Enhanced photocatalytic NO removal and toxic NO2 production inhibition over ZIF-8-derived ZnO nanoparticles with controllable amount of oxygen vacancies

In Situ-Derived N-Doped ZnO from ZIF-8 for Enhanced Ethanol Sensing in ZnO/MEMS Devices

Microelectromechanical systems (MEMS) gas sensors have numerous advantages such as compact size, low power consumption, ease of integration, etc., while encountering challenges in sensitivity and high resistance because of their low sintering temperature. This work utilizes the in situ growth of Zeolitic Imidazolate Framework-8 (ZIF-8) followed by its conversion to N-doped ZnO. The results obtained from scanning electron microscopy (SEM) and transmission electron microscopy (TEM) indicate that the in situ derivation of ZIF-8 facilitates the adhesion of ZnO particles, forming an island-like structure and significantly reducing the interfaces between these particles. Furthermore, powder X-ray diffraction (XRD) analysis, elemental mapping, and X-ray photoelectron spectroscopy (XPS) analysis confirm the conversion of ZIF-8 to ZnO, the successful incorporation of N atoms into the ZnO lattice, and the creation of more oxygen vacancies. The ZIF-8-derived N-doped ZnO/MEMS sensor (ZIF (3)-ZnO/MEMS) exhibits remarkable gas sensitivity for ethanol detection. At an operating temperature of 290 °C, it delivers a substantial response value of 80 towards 25 ppm ethanol, a 13-fold enhancement compared with pristine ZnO/MEMS sensors. The sensor also exhibits an ultra-low theoretical detection limit of 11.5 ppb to ethanol, showcasing its excellent selectivity. The enhanced performance is attributed to the incorporation of N-doped ZnO, which generates abundant oxygen vacancies on the sensor's surface, leading to enhanced interaction with ethanol molecules. Additionally, a substantial two-order-of-magnitude decrease in the resistance of the gas-sensitive film is observed. Overall, this study provides valuable insights into the design and fabrication strategies applicable to high-performance MEMS gas sensors in a broader range of gas sensing.

Metal-organic framework heterojunctions for photocatalysis

Heterojunctions combining two photocatalysts of staggered conduction and valence band energy levels can increase the photocatalytic efficiency compared to their individual components. This activity enhancement is due to the minimization of undesirable charge recombination by the occurrence of carrier migration through the heterojunction interface with separated electrons and holes on the reducing and oxidizing junction component, respectively. Metal-organic frameworks (MOFs) are currently among the most researched photocatalysts due to their tunable light absorption, facile charge separation, large surface area and porosity. The present review summarizes the current state-of-the-art in MOF-based heterojunctions, providing critical comments on the construction of these heterostructures. Besides including examples showing the better performance of MOF heterojunctions for three important photocatalytic processes, such as hydrogen evolution reaction, CO2 photoreduction and dye decolorization, the focus of this review is on describing synthetic procedures to form heterojunctions with MOFs and on discussing the experimental techniques that provide evidence for the operation of charge migration between the MOF and the other component. Special attention has been paid to the design of rational MOF heterojunctions with small particle size and controlled morphology for an appropriate interfacial contact. The final section summarizes the achievements of the field and provides our views on future developments.

Ultrafast synthesis of halogen-doped Ru-based electrocatalysts with electronic regulation for hydrogen generation in acidic and alkaline media

Developing a facile and time-saving method for preparing hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) electrocatalysts can accelerate the practical applications of hydrogen energy. In this study, halogen (X = F, Cl, Br and I) doped Ru-RuO₂ on carbon cloth (CC) (X-Ru-RuO₂/MCC) was synthesized via an ultrafast microwave-assisted method for 30 s. Particularly, the doped Br (Br-Ru-RuO₂/MCC) significantly improved the electrocatalytic performances of the catalyst through the regulation of electronic structures. Then, the Br-Ru-RuO₂/MCC catalyst featured HER overpotentials of 44 mV and 77 mV in 1.0 M KOH and 0.5 M H₂SO₄, and the OER overpotential of 300 mV at 10 mA cm^-2 in 1.0 M KOH. This study provides a novel method for developing of halogen-doped catalysts.

Microwave-Assisted Rapid Substitution of Ti for Zr to Produce Bimetallic (Zr/Ti)UiO-66-NH2 with Congenetic "Shell-Core" Structure for Enhancing Photocatalytic Removal of Nitric Oxide

Efficient nitric oxide (NO) removal without nitrogen dioxide (NO₂ ) emission is desired for the control of air pollution. Herein, a series of (Zr/Ti)UiO-66-NH₂ with congenetic shell-core structure, denoted as Ti-UION, are rapidly synthesized by microwave-assisted post-synthetic modification for NO removal. The optimal Ti-UION (i.e., 2.5Ti-UION) exhibits the highest activity of 80.74% without NO₂ emission with moisture, which is 21.65% greater than that of the UiO-66-NH₂ . The NO removal efficiency of 2.5Ti-UION further increases to 95.92% without photocatalyst deactivation under an anhydrous condition. This is because selectively produced NO₂ in photocatalysis is completely adsorbed into micropores, refreshing active sites for subsequent reaction. In addition, the enhanced photocatalytic activity after Ti substitution is due to the presence of Ti electron acceptor, the potential difference between the shell and core of Ti-UION crystal, and the high conductivity of TiO units. Additionally, the improved adsorption of gas molecules not only favors NO oxidation, but also avoids the emission of NO₂ . This work provides a feasible strategy for rapid metal substitution in metal-organic frameworks and insights into enhanced NO photodegradation.

Synthesis and comparative study of the structural and optical properties of binary ZnO-based composites for environmental applications

The development of photoactive systems to solve serious environmental problems is a key objective of researchers and remains a real challenge. Herein, n-p heterojunction ZnO-based composites were developed to achieve better photocatalytic performance in methylene blue (MB) degradation under natural solar irradiation. The hydrothermal technique was used to synthesize zinc oxide (ZnO)/metal oxide (MO) composites, with a molar ratio of 1 : 1 (MO = Mn₃O₄; Fe₃O₄; CuO; NiO). Various characterization techniques were used for the analysis of the structural, morphological and optical properties. X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDX) Diffuse Reflectance Spectroscopy analysis (DRS), and Diffuse Reflectance Spectroscopy analysis (DRS) validated the presence of two phases for each sample, excluding any impurities. Indeed, the ZnO structure was not affected by the coupling with MO, confirming that MO was well dispersed on the surface of the ZnO crystalline lattice for each composite. Eventually, the photocatalytic performance evaluation test of the synthesized photocatalysts was carried out on aqueous MB solution. According to the results, the ZnO/Fe₃O₄ nano-catalyst showed the best photodegradation efficiency. This result suggests that the formation of Fe₃O₄/ZnO as a p/n heterojunction reduces the recombination of photo-generated electron/hole pairs and broadens the solar spectral response range, resulting in significant photocatalytic efficiency. Meanwhile, the possible mechanism for degradation of the MB was discussed.

Synergistic effect of modified pore and heterojunction of MOF-derived α-Fe2O3/ZnO for superior photocatalytic degradation of methylene blue

Mesoporous heterojunction MOF-derived α-Fe₂O₃/ZnO composites were prepared by a simple calcination of α-Fe₂O₃/ZIF-8 as a sacrificial template. The optical properties confirm that coupling of both the modified pore and the n-n heterojunction effectively reduces the possibility of photoinduced charge carrier recombination under irradiation. The mesoporous Fe(25)ZnO with 25% loading of α-Fe₂O₃ exhibited the best performance in MB degradation, up to ∼100% after 150 minutes irradiation, higher than that of pristine ZnO and α-Fe₂O₃. Furthermore, after three cycles reusability, mesoporous Fe(25)ZnO still showed an excellent stability performance of up to 95.42% for degradation of MB. The proposed photocatalytic mechanism of mesoporous Fe(25)ZnO for the degradation of MB corresponds to the n-n heterojunction system. This study provides a valuable reference for preparing mesoporous MOF-derived metal oxides with an n-n heterojunction system to enhance MB photodegradation.

Construction dual vacancies to regulate the energy band structure of ZnIn2S4 for enhanced visible light-driven photodegradation of 4-NP

Most of the intrinsic photocatalysts with visible light response can only generate one active radical due to the limitation of their band structures, which is immediate cause limiting their photocatalytic degradation performance. In this work, ZnIn₂S₄ with Zn vacancy and S vacancy (V_Zn+S-ZnIn₂S₄) was prepared for the first time. As expected, the V_Zn+S-ZnIn₂S₄ exhibits remarkable photocatalytic performance for 4-Nitrophenol (4-NP) degradation under visible light and the apparent rate constant is about 11 times that of pristine ZnIn₂S₄. The construction of dual vacancies can regulate the energy band structure of the ZnIn₂S₄, enabling it to generate •OH and •O₂^- simultaneously. Meanwhile, dual vacancies system can also extremely improve the separation efficiency of carriers. It is worth noting that Zn vacancy and S vacancy can capture photogenerated holes and photogenerated electrons, respectively, which is beneficial for photogenerated carriers to participate in radical generation reactions. In addition, a possible 4-NP degradation pathway was proposed based on HPLC-MS analysis. This work provides a new way to construct photocatalysts for photodegradation of pollutants in wastewater.

Bimetallic CoCu-ZIF material for efficient visible light photocatalytic fuel denitrification

Effective design of photocatalysts is an effective method to improve the separation of photogenerated carriers, which improves the photocatalytic performance of photocatalysts. In this work, CoCu-ZIF materials with bimetallic structure were synthesized at room temperature for efficient photocatalytic fuel denitrification. The properties and structures of CoCu-ZIF photocatalysts can be effectively controlled by adjusting the molar ratio of cobalt to copper. The as-prepared CoCu-ZIF photocatalysts were characterized by XRD, FT-IR, SEM, TEM, UV-vis, Raman, BET and other techniques. The photoactivity of CoCu-ZIF for the denitrogenation of NCCs has been evaluated using visible light (λ ≥ 420 nm). The results indicate that Co₈Cu₂-ZIF photocatalysts exhibit excellent photocatalytic properties, in which the denitrification rate almost reached 80% after 4 hours under visible light irradiation, which is higher than the degradation ability of ZIF-67 (38%). Transient photoelectrochemical experiments and EIS Nyquist plots indicate that Co₈Cu₂-ZIF with unique structure efficiently improves the separation and transfer of photogenerated electron-hole pairs. Moreover, a possible reaction mechanism was proposed by LC-MS analysis.

Facile Fabrication of Durable Biochar/H2-TiO2 for Highly Efficient Solar-Driven Degradation of Enrofloxacin: Properties, Degradation Pathways, and Mechanism

Widespread application of TiO₂ for degradation of antibiotics is restricted by mainly the low photodegradation efficiency under solar irradiation. To expand the application of TiO₂, the key factors that should be improved are visible-light response, yield of electrons and holes, and durability. Herein, we report a visible-light responsive and durable sugarcane-bagasse-derived biochar supported hydrogenated TiO₂ (HSCB/H₂-TiO₂) photocatalyst with higher electron production fabricated by a facile one-pot hydrogenation. Mild hydrogenation temperature preserved the lotus-stem-like structure of sugarcane bagasse and gave the photocatalyst great separability. The superior durability of HSCB/H₂-TiO₂ was demonstrated by 12 rounds of repeated degradation of methylene blue (MB). In addition, the electron paramagnetic resonance (EPR) results demonstrated that the biochar skeleton contains abundant persistent free radicals (PFRs), which can provide excess electrons to form more ^•O₂ ^-. Meanwhile, radical quenching experiment and EPR radical trapping results also revealed that ^•O₂ ^- was the most dominant species for enrofloxacin (ENR) degradation. Thus, the as-fabricated photocatalyst shows excellent solar-driven degradation of ENR, and 95.6% of ENR was degraded in 180 min under simulated solar irradiation. Possible ENR degradation pathways and mechanism are also proposed based on the identified intermediates.

The Oxygen Vacancy Defect of ZnO/NiO Nanomaterials Improves Photocatalytic Performance and Ammonia Sensing Performance

In this paper, ZnO/NiO composites rich in oxygen vacancies are prepared by the solvothermal method and reduction method. In the test, through the use of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscope (TEM), diffuse reflectance spectroscopy (DRS), photoluminescence spectroscopy (PL), and electron paramagnetic resonance (EPR), we effectively prove the existence of phase, morphology and oxygen vacancies in the material. Through the photocatalysis test and gas sensitivity test, it is found that 10% Ni doped OZN-10 has the best photocatalytic activity and gas sensitivity characteristics. The degradation rate of methylene blue (MB) was 98%. The gas sensitivity test shows that OZN-10 has good selectivity, good response performance (3000 ppm, 27,887%) and excellent response recovery time (response time: 50 s, recovery time: 5–7 s) for saturated NH₃ gas at standard atmospheric pressure (101.325 KPa) and room temperature (25 °C). The synergistic effect of oxygen vacancy as the center of a trap and p–n heterojunction forming an electric potential field at the interface is explained, and the mechanism of improving photocatalysis and gas sensitivity is analyzed. This work will provide an innovative vision for dual-performance oxygen vacancy modification of heterojunctions through photocatalysis.

A review of graphene-TiO2 and graphene-ZnO nanocomposite photocatalysts for wastewater treatment

Technologies for wastewater remediation have been growing ever since the environmental and health concern is realized. Development of nanomaterials has enabled mankind to have different methods to treat the various kinds of inorganic and organic pollutants present in wastewater from many resources. Among the many materials, semiconductor materials have found many environmental applications due to their outstanding photocatalytic activities. TiO₂ and ZnO are more effectively used as photocatalyst or adsorbents in the withdrawal of inorganic as well as organic wastes from the wastewater. On the other hand, graphene is tremendously being investigated for applications in environmental remediation in view of the superior physical, optical, thermal, and electronic properties of graphene nanocomposites. In this work, graphene-TiO₂ and graphene-ZnO nanocomposites have been reviewed for photocatalytic wastewater treatment. The various preparation techniques of these nanocomposites have been discussed. Also, different design strategies for graphene-based photocatalyst have been revealed. These nanocomposites exhibit promising applications in most of the water purification processes which are reviewed in this work. Along with this, the development of these nanocomposites using biomass-derived graphene has also been introduced. PRACTITIONER POINTS: Graphene-TiO₂ and graphene-ZnO nanocomposites are effective for wastewater treatment through photocatalysis. These nanocomposite photocatalysts have been used in the form of membrane as well as antibacterial agents. Synthetic strategies and design considerations of graphene-based photocatalyst play a major role. Biomass-derived graphene-TiO₂ and graphene-ZnO nanocomposites have also found application in wastewater treatment.

A photo-renewable ZIF-8 photo-electrochemical sensor for the sensitive detection of sulfamethoxazole antibiotic

Electroanalysis is an effective monitoring method for organic pollution in environmental samples. However, chemical fouling with the formation of non-conductive fouled films easily occurs on the surface of the electrode during organic pollution detection that would inactivate the electrode and affect the detecting sensitivity of organic pollution. In this work, we found that zeolitic imidazolate framework-8 (ZIF-8) electrode can achieve effective degradation of non-conductive fouled films under the light illumination during electrochemical detection of some typical organic pollution (sulfamethoxazole (SMX), Bisphenol A (BPA) and diclofenac sodium (DS)). Profiting from the charge transfer capability and photoelectric characteristics, ZIF-8 electrode exhibits a lower detection limitation for organic pollution detection and superior regeneration property. The nice detection and superior regenerated property are mainly due to non-selective superoxide radical (·O₂^-) and hydroxyl radicals (·OH) mediation produced by ZIF-8 electrode under light illumination that can mineralize anodic fouled products and resume surface reactive sites. Compared with the single electrochemical determination, photo-assisted electroanalysis provides a stable monitoring and a renewable pathway for practical applications.

Highly-efficient visible-light-driven photocatalytic H2 evolution integrated with microplastic degradation over MXene/ZnxCd1-xS photocatalyst

The development of highly-efficient photocatalyst for H₂ production integrated with microplastic degradation is significant to meet the demand for clean energy and resolve "white pollution". Herein, a series of MXene/Zn_xCd_1-xS photocatalysts were successfully fabricated for H₂ evolution integrated with degradation of polyethylene terephthalate (PET). The resultant photocatalysts exhibited excellent photocatalytic performance, and the best photocatalytic H₂ evolution rate can reach 14.17 mmol·g^-1·h^-1 in alkaline PET alkaline solution. What's more, the PET was also converted to the useful organic micromolecule, including glycolate, acetate, ethanol, etc. The highly-efficient photocatalytic performance of MXene/Zn_xCd_1-xS photocatalysts can be attributed to the enhanced separation ability of photocarriers and optimum band structure with enhanced oxidation capacity of valence band. Finally, the photocatalytic mechanism was investigated in detail. Overall, this work supplied a new useful guidance for solving the energy problem and microplastic pollution issues, simultaneously.

Influence of calcination temperature on photocatalytic H2O2productivity of hierarchical porous ZnO microspheres

Photocatalytic production of H₂O₂from water and atmospheric oxygen has been recognized as a green and sustainable chemical process, due to the abundance of raw materials and sustainable solar energy. Herein, flower-like hierarchical ZnO microspheres were prepared by hydrothermal method followed by calcination at different temperatures, and their photocatalytic performance in H₂O₂production was examined under simulated sunlight irradiation. The calcination temperature plays a vital role in the structure, morphology, and surface area of the final ZnO products as well as their optical and electrochemical properties, which are determining factors in their photocatalytic activity. The ZnO calcined at 300 °C (Zn-300) exhibits the highest activity and optimal stability, showing productivity of 2793μmol l^-1within 60 min of irradiation, which was 6.5 times higher than that of uncalcined ZnO precursor. The remarkable photocatalytic activity is attributed to enhanced light utilization, large surface area, abundant exposed active sites, improved separation efficiency, and prolonged carrier lifespan. Moreover, the results from cycling experiments indicate the as-prepared ZnO samples exhibit good stability and long-time performance. This work provides useful information for the preparation of hierarchical ZnO photocatalysts.

Microstructure and Properties of Ag-Doped ZnO Grown Hydrothermally on a Graphene-Coated Polyethylene Terephthalate Bilayer Flexible Substrate

Ag-doped ZnO nanorods growth on a PET-graphene substrate (Ag-ZnO/PET-GR) with different Ag-doped content were synthesized by low-temperature ion-sputtering-assisted hydrothermal synthesis method. The phase composition, morphologies of ZnO, and electrical properties were analyzed. Ag-doping affects the initially perpendicular growth of ZnO nanorods, resulting in oblique growth of ZnO nanorods becoming more obvious as the Ag-doped content increases, and the diameter of the nanorods decreasing gradually. The width of the forbidden band gap of the ZnO films decreases with increasing Ag-doped content. For the Ag-ZnO/PET-GR composite structure, the Ag-ZnO thin film with 5% Ag-doped content has the largest carrier concentration (8.1 × 10¹⁸ cm⁻³), the highest mobility (67 cm² · V⁻¹ · s⁻¹), a small resistivity (0.09 Ω·cm), and impressive electrical properties.