMS + NIR system. The utilization of near-infrared (NIR) light with strong photothermal effect in photo-Fenton or Fenton-like systems for the degradation of organic pollutants is always ignored. In this study, we have synthesized a novel photothermal Fenton-like catalyst (Co 3 O 4 /PDA) to activate peroxymonosulfate (PMS) for the degradation of antibiotics. The Co 3 O 4 in the nanocomposite acts as a PMS activator, while unexpectedly, PDA was found to accelerate the redox cycle of Co2+/Co3+. Both Co 3 O 4 and PDA are black-colored photothermal materials with excellent NIR absorption properties. During the degradation process, the light-to-heat conversion ability of Co 3 O 4 and PDA enables a higher temperature, which innovatively boosts the rate of PMS catalytic activation. Mechanistic studies revealed that the increased temperature effectively promotes the heterogeneous catalytic PMS activation by Co 3 O 4 /PDA. As a result, the Co 3 O 4 /PDA + PMS + NIR system exhibits greatly enhanced degradation performance. Furthermore, this degradation system demonstrates strong tolerance to different operation conditions, and the catalyst exhibits good stability in cycling tests. These results demonstrate the favorable application prospects of this photothermal Fenton-like system in environmental water remediation. The findings also contribute valuable insights towards the development of efficient strategies for the photothermal activation of PMS. [ABSTRACT FROM AUTHOR]

[Display omitted] • C/Bi 3 O 4 I achieved 100% pyrene elimination within 20 min. • Carbon doping dramatically elevated internal electric field and photolysis activity. • Effect of halogen type on photocatalytic activity of Bi 3 O 4 X was thoroughly studied. • Doped C implanted into [X] layers and induced impurity state formation above VB. • Formation rate of free radicals and their contribution on degradation were measured. A series of C-doped Bi 3 O 4 X (X = Cl, Br, I) photocatalysts with layer-stacked structure was synthesized using glucose as carbon source, and the carbon doping and halogen species on harvesting broader solar spectrum and promoting charge carrier separation were systematically investigated. For pyrene photolysis, the photodegradation rate of pristine materials followed the order of Bi 3 O 4 I > Bi 3 O 4 Br > Bi 3 O 4 Cl, which was attributed to the difference in electronegativity of the halogen elements. The doped carbon boosted photocatalytic performance and the optimal C/Bi 3 O 4 I achieved 100% pyrene removal within 20 min, which primarily benefited from the dramatic improvement of the internal electric field (IEF). The improved IEF further increased the separation and transfer efficiency of photogenerated charge carriers. XRD and XPS characterizations confirmed that the doped carbon implanted into the lattice of [X] layers, and mainly affected the X np states. The X np orbitals contributed to the valence band (VB) of Bi 3 O 4 X, thus the local occupied states induced by doped carbon formed above VB and significantly decreased the VB potential. Meanwhile, the doped carbon narrowed the band gap and greatly improved visible light utilization. The O 2 −, h+ and OH were identified as dominant active species for pyrene degradation, and the generation rate of O 2 − and OH was further measured by the probe technique. Moreover, the photodegradation pathways of pyrene were proposed and the ecotoxicity of intermediates was assessed. This study reveals the effect of halogen species on photocatalytic activity and provides guidance for enhancing IEF by doping inorganic element. [ABSTRACT FROM AUTHOR]

A novel heterojunction photocatalyst with well-dispersed TiO 2 nanoparticles on Co-Doped g-C 3 N 4 has been successfully fabricated by an in situ generation solvothermal method. The photocatalytic performance of the as-prepared TiO 2 /Co-g-C 3 N 4 catalyst was evaluated by the degradation of phenol in the presence of Cr 6 + under simulated sunlight irradiation. The catalyst exhibited markedly enhanced photocatalytic activity for degradation of phenol but obviously decreased photocatalytic activity for reduction of Cr 6 + compared with common TiO 2 /g-C 3 N 4 catalyst. The mechanism for this photocatalytic performance was proposed. [ABSTRACT FROM AUTHOR]

Carbon dots (CD) and Ti species were assembled in the mesoporous silica matrix by a one-pot co-condensation strategy. The CD and Ti in the silica matrix were demonstrated to interact in two ways: on the one hand, part of the carbon in CD was doped into the Ti species; on the other hand, CD worked as photosensitizers for Ti species. The synergy effect between CD and Ti in the silica matrix along with the unique physical properties of the composite including ordered pore channels, large surface area and wide-range light absorption from UV to near IR made this composite be an excellent photocatalyst, as demonstrated by the photocatalytic degradation of azo dye acid orange 7. In addition to the degradation of organic pollutant, this newly developed mesoporous composite is expected to have promising applications in various areas related to environment and energy such as photoreduction of CO 2 and photocatalytic H 2 production. [ABSTRACT FROM AUTHOR]

g-C 3 N 4 modified TiO 2 composites were prepared through a simple calcination process of anatase and cyanamide. The as-prepared samples were characterized by X-ray diffraction (XRD), diffuse reflectance spectrophotometry (DRS), fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), thermogravimetry differential thermal analysis (TG–DTA) and X-ray photoelectron spectroscopy (XPS), proving a successful modification of TiO 2 with g-C 3 N 4 . Photodegradation of acid orange 7 (AO7) was used to evaluate the photocatalytic activities of the composites, showing excellent activity of them under both visible and UV light. In addition, base treatment was then introduced to investigate the interaction between g-C 3 N 4 and TiO 2 . After removing the g-C 3 N 4 modified on TiO 2 by base, no nitrogen doping is found in TiO 2 lattice, demonstrating the g-C 3 N 4 was surface attached on TiO 2 and attributing all improvement of photocatalytic activity of g-C 3 N 4 /TiO 2 composite to the synergy between the two semiconductors. [ABSTRACT FROM AUTHOR]

A strategy combining generation promotion and self-decomposition suppression for H 2 O 2 production is developed by utilizing hydroxyls-grafted g-C 3 N 4 nanotube. • g-C 3 N 4 nanotube with in situ Grafted OH was prepared by rational experimental method. • The metal-free ACNT catalyst shows ultra high H 2 O 2 production in spontaneous system. • The nanotube structure and surface OH synergistic promote the H 2 O 2 production. • Multipe mechanisms of action for surface OH are studied and proposed. An active and inexpensive photocatalyst for H 2 O 2 production is desirable for industrial applications. However, obtaining high photocatalytic activity from metal-free catalysts without the use of sacrificial electron donors is difficult. Herein, g-C 3 N 4 (CN) nanotubes functionalized with surface > OH groups that are grafted in situ were successfully synthesized via a novel alkalinization process. The nanotube structures provide a large surface area and improved mass transfer properties. In situ grafted > OH groups can capture photogenerated holes to promote separation of photogenerated charge, enabling the ready availability of electrons and hydrogen ions for H 2 O 2 production. Further, the surface > OH groups help to suppress H 2 O 2 self-decomposition. Consequently, a high rate of 240.36 μmol h−1 g−1 of H 2 O 2 production can be achieved without sacrificial agents, which is the highest H 2 O 2 production in a spontaneous system for metal-free photocatalysts. This work provides a new strategy for an efficient and spontaneous H 2 O 2 production method using a metal-free CN photocatalyst. [ABSTRACT FROM AUTHOR]

[Display omitted] • A highly solar active TiO 2 /WO 3 /CQDs was prepared by hydrothermal-calcining method. • TiO 2 /WO 3 /CQDs had narrowed band gap and improved electrons-holes separation. • CQDs induced reverse transfer of electrons and holes under the internal electric field. • The promoted charge separation is superior to enhanced visible light utilization. • High antibiotics mineralization was achieved and the toxicity evolution was evaluated. A highly solar active nanocomposite with sheet-like WO 3 skeleton and evenly loaded TiO 2 and carbon quantum dots (CQDs) was synthesized by facile hydrothermal-calcining process, which showed 3.1- and 46.6- times activity on antibiotic (cephalexin) degradation than TiO 2 and WO 3 , respectively. The construction of TiO 2 /WO 3 heterojunction narrowed the band gap and facilitated the electrons-holes separation. The π conjugated CQDs was found to further improve the charge separation and extend the visible light response by photosensitization. However, the promoted charge separation predominantly contributed to the improved photocatalytic activity. The contributions of detected reactive species follows the order of: O 2 –>1O 2 > OH> h+. The cephalexin degradation mechanism and pathway were proposed based on DFT (density functional theory) calculation and experimental analysis. The photocatalytic mineralization efficiency can reach 92.4% in 4 h, indicating the efficient reduction of ecotoxicity of cephalexin and its intermediates. This new composite proved to have great potentials for emerging contaminants degradation in water. [ABSTRACT FROM AUTHOR]

• Ultrathin g-C 3 N 4 nanosheets with hierarchical pores and desirable energy band for H 2 O 2 production has been prepared. • A high H 2 O 2 production rate of 1083 μmol g−1 h−1 has been achieved. • The optimization of energy band via P doping changed the reaction route for H 2 O 2 production. • Unique morphology acts in synergy with suitable energy band to enhance the H 2 O 2 production. H 2 O 2 production through photocatalysis has been considered as a sustainable technique. Here, we report ultrathin g-C 3 N 4 nanosheets with hierarchical pores and desirable energy band for high-efficiency photocatalytic H 2 O 2 production. The resultant catalyst showing an ultra-high H 2 O 2 production rate of 1083 μmol g−1 h−1, which is about seven times higher than that of bulk g-C 3 N 4 and represents one of the most active photocatalysts for H 2 O 2 production. DFT calculation and experimental studies revealed that the suitable energy band structure achieved by a phosphorus doping in g-C 3 N 4 leading to the efficient yielding two-electron reduction of O 2 to form H 2 O 2. Meanwhile, the particular morphology provided a large accessible surface area, multi-mass transport channels and short charge transfer distance. The synergy effect of desirable energy band and hierarchical porous nanosheets bring the excellent photocatalytic activity. In addition, the simple preparation procedure and the non-metal properties make it have great potential for the practical application. [ABSTRACT FROM AUTHOR]