Photocatalytic Cr(VI) reduction using amine-functionalized ZIF-8 through visible light irradiation.
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Hexavalent chromium (Cr(VI)) is one of the most toxic and carcinogenic species ...known to living beings, the environment, and our eco-system. Thus, it is urgent to develop a facile and effective approach for Cr(VI) removal. Zinc-based zeolitic imidazolate frameworks (ZIF-8), a typical metal organic framework, have high porosity, large specific surface area, high chemical stability, and abundant surface grafting sites. These sites can be easily modified with ethylenediamine (EDA) using a solvothermal process to generate a material that can serve as a potential candidate for photocatalytic Cr(VI) reduction under visible light irradiation. Various EDA contents and synthetic conditions were adopted in an attempt to investigate the correlation between ZIF-8 amine-functionalization and photocatalytic Cr(VI) reduction. The amine functionalization and the grafting sites on ZIF-8 were determined to be located at the –CH3 site of the 2-methylimidazole chains via X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and nuclear magnetic resonance spectroscopy (NMR). Under optimum conditions, amine-functionalized ZIF-8 exhibited a normalized rate constant (k/specific surface area, kSSA), which was 9.8 times higher than that of unmodified ZIF-8 one for photocatalytic Cr(VI) reduction. The increased catalytic activity and range of visible light absorption of amine-functionalized ZIF-8 can be attributed to the increase in electron density due to the lone pairs of the surface grafted amines. In summary, amine-functionalized ZIF-8 could serve as a promising visible-light-active photocatalyst for environmental remediation.
In this study, the photocatalytic activity of graphitic carbon nitride (g-C3N4) was improved by codoping S and P on the interstitial and carbon sites of its aromatic heptazine units, respectively. ...X-ray photoelectron spectra and X-ray diffraction measurements demonstrated that P covalently bonded with N because of the substitution of C with P and that S was located at the interstitial sites with relatively weak bonding with N in the heptazine units of the g-C3N4 structure. The samples of g-C3N4 codoped with P and S (PSCN) displayed low photoluminescence emission intensity, suggesting suppressed recombination of photogenerated charges, and thus increased photocatalytic degradation activity. In particular, PC (C replaced by P) and Si (interstitial doping of S) doping on g-C3N4 was prominently able to enhance the charge transfer through the NSNCNP pathway to cross the heptazine units for the photocatalytic reaction, making PSCN a highly visible-active and effective metal-free photocatalyst.
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Solar‐driven photocatalysis with graphitic carbon nitride (g‐C3N4) is considered to be the most promising approach for the generation of H2 from water, the degradation of organic pollutants, and the ...reduction of CO2. However, bulk g‐C3N4 exhibits several drawbacks, such as a low specific surface area, high defect density, and fast charge recombination, which result in low photocatalytic performance. The construction of 3D porous hydrogels for g‐C3N4 through nanostructural engineering is a rapid, feasible, and cost‐effective technique to improve the adsorption capability, stability, and separability of the hydrogel composite; to increase the number of active sites; and to create an internal conductive path for facile charge transfer and high photocatalytic activity. This minireview summarizes recent progress in photocatalytic water splitting and dye degradation by using g‐C3N4‐based hydrogels, with respect to state‐of‐the‐art methods for synthesis, preparation, modification, and multicomponent coupling. Furthermore, comprehensive outlooks, future challenges, and concluding remarks regarding the use of g‐C3N4‐based hydrogels as highly efficient photocatalysts are presented.
Gelling to split: Recent developments in graphitic carbon nitride (g‐C3N4)‐based hydrogels, with regard to synthesis, preparation, and multicomponent coupling, and their photocatalytic reactions, including water splitting, wastewater purification, and CO2 reduction, are reviewed. Particular focus is given to g‐C3N4‐based hydrogels as photocatalysts.
In this study, graphitic carbon nitride (g-C3N4) nanosheets (CNns) were modified using polyaniline (PANI) codoped with an inorganic (hydrochloric acid, HCl) and an organic (phytic acid, PA) acid. Our ...results revealed that these samples exhibited extended visible-light absorption and a three-dimensional (3D) hierarchical structure with a large specific surface area. They also inhibited photoluminescence emission, reduced electrical resistance, and provided abundant free radicals, resulting in high photocatalytic performance. The PANI/g-C3N4 sample demonstrated outstanding photocatalytic activity of a Cr(VI) removal capacity of 4.76 mg·min–1·gc –1, which is the best record for the reduction of a 100 ppm K2Cr2O7 solution. Moreover, g-C3N4 coupled with PANI monotonically doped with HCl or PA did not demonstrate increased activity, suggesting that the codoping of HCl and PA plays a significant role in enhancing the performance. The improved photocatalytic activity of PANI/g-C3N4 can be attributed to the interchain and intrachain doping of PA and HCl over PANI, respectively, to create a 3D connected network and synergistically increase the electrical conductivity. Therefore, new insights into g-C3N4 coupled with PANI and codoped by HCl and PA may have excellent potential for the design of g-C3N4-based compounds for efficient photocatalytic reactions.
S-doped g-C3N4 can serve as an efficient O2-evolved photocatalyst in a Z-scheme system.
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In this study, an S-doped g-C3N4 nanosheet was prepared as a photocatalyst for effective oxygen ...evolution reaction. Sulfur plays a crucial role in S-doped g-C3N4 not only in increasing the charge density but also in reducing the energy band gap of S-doped g-C3N4 via substitution of nitrogen sites. S-doped g-C3N4 can serve as an oxygen-evolved photocatalyst, when combined with Ru/SrTiO3:Rh in the presence of Co(bpy)33+/2+ as an electron mediator, enables photocatalytic overall water splitting under visible light irradiation with hydrogen and oxygen production rates of 24.6 and 14.5 μmol-h−1, respectively. Moreover, the photocatalytic overall water splitting to produce H2 and O2 using this Z-scheme system could use for five runs to at least 94.5 h under visible light irradiation. On the other hand, S-doped g-C3N4 can reduce biofouling by bacteria such as Escherichia coli by more than 70%, by simply incubating the S-doped g-C3N4 sample with bacterial solution under light irradiation. Our results suggest that S-doped g-C3N4 is a potentially effective, green, and promising material for a variety of photocatalytic applications.
In this study, hydroxyl-modified/Na-intercalated g-C3N4 was used as an effective material for the removal of gaseous formaldehyde (HCHO) through adsorption and photocatalytic oxidation. In a simple ...reflux reaction using different NaOH concentrations, the surface-grafted hydroxyl (‒OH) groups of g-C3N4 formed internal hydrogen bonds with gaseous HCHO, while the Na atoms intercalated into the layered structure of g-C3N4 to create an electron transfer path (layer‒Na‒layer) to improve charge separation for photocatalytic oxidation. The hydroxylation as well as Na intercalation of g-C3N4 significantly increased its HCHO removal efficiency compared with that of bare g-C3N4. The HCHO removal capacity of the modified g-C3N4 reached approximately 0.12 ppm g−1 min−1, which was twice that of bare g-C3N4 (<0.06 ppm g−1 min−1). Moreover, the modified g-C3N4 was reused at least thrice without any decline in activity. This work provides a facile, simple, and fast approach to the modification of g-C3N4 via hydroxylation and Na intercalation to enhance HCHO adsorption as well as electron transfer, rendering it a promising material for indoor HCHO removal.
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•Hydroxyl-modified and Na-intercalated g-C3N4 was prepared via treatment with NaOH.•Hydroxylation of g-C3N4 significantly improved HCHO adsorption.•Na intercalation in g-C3N4 created an electron transfer path for charge migration.•Hydroxyl-modified and Na-intercalated g-C3N4 exhibited high HCHO removal performance.
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Graphitic carbon nitride (g-C3N4), comprising tri-s-triazine (C6N7) units, is a promising visible-light-active photocatalyst and is generally synthesized using thermal condensation. ...This study reports a time-saving microwave-assisted solvothermal synthesis (MW) in which powders were obtained at temperatures as low as 180°C within 60–120min. Images obtained using scanning electron microscopy and transmission electron microscopy demonstrated that the MW g-C3N4 particles had a uniform microspheric shape, whereas g-C3N4 prepared with conventional thermal condensation (TC) and solvothermal (ST) approaches exhibited plate-like layered structures with buckles on their surface. X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy indicated that TC g-C3N4 had terminal NH2 groups on the periphery of its tri-s-triazine units, whereas some cyano moieties were present on MW and ST g-C3N4. In a photocatalytic test, MW g-C3N4 was discovered to have enhanced photocatalytic activity in the degradation of methyl orange dye and phenol solution under visible light irradiation, which can be ascribed to its reduced charge recombination and increased light harvesting in the visible region, as evidenced by UV–vis and photoluminescence spectroscopy. The terminal NH2 groups attached to TC g-C3N4 may have consumed the photoinduced charges, thus reducing the photocatalytic activity and increasing recombination of the sample. The facile and time-saving microwave-assisted solvothermal method presented herein can thus be used for the synthesis of visible-light-active g-C3N4, which has potential photocatalysis applications.
A lanthanum-modified zeolitic imidazolate framework (La/ZIF-8) was developed to produce glycerol carbonate using CO2 and glycerol as raw materials. La/ZIF-8 provides a high catalytic glycerol ...conversion efficiency owing to its surface-attached nanoclusters of La2O3, which can be viewed as La3+–O2− pairs that strengthen the Lewis basicity and acidity, and the large specific surface area of ZIF-8. The catalytic glycerol conversion and the yield of glycerol carbonate were 46.5% and 35.3%, respectively, using CH3CN as a dehydrating agent. With increase in the amount of CH3CN, the water molecules could react with CH3CN to reduce the selectivity. When an inorganic dehydrating agent, MgCO3, was used to physically adsorb and remove water molecules in the reaction, the selectivity of the reaction could be increased to over 95%, which is the highest ever reported. Reaction kinetics analysis also revealed that the activation energy of using MgCO3 (5.4 kJ mol−1) as a dehydrating agent is lower than that using CH3CN (7.8 kJ mol−1). Moreover, the La/ZIF-8 could be recycled and reused at least three times with high catalytic performance. This study provides an effective material with dual Lewis basicity and acidity for CO2/glycerol conversion and significantly improves the catalytic performance using an inorganic dehydrating agent.
Metal-free, visible light-responsive, carbon-based graphitic carbon nitride (g-C
3
N
4
) is a well-known photocatalyst. It is necessary to develop a g-C
3
N
4
for use as a photocatalyst in a green, ...economically viable, and effective manner. In this study, a waste-derived g-C
3
N
4
was successfully prepared from the mixture of dicyandiamide (DICY) and mushroom waste (MW). The waste-derived g-C
3
N
4
(1MDCN) exhibited enhanced photocatalytic activity compared to the conventional dicyandiamide-derived g-C
3
N
4
for the degradation of an azo dye, methyl blue (MB), which is a most commonly used dye in the pharmaceutical industry. Under visible light irradiation, the improved activity could be attributed to its greater graphitic degree and fewer pyrrolic-N forming repeated tri-s-triazine units oriented along the 100 plane, which was evidenced by high-resolution X-ray photoelectron spectroscopy. In addition, the waste-derived g-C
3
N
4
in this study is less harmful and exhibited good cell viability for Chinese hamster ovary cells, with over 50% of the cells surviving, whereas more than 85% of the cells were killed using melamine-derived g-C
3
N
4
at a concentration of 1000 μg mL
−1
. The present study shows that g-C
3
N
4
derived from the mixture of DICY and MW can be a harmless, economically viable, and effective photocatalyst for environmental remediation.
Graphic Abstract
In the present study, carbon-dots (CDs) were derived from the thermal oxidation of an agricultural waste, bitter tea residue, to obtain different sp2/sp3 ratios and electronic structures for metal ...sensing. The CDs obtained from calcination at 700 °C exhibited the highest photoluminescence (PL) quantum yield (QY) of 11.8% among all the samples treated at different temperatures. These CDs had a high degree of graphitization, which resulted in a strong π-π* electron transition, and hence in a high QY. The strong photoluminescence of the CDs could be used to sense the metal ions Ag+, Sr2+, Fe2+, Fe3+, Co2+, Ni2+, Cu2+, and Sn2+ by monitoring their PL intensity at an excitation wavelength of 320 nm. The metals inhibited the PL intensity in the order Ag+ > Fe2+, Fe3+, Ni2+ > Sr2+, Co2+, Cu2+, Sn2+, which demonstrated that the CDs exhibited high metal ion detection capability and selectivity. The detection of Fe3+ using CDs was performed in the range of 10–100 ppm with a LOD (limit of detection) value of 0.380 ppm. Theoretical calculations demonstrated that Ag+, Sr2+, and Sn2+ induced charge transfer excitation and that Fe2+ and Ni2+ induced d-d transitions via complexation with the sp2 clusters. The charge transfer excitation and d-d transitions hindered the π-π* transition of the sp2 clusters, leading to a quenching effect. On the other hand, Li+, Na+, and K+ ions did not alter the π-π* transition of the sp2 clusters, resulting in a negligible quenching effect. In summary, the oxidation level and electronic structure of CDs derived from bitter tea residue could be tailored, and the CDs were shown to be a facile, sustainable, and eco-friendly material for metal sensing.
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•Bitter tea residue-derived carbon dots are prepared from the thermal oxidation method.•Carbon dots exhibited high metal ion detection capability and selectivity.•Cstarbon dots exhibited a photoluminescence quantum yield of 11.8%.
