Direct Z-scheme system of UiO-66 cubes wrapped with Zn0.5Cd0.5S nanoparticles for photocatalytic hydrogen generation synchronized with organic pollutant degradation.

Optimized fabrication of Z-scheme photocatalyst based on MOF materials offers sustainable energy generation and environmental improvement due to their attractive properties. The Z-scheme heterojunctions consisting of UiO-66 cubes covered with Zn0.5Cd0.5S nanoparticles were fabricated by a facile solvothermal method. Thanks to the Z-scheme carrier transport under simulated sunlight irradiation, UiO-66@Zn0.5Cd0.5S exhibited enhanced photocatalytic performance of H2 generation synchronized with organic pollutant degradation in fluoroquinolone antibiotic wastewater. Synergistically, the highest comprehensive performance was obtained in ciprofloxacin solution. The H2 yield reached 224 µmol∙ g-1∙ h-1 and simultaneously the removal efficiency was up to 83.6 %. The degradation pathways revealed that the process of piperazine ring cleavage and decarboxylation also generates H protons, further promoting the production of H2. Therefore, the effective spatial separation and transfer of the photoinduced carriers are attributed to the good band structure, large specific surface area, and cooperative reduction and oxidation reactions of UiO-66@Zn0.5Cd0.5S, resulting in significant photocatalytic activity. The toxicity assessment of antibiotics and intermediate products during the photocatalytic reaction also verifies the reduction of environmental risk. This study highlights a promising way to expand the application of the MOFs-based photocatalyst in clean energy conversion coupling with water remediation.

Fabrication of CdLa2S4@La(OH)3@Co3S4 Z-scheme heterojunctions with dense La, S-dual defects for robust photothermal assisted photocatalytic performance.

Optimize the separation and transport mechanism of photogenerated carriers in heterojunction composites, and make full use of the active sites of each material are key factors to enhance photocatalytic activity. Herein, we successfully synthesize defective CdLa2S4@La(OH)3@Co3S4 (CLS@LOH@CS) Z-scheme heterojunction photocatalysts through a facile solvothermal method, which show broad-spectrum absorption and excellent photocatalytic activity. La(OH)3 nanosheets not only greatly increase the specific surface area of photocatalyst, but also can be coupled with CdLa2S4 (CLS) and form Z-scheme heterojunction by converting irradiation light. In addition, Co3S4 with photothermal properties is obtained by in-situ sulfurization method, which can release heat to improve the mobility of photogenerated carriers, and also be used as a cocatalyst for hydrogen production. Most importantly, the formation of Co3S4 leads to a large number of sulfur vacancy defects in CLS, and thus improving the separation efficiency of photogenerated electrons and holes, and increasing the catalytic active sites. Consequently, the maximum hydrogen production rate of CLS@LOH@CS heterojunctions can reach 26.4 mmol g-1h-1, which is 293 times than pristine CLS (0.09 mmol g-1h-1). This work will provide a new horizon for synthesizing high efficiency heterojunction photocatalysts through switching the separation and transport modes of photogenerated carrier.

Oxygen Vacancy-Induced Construction of CoO/h-TiO2 Z-Scheme Heterostructures for Enhanced Photocatalytic Hydrogen Evolution.

Environmentally friendly catalysts with excellent performance and low cost are critical for photocatalysis. Herein, using hydrogenated TiO2 (h-TiO2) nanosheets with enriched oxygen vacancies as the support, two-dimensional CoO/h-TiO2 Z-scheme heterostructures are fabricated for hydrogen production through photocatalytic water splitting. It is revealed that the oxygen vacancies in h-TiO2 can inhibit the oxidation of Co2+ into high-valence Co3+ during the hydrothermal reaction and thermal treatment processes. A CoO/h-TiO2 Z-scheme heterostructure possesses a space charge region and a built-in electric field at the interface, and oxygen vacancies in h-TiO2 can provide more reactive sites, which synergistically improve the separation and transportation of photogenerated carriers. As a result, the photocatalytic hydrogen evolution rate achieves 129.75 µmol·h-1 (with 50 mg of photocatalysts) on the optimized CoO/h-TiO2 heterostructures. This work provides a new design idea for the preparation of excellent TiO2-based photocatalysts.

In Situ Growth of Nitrogen-Doped Carbon Nanotubes Based on Hierarchical Ni@C Microspheres for High Efficiency Bisphenol A Removal through Peroxymonosulfate Activation.

N-doped carbon nanotubes (NCNTs) are promising metal-free heterogeneous catalysts toward peroxymonosulfate (PMS) activation in advanced oxidation processes for wastewater remediation. However, conventional CNTs always suffer from serious agglomeration and low N content, which renders their design synthesis as an important topic in the related field. With hierarchical Ni@C microspheres as a nutritious platform, we have successfully induced in situ growth of NCNTs on their surface by feeding melamine under high-temperature inert atmospheres. These as-grown NCNTs with a small diameter (ca. 20 nm) are firmly rooted in Ni@C microspheres and present loose accumulation on their surface, and their relative content can be tailored easily by manipulating the mass ratio of melamine to Ni@C microspheres. The investigation on bisphenol A (BPA) removal reveals that the loading amount of NCNTs affects the catalytic performance greatly, and the optimum ratio of melamine to Ni@C microspheres is 5.0 because the corresponding MNC-5.0 possesses sufficient surface N sites and moderate electron transfer, resulting in powerful PMS activation and sufficient utilization of reactive oxidative species (ROS). MNC-5.0 also addresses its advantages as compared with other NCNTs from post treatment and spontaneous growth strategies. The primary ROS responsible for BPA degradation are identified as hydroxyl radical, sulfate radical, superoxide radical, and singlet oxygen through quenching experiments and electron paramagnetic resonance, and the corresponding catalytic mechanism is also put forward based on these results.

O, S-Dual-Vacancy Defects Mediated Efficient Charge Separation in ZnIn2S4/Black TiO2 Heterojunction Hollow Spheres for Boosting Photocatalytic Hydrogen Production.

Defective ZnIn2S4 nanosheets/mesoporous black TiO2 heterojunction hollow spheres (H-ZIS/b-TiO2) are prepared through hydrothermal and surface low-temperature hydrogenation strategies, which show broad-spectrum response and excellent charge separation efficiency. This H-ZIS/b-TiO2 flower-like heterojunction hollow spheres with a narrow band gap of ∼1.88 eV expand the light response to visible light and show excellent photocatalytic hydrogen evolution rate (278 µmol h-1 50 mg-1) under visible-light irradiation, which is 1.5 times as high as that of ZnIn2S4/black TiO2 heterojunction hollow spheres (ZIS/b-TiO2) (181 µmol h-1 50 mg-1). The excellent photocatalytic performance is due to the formation of O, S dual vacancies in b-TiO2 and H-ZIS providing more active sites for photocatalytic reaction and improving the charge separation efficiency, heterojunctions promoting transport of photogenerated carriers, and the hollow structure increasing light utilization by reflecting light. The novel heterojunction hollow sphere with high performance has broad application prospects in the field of energy.

Polyaniline: A New Metal-Free Catalyst for Peroxymonosulfate Activation with Highly Efficient and Durable Removal of Organic Pollutants.

Metal-free heterogeneous catalysts are receiving more and more attention for wastewater remediation by activating peroxymonosulfate (PMS) due to their environmental benign. However, carbon-based materials as the most typical metal-free heterogeneous always suffer from poor durability. Inspired by the fact that a conjugated system may facilitate the electron transfer during PMS activation, we innovatively select polyaniline (PANI) as a new PMS activator and investigate its catalytic performance in detail. It is found that PANI can display better catalytic performance than traditional metal-based catalysts and popular N-doped carbocatalysts in methyl orange (MO) degradation. More importantly, PANI is not only universal for various pollutants degradation but also maintains its catalytic performance in repeated degradation experiments. The stable N sites in the conjugated chains and the oxidation-resistance benzene rings as the building units are considered to be responsible for such an excellent durability. In addition, the influences of some routine factors and actual water backgrounds are comprehensively checked and analyzed. The quenching experiments and electron paramagnetic resonance confirm that MO degradation is achieved through both radical and nonradical pathways, where SO4•- and 1O2 are primary reactive species. The reaction mechanism is also proposed with the assistance of X-ray photoelectron spectroscopy.

Improved charge separation of NiS nanoparticles modified defect-engineered black TiO2 hollow nanotubes for boosting solar-driven photocatalytic H2 evolution.

NiS nanoparticles modified black TiO2 hollow nanotubes (NBTNs) are successfully synthesized via surface hydrogenation and the facile solvothermal method. The unique structure with intensified surface and interface characteristics endow NBTNs with more catalytic sites, and increase charge carrier separation efficiency with an extended charge lifetime, overwhelmingly promoting its photocatalytic performance. The resultant NBTNs possess a relatively high surface area and pore size of ∼89 m2 g-1 and ∼9.8 nm, respectively. The resultant NBTNs exhibit an excellent solar-driven photocatalytic hydrogen rate (3.17 mmol h-1 g-1), which is almost as high as that of Pt as cocatalyst, in which the apparent quantum yield of 5.4% (420 nm) is recorded for the NBTNs sample. Moreover, the turnover number can be up to 116 000 within 48 h and the turnover frequency is 2400 for NiS. This novel strategy could provide a better understanding of cocatalyst photocatalytic mechanisms, and a scheme simultaneously regulating the morphology and structure of photocatalysts for promoting H2 generation.

Homojunction and defect synergy-mediated electron-hole separation for solar-driven mesoporous rutile/anatase TiO2 microsphere photocatalysts.

The photocatalytic hydrogen evolution of TiO2 is deemed to be one of the most promising ways of converting solar energy to chemical energy; however, it is a challenge to improve the photo-generated charge separation efficiency and enhance solar utilization. Herein, black mesoporous rutile/anatase TiO2 microspheres with a homojunction and surface defects have been successfully synthesized by an evaporation-induced self-assembly, solvothermal and high-temperature surface hydrogenation method. The H500-BMR/ATM (HX-BMR/ATM, where X means the different hydrogen calcination temperatures) materials not only possess a mesoporous structure and relatively high specific surface area of 39.2 m2 g-1, but also have a narrow bandgap (∼2.87 eV), which could extend the photoresponse to the visible light region. They exhibit high photocatalytic hydrogen production (6.4 mmol h-1 g-1), which is much higher (approximately 1.8 times) than that of pristine mesoporous rutile/anatase TiO2 microspheres (3.58 mmol h-1 g-1). This enhanced photocatalytic hydrogen production property is attributed to the synergistic effect of the homojunction and surface defects in improving efficient electron-hole separation and high utilization of solar light. This work proposes a new approach to improve the performance of photocatalytic hydrogen production and probably offers a new insight into fabricating other high-performance photocatalysts.

Surface Plasmon Resonance-Enhanced Visible-NIR-Driven Photocatalytic and Photothermal Catalytic Performance by Ag/Mesoporous Black TiO2 Nanotube Heterojunctions.

Ag/mesoporous black TiO2 nanotubes heterojunctions (Ag-MBTHs) were fabricated through a surface hydrogenation, wet-impregnation and photoreduction strategy. The as-prepared Ag-MBTHs possess a relatively high specific surface area of ≈85 m2 g-1 and an average pore size of ≈13.2 nm. The Ag-MBTHs with a narrow band gap of ≈2.63 eV extend the photoresponse from UV to the visible-light and near-infrared (NIR) region. They exhibit excellent visible-NIR-driven photothermal catalytic and photocatalytic performance for complete conversion of nitro aromatic compounds (100 %) and mineralization of highly toxic phenol (100 %). The enhancement can be attributed to the mesoporous hollow structures increasing the light multi-refraction, the Ti3+ in frameworks and the surface plasmon resonance (SPR) effect of plasmonic Ag nanoparticles favoring light-harvesting and spatial separation of photogenerated electron-hole pairs, which is confirmed by transient fluorescence. The fabrication of this SPR-enhanced visible-NIR-driven Ag-MBTHs catalyst may provide new insights for designing other high-performance heterojunctions as photocatalytic and photothermal catalytic nanomaterials.

Prussian Blue Microcrystals with Morphology Evolution as a High-Performance Photo-Fenton Catalyst for Degradation of Organic Pollutants.

The morphology-dependent property of crystal materials has aroused extensive attention and raised high requirements for subtly tailoring the morphology of micro-/nanocrystals. Herein, we develop an in situ etching method for preparation of Prussian blue (PB) microcrystals with morphology evolution by progressively increasing the concentration of chloroplatinic acid in the reaction system. These PB microcrystals with controllable morphologies are employed as photo-Fenton reagents to degrade organic pollutants. PB hexapods (PB-hpds) and PB hexapod stars present superior catalytic performance to pristine PB microcubes and other PB intermediates with truncated corners or edges because of their high specific surface areas and adequate exposure of FeIII-NC coordination active sites. In the reusability test, the reused PB-hpds present more efficient catalytic performance for rhodamine B decomposition compared with the pristine catalyst. According to more investigations, the reasonable mechanism is proposed that FeIII-NC exhibits higher catalytic activity than FeII-CN in the specific coordination environment. The increased content of surface FeIII-NC coordination active sites is the key factor in accelerating the decomposition of H2O2 and enhancing the photo-Fenton performance of PB-hpds. Several operating parameters including catalyst dosage, H2O2 concentration, pH value, and reaction temperature are evaluated in detail. Classical quenching experiments and electron paramagnetic resonance measurements further reveal that HO• should be responsible for high performance of catalysts. This work will be significant for tailoring the morphology of the materials and arousing more attention to enhance the stability and reusability of catalysts.

Synthesis of Particulate Hierarchical Tandem Heterojunctions toward Optimized Photocatalytic Hydrogen Production.

Photocatalytic hydrogen production using semiconductors is identified as one of the most promising routes for sustainable energy; however, it is challenging to harvest the full solar spectrum in a particulate photocatalyst for high activity. Herein, a hierarchical hollow black TiO2 /MoS2 /CdS tandem heterojunction photocatalyst, which allows broad-spectrum absorption, thus delivering enhanced hydrogen evolution performance is designed and synthesized. The MoS2 nanosheets not only function as a cost-effective cocatalyst but also act as a bridge to connect two light-harvesting semiconductors into a tandem heterojunction where the CdS nanoparticles and black TiO2 spheres absorb UV and visible light on both sides efficiently, coupling with the MoS2 cocatalyst into a particulate photocatalyst system. Consequently, the photocatalytic hydrogen rate of the black TiO2 /MoS2 /CdS tandem heterojunction is as high as 179 µmol h-1 per 20 mg photocatalyst under visible-light irradiation, which is almost 3 times higher than that of black TiO2 /MoS2 heterojunctions (57.2 µmol h-1 ). Most importantly, the stability of CdS nanoparticles in the black TiO2 /MoS2 /CdS tandem heterojunction is greatly improved compared to MoS2 /CdS because of the formation of tandem heterojunctions and the strong UV-absorbing effect of black TiO2 . Such a tandem architectural design provides new ways for synthesizing particulate photocatalysts with high efficiencies.

Surface defect-mediated efficient electron-hole separation in hierarchical flower-like bismuth molybdate hollow spheres for enhanced visible-light-driven photocatalytic performance.

It is desirable to develop an efficient visible-light-driven photocatalyst for practical application to degrade highly-noxious pollutants. Herein, the hydrogenation hierarchical flower-like Bi2MoO6 hollow spheres (H-BMO-X, where X represents the different hydrogen calcination temperatures) have been successfully fabricated by a solvothermal-surface hydrogenation process. The as-prepared nano-photocatalyst H-BMO-300 clearly exhibits a photocatalytic reaction apparent rate constant k for high-noxious pollutants by ∼3-times higher than pristine Bi2MoO6. Moreover, the resultant H-BMO-300 sample with a narrow bandgap of ∼2.70 eV possesses surface oxygen vacancy defects. Based on the scanning Kelvin probe and surface photovoltage spectroscopy, it is deduced that the photocatalytic activities are attributed to the surface oxygen vacancy of H-BMO-X favoring the electron-hole pair's separation. The enhanced photocatalytic performance can be ascribed to the synergistic effect of surface defects favoring efficient electron-hole separation and the hollow hierarchical structure benefiting the utilization of visible light, which provides more surface-active sites. This work provides a viable route to perceptibly enhance the photocatalytic activities of H-BMO-300 for environmental remediation with good mineralization properties.