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2021 Volume 37 Issue 8
2021, 37(8): 200708
doi: 10.3866/PKU.WHXB202007086
Abstract:
In-depth understanding of the mechanisms of hydrogen sulfide (H2S) adsorption on catalysts during desulfurization from industrial waste gas streams is important for developing effective catalysts to be used in the decomposition of H2S. In this work, the dissociation behavior of H2S adsorbed on a single-atom catalyst (Ti or V-decorated Ti2CO2 surface) was investigated by performing density functional theory (DFT) calculations. The corresponding diffusion behavior revealed that Ti or V atoms could be dispersed on the Ti2CO2 monolayer, without aggregation in the form of single atoms. In addition, analyses of the partial density of states (PDOS), Hirshfeld charges, and electron density difference indicated that the decorated Ti or V atoms led to charge redistribution on the Ti2CO2 surface and significantly improved the interaction between the H2S gas molecules and Ti2CO2, thereby enhancing the catalytic activity of V/Ti2CO2. In order to gain a deeper understanding of the mechanism of H2S decomposition (H2S → HS* + H* → H2 + S*), a comparative analysis of the results for the decomposition of H2S on the Ti/Ti2CO2 and V/Ti2CO2 surfaces was carried out. The catalytic dissociation behavior of H2S is explained as follows: once H2S is adsorbed on the V/Ti2CO2 or Ti/Ti2CO2 surface, it spontaneously dissociates into HS*/H* without any energy barrier on the catalyst surface. Subsequently, the V atoms would not only promote the cleavage of the H-S bond, but also play a major role in the formation of S atoms. Moreover, the rate-limiting step for the entire process proceeded on the Ti/Ti2CO2 surface with an energy barrier of 0.86 eV, while that for V/Ti2CO2 was 0.28 eV, indicating that the H2S molecules easily dissociated into S and H2 on the V/Ti2CO2 surface at room temperature. The reaction time for H2S decomposition on the V/Ti2CO2 surface at 500 K was 65.79 ns, which was almost two orders of magnitude higher than that at room temperature. Thus, the decomposition of H2S on the V-doped Ti2CO2 surface is associated very fast kinetics. Furthermore, the S atoms can form elemental sulfur with aggregation on the V/Ti2CO2 surface to promote recycling reactions. Compared with previously reported catalytic systems, the single-atom catalyst (SAC) V/Ti2CO2 catalyst has greater application prospects in terms of sustainable economy or removal efficiency for H2S treatment. Our results suggest that V-doped Ti2CO2 is an excellent candidate for a highly effective non-noble metal catalyst applicable to H2S decomposition. ![]()
In-depth understanding of the mechanisms of hydrogen sulfide (H2S) adsorption on catalysts during desulfurization from industrial waste gas streams is important for developing effective catalysts to be used in the decomposition of H2S. In this work, the dissociation behavior of H2S adsorbed on a single-atom catalyst (Ti or V-decorated Ti2CO2 surface) was investigated by performing density functional theory (DFT) calculations. The corresponding diffusion behavior revealed that Ti or V atoms could be dispersed on the Ti2CO2 monolayer, without aggregation in the form of single atoms. In addition, analyses of the partial density of states (PDOS), Hirshfeld charges, and electron density difference indicated that the decorated Ti or V atoms led to charge redistribution on the Ti2CO2 surface and significantly improved the interaction between the H2S gas molecules and Ti2CO2, thereby enhancing the catalytic activity of V/Ti2CO2. In order to gain a deeper understanding of the mechanism of H2S decomposition (H2S → HS* + H* → H2 + S*), a comparative analysis of the results for the decomposition of H2S on the Ti/Ti2CO2 and V/Ti2CO2 surfaces was carried out. The catalytic dissociation behavior of H2S is explained as follows: once H2S is adsorbed on the V/Ti2CO2 or Ti/Ti2CO2 surface, it spontaneously dissociates into HS*/H* without any energy barrier on the catalyst surface. Subsequently, the V atoms would not only promote the cleavage of the H-S bond, but also play a major role in the formation of S atoms. Moreover, the rate-limiting step for the entire process proceeded on the Ti/Ti2CO2 surface with an energy barrier of 0.86 eV, while that for V/Ti2CO2 was 0.28 eV, indicating that the H2S molecules easily dissociated into S and H2 on the V/Ti2CO2 surface at room temperature. The reaction time for H2S decomposition on the V/Ti2CO2 surface at 500 K was 65.79 ns, which was almost two orders of magnitude higher than that at room temperature. Thus, the decomposition of H2S on the V-doped Ti2CO2 surface is associated very fast kinetics. Furthermore, the S atoms can form elemental sulfur with aggregation on the V/Ti2CO2 surface to promote recycling reactions. Compared with previously reported catalytic systems, the single-atom catalyst (SAC) V/Ti2CO2 catalyst has greater application prospects in terms of sustainable economy or removal efficiency for H2S treatment. Our results suggest that V-doped Ti2CO2 is an excellent candidate for a highly effective non-noble metal catalyst applicable to H2S decomposition.
2021, 37(8): 200904
doi: 10.3866/PKU.WHXB202009045
Abstract:
Metal-organic frameworks (MOFs) are of significant interest for photocatalysis using visible light, but they are typically limited by the instability and high recombination ratio of photoexcited pairs. Integrating MOFs into an inorganic semiconductor is one of the most widespread methods to promote their activity. In this study, a core-shell structured MOF@TiO2 (NH2-UiO-66@TiO2) was synthesized as an efficient photocatalyst for the degradation of toluene. Pristine NH2-UiO-66 was synthesized by a hydrothermal method as the core, which was then coated with an amorphous TiO2 shell. Compared with pristine NH2-UiO-66 and other samples prepared by the direct mixing of NH2-UiO-66 and TiO2, NH2-UiO-66@TiO2 exhibited a higher degradation rate of toluene. Using NH2-UiO-66@TiO2 as a catalyst, the degradation efficiency of toluene reached 76.7% within 3 h, which is 1.48 times higher than that of NH2-UiO-66. The degradation performance was also stable in four repeated reuse experiments, and the slight deactivation was reactivated after washing with ethanol. A series of characterization methods were used to determine the physicochemical properties of NH2-UiO-66@TiO2, including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Using the measured physicochemical properties, the photocatalytic mechanism of NH2-UiO-66@TiO2 was explored. NH2-UiO-66 is an ideal photocatalyst, with visible-light response and a huge specific surface area (914.9 m2·g-1), which is favorable for the utilization of sunlight as well as the absorption of pollutants in indoor air. In addition, a new interface formed between the two components (NH2-UiO-66 and TiO2), which efficiently broaden the light absorption area and enhanced the utilization of photogenerated species. The photogenerated holes and electrons could transfer through the interlayer as soon as they were formed. It is speculated that holes would transfer to the HOMO of NH2-UiO-66, and then combine with H2O molecules to form hydroxyl radicals (·OH). At the same time, more electrons tended to combine with oxygen molecules in the conduction band of TiO2 rather than recombine with holes. Consequently, the recombination rate of electrons and holes decreased, while the quantity of oxygen radicals and hydroxyl radicals increased. Toluene was efficiently oxidized by these two types of radicals. Owing to the outstanding properties mentioned above, the strategy of constructing NH2-UiO-66@TiO2 is considered to be an effective approach. This work may provide new insights into the design of core-shell structured MOF@photocatalysts for the photocatalytic degradation of indoor air pollutants. ![]()
Metal-organic frameworks (MOFs) are of significant interest for photocatalysis using visible light, but they are typically limited by the instability and high recombination ratio of photoexcited pairs. Integrating MOFs into an inorganic semiconductor is one of the most widespread methods to promote their activity. In this study, a core-shell structured MOF@TiO2 (NH2-UiO-66@TiO2) was synthesized as an efficient photocatalyst for the degradation of toluene. Pristine NH2-UiO-66 was synthesized by a hydrothermal method as the core, which was then coated with an amorphous TiO2 shell. Compared with pristine NH2-UiO-66 and other samples prepared by the direct mixing of NH2-UiO-66 and TiO2, NH2-UiO-66@TiO2 exhibited a higher degradation rate of toluene. Using NH2-UiO-66@TiO2 as a catalyst, the degradation efficiency of toluene reached 76.7% within 3 h, which is 1.48 times higher than that of NH2-UiO-66. The degradation performance was also stable in four repeated reuse experiments, and the slight deactivation was reactivated after washing with ethanol. A series of characterization methods were used to determine the physicochemical properties of NH2-UiO-66@TiO2, including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Using the measured physicochemical properties, the photocatalytic mechanism of NH2-UiO-66@TiO2 was explored. NH2-UiO-66 is an ideal photocatalyst, with visible-light response and a huge specific surface area (914.9 m2·g-1), which is favorable for the utilization of sunlight as well as the absorption of pollutants in indoor air. In addition, a new interface formed between the two components (NH2-UiO-66 and TiO2), which efficiently broaden the light absorption area and enhanced the utilization of photogenerated species. The photogenerated holes and electrons could transfer through the interlayer as soon as they were formed. It is speculated that holes would transfer to the HOMO of NH2-UiO-66, and then combine with H2O molecules to form hydroxyl radicals (·OH). At the same time, more electrons tended to combine with oxygen molecules in the conduction band of TiO2 rather than recombine with holes. Consequently, the recombination rate of electrons and holes decreased, while the quantity of oxygen radicals and hydroxyl radicals increased. Toluene was efficiently oxidized by these two types of radicals. Owing to the outstanding properties mentioned above, the strategy of constructing NH2-UiO-66@TiO2 is considered to be an effective approach. This work may provide new insights into the design of core-shell structured MOF@photocatalysts for the photocatalytic degradation of indoor air pollutants.
2021, 37(8): 201000
doi: 10.3866/PKU.WHXB202010001
Abstract:
Vacuum ultraviolet irradiation coupled with photocatalytic oxidation (VUV-PCO) is an efficient and promising method for eliminating pollutants at room temperature; it involves three processes: vacuum ultraviolet (VUV) photolysis, photocatalytic oxidation (PCO), and ozone catalytic oxidation. Herein, toluene was chosen as the representative volatile organic compound (VOC), which is one of the most important precursors to form fine particulate matter and photochemical smog, because of its high toxicity and extensive existence in industries. All experiments were performed in a fixed-bed continuous-flow reactor that contained units for VUV photolysis and PCO. Mesoporous P-Mn-TiO2 was prepared by one-step hydrolysis and used as a catalyst for the oxidation of gaseous toluene under VUV irradiation through the VUV-PCO process. The as-prepared P-Mn-TiO2 samples were characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), ultraviolet-visible light (UV-Vis) spectroscopy, and X-ray diffraction (XRD) analysis to determine the physicochemical properties of the catalysts and to determine the mechanisms of Mn doping and phosphoric acid modification and the effects of these processes on photocatalytic activity, ozone catalytic activity, and adsorption performance. The results indicated that the synergistic effect of phosphoric acid modification and Mn doping can improve the ozone catalytic activity and photocatalytic performance by increasing the number of oxygen active sites, completely eliminating the outlet ozone, and simultaneously promoting the efficient degradation of toluene. Moreover, doping TiO2 with Mn3+ significantly enhanced light harvesting, and numerous oxygen vacancies can be generated on the catalyst surface because of the presence of doped Mn3+ in the lattice, which adsorbs and transforms the oxygen species for toluene degradation. In addition, modification with an appropriate amount of phosphate groups can facilitate O2 and O3 adsorption on the TiO2 surface that can favor photo-induced charge carrier separation, thereby significantly improving the photocatalytic and ozone catalytic activities. The excellent catalytic performance of mesoporous P-Mn-TiO2 for toluene degradation and outlet ozone elimination was ascribed to the formation of highly reactive oxidizing species such as O(1D), O(3P), and ·OH via the catalytic decomposition of O3 adsorbed on the oxygen vacancy sites containing Mn and phosphate groups on the catalyst surface. In the VUV-PCO process, toluene was first destructed via VUV photolysis and oxidized by residual O3 generated from VUV photolysis and the active oxygen species formed in the presence of the catalyst. Finally, toluene and the generated intermediate products were oxidized and degraded to CO2 and H2O through VUV-PCO. In addition, the outlet ozone byproduct was simultaneously eliminated by the multifunctional catalyst. ![]()
Vacuum ultraviolet irradiation coupled with photocatalytic oxidation (VUV-PCO) is an efficient and promising method for eliminating pollutants at room temperature; it involves three processes: vacuum ultraviolet (VUV) photolysis, photocatalytic oxidation (PCO), and ozone catalytic oxidation. Herein, toluene was chosen as the representative volatile organic compound (VOC), which is one of the most important precursors to form fine particulate matter and photochemical smog, because of its high toxicity and extensive existence in industries. All experiments were performed in a fixed-bed continuous-flow reactor that contained units for VUV photolysis and PCO. Mesoporous P-Mn-TiO2 was prepared by one-step hydrolysis and used as a catalyst for the oxidation of gaseous toluene under VUV irradiation through the VUV-PCO process. The as-prepared P-Mn-TiO2 samples were characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), ultraviolet-visible light (UV-Vis) spectroscopy, and X-ray diffraction (XRD) analysis to determine the physicochemical properties of the catalysts and to determine the mechanisms of Mn doping and phosphoric acid modification and the effects of these processes on photocatalytic activity, ozone catalytic activity, and adsorption performance. The results indicated that the synergistic effect of phosphoric acid modification and Mn doping can improve the ozone catalytic activity and photocatalytic performance by increasing the number of oxygen active sites, completely eliminating the outlet ozone, and simultaneously promoting the efficient degradation of toluene. Moreover, doping TiO2 with Mn3+ significantly enhanced light harvesting, and numerous oxygen vacancies can be generated on the catalyst surface because of the presence of doped Mn3+ in the lattice, which adsorbs and transforms the oxygen species for toluene degradation. In addition, modification with an appropriate amount of phosphate groups can facilitate O2 and O3 adsorption on the TiO2 surface that can favor photo-induced charge carrier separation, thereby significantly improving the photocatalytic and ozone catalytic activities. The excellent catalytic performance of mesoporous P-Mn-TiO2 for toluene degradation and outlet ozone elimination was ascribed to the formation of highly reactive oxidizing species such as O(1D), O(3P), and ·OH via the catalytic decomposition of O3 adsorbed on the oxygen vacancy sites containing Mn and phosphate groups on the catalyst surface. In the VUV-PCO process, toluene was first destructed via VUV photolysis and oxidized by residual O3 generated from VUV photolysis and the active oxygen species formed in the presence of the catalyst. Finally, toluene and the generated intermediate products were oxidized and degraded to CO2 and H2O through VUV-PCO. In addition, the outlet ozone byproduct was simultaneously eliminated by the multifunctional catalyst.
2021, 37(8): 201004
doi: 10.3866/PKU.WHXB202010042
Abstract:
Excessive use of ciprofloxacin (CIP) has proven to be a significant threat to the ecological environment. In this work, a novel Fe-free photo-electro-Fenton system was designed for the degradation of CIP in water. The NiO/g-C3N4 composites were synthesized by a simple solvothermal method. The crystalline phases and chemical compositions of the different catalysts were determined via X-ray diffraction (XRD) analysis. Fourier transform infrared (FT-IR) spectroscopy further confirmed the molecular structures of the different composites. The results proved the successful synthesis of NiO/g-C3N4 composites. The morphology of the material was obtained using scanning electron microscopy (SEM), which showed that the structure of the optimal NiO/g-C3N4-60% was two-dimensional and flower-like. The transmission electron microscopy (TEM) analysis further proved that the NiO/g-C3N4-60% possessed a layered structure. Owing to the layered structure, the NiO/g-C3N4-60% boasts of a large specific surface area and abundant active sites, which were beneficial for the transmission of electrons and oxidation of CIP. Furthermore, it was evident from the X-ray photoelectron spectroscopy (XPS) analysis that the Ni2+ and Ni3+ coexisted, and there was low coordination oxygen with defects in the NiO/g-C3N4-60% composite. The electron paramagnetic resonance (EPR) spectrum also proved the existence of oxygen vacancies, which not only facilitated the activation of H2O2, but also promoted the formation of stable mixed valence states of metal ions. UV-vis diffuse reflection spectrum (UV-Vis DRS), photoluminescence (PL), and electrochemical tests showed that NiO/g-C3N4-60% exhibited the strongest light absorption capacity, lowest charge transfer resistance, and fastest charge separation efficiency, which was beneficial for the generation of active species and the rapid degradation of CIP. Therefore, the flower-like NiO/g-C3N4-60% composites exhibited photoelectric synergy in the photo-electro-Fenton process. They not only effectively decomposed the H2O2 produced in the electro-Fenton process into ·OH by the conversion of Ni3+/Ni2+, but also generated photogenerated electrons and holes to promote the production of ·OH, ·O2–, and h+ under light irradiation to improve the degradation efficiency of CIP. When the optimal NiO/g-C3N4-60% served as a catalyst in the photo-electro-Fenton system, the degradation efficiency of CIP reached approximately 100% in 90 min and the mineralization efficiency reached 82.0% in 120 min. In addition, compared with the traditional Fenton system (the optimal pH value of which is 2.8–3.5), the novel photo-electro-Fenton system possessed a wider range of pH, with a final CIP degradation efficiency of 78.8% at a pH value of 6. The NiO/g-C3N4-60% also demonstrated excellent structural stability in the photo-electro-Fenton system. After five consecutive cycles, the degradation efficiency was maintained at 96.3%. Based on the results of high-performance liquid chromatography-mass spectrometry (HPLC-MS), two possible pathways for CIP degradation were proposed. This study provides a theoretical basis for the rapid degradation of antibiotics in wastewater. ![]()
Excessive use of ciprofloxacin (CIP) has proven to be a significant threat to the ecological environment. In this work, a novel Fe-free photo-electro-Fenton system was designed for the degradation of CIP in water. The NiO/g-C3N4 composites were synthesized by a simple solvothermal method. The crystalline phases and chemical compositions of the different catalysts were determined via X-ray diffraction (XRD) analysis. Fourier transform infrared (FT-IR) spectroscopy further confirmed the molecular structures of the different composites. The results proved the successful synthesis of NiO/g-C3N4 composites. The morphology of the material was obtained using scanning electron microscopy (SEM), which showed that the structure of the optimal NiO/g-C3N4-60% was two-dimensional and flower-like. The transmission electron microscopy (TEM) analysis further proved that the NiO/g-C3N4-60% possessed a layered structure. Owing to the layered structure, the NiO/g-C3N4-60% boasts of a large specific surface area and abundant active sites, which were beneficial for the transmission of electrons and oxidation of CIP. Furthermore, it was evident from the X-ray photoelectron spectroscopy (XPS) analysis that the Ni2+ and Ni3+ coexisted, and there was low coordination oxygen with defects in the NiO/g-C3N4-60% composite. The electron paramagnetic resonance (EPR) spectrum also proved the existence of oxygen vacancies, which not only facilitated the activation of H2O2, but also promoted the formation of stable mixed valence states of metal ions. UV-vis diffuse reflection spectrum (UV-Vis DRS), photoluminescence (PL), and electrochemical tests showed that NiO/g-C3N4-60% exhibited the strongest light absorption capacity, lowest charge transfer resistance, and fastest charge separation efficiency, which was beneficial for the generation of active species and the rapid degradation of CIP. Therefore, the flower-like NiO/g-C3N4-60% composites exhibited photoelectric synergy in the photo-electro-Fenton process. They not only effectively decomposed the H2O2 produced in the electro-Fenton process into ·OH by the conversion of Ni3+/Ni2+, but also generated photogenerated electrons and holes to promote the production of ·OH, ·O2–, and h+ under light irradiation to improve the degradation efficiency of CIP. When the optimal NiO/g-C3N4-60% served as a catalyst in the photo-electro-Fenton system, the degradation efficiency of CIP reached approximately 100% in 90 min and the mineralization efficiency reached 82.0% in 120 min. In addition, compared with the traditional Fenton system (the optimal pH value of which is 2.8–3.5), the novel photo-electro-Fenton system possessed a wider range of pH, with a final CIP degradation efficiency of 78.8% at a pH value of 6. The NiO/g-C3N4-60% also demonstrated excellent structural stability in the photo-electro-Fenton system. After five consecutive cycles, the degradation efficiency was maintained at 96.3%. Based on the results of high-performance liquid chromatography-mass spectrometry (HPLC-MS), two possible pathways for CIP degradation were proposed. This study provides a theoretical basis for the rapid degradation of antibiotics in wastewater.
2021, 37(8): 200801
doi: 10.3866/PKU.WHXB202008010
Abstract:
Layered graphitic carbon nitride (g-C3N4) is a typical polymeric semiconductor with an sp2 π-conjugated system having great potential in energy conversion, environmental purification, materials science, etc., owing to its unique physicochemical and electrical properties. However, bulk g-C3N4 obtained by calcination suffers from a low specific surface area, rapid charge carrier recombination, and poor dispersion in aqueous solutions, which limit its practical applications. Controlling the size of g-C3N4 (e.g., preparing g-C3N4 nanosheets) can effectively solve the above problems. Compared with the bulk material, g-C3N4 nanosheets have a larger specific surface area, richer active sites, and a larger band gap due to the quantum confinement effect. As g-C3N4 has a layered structure with strong in-plane C-N covalent bonds and weak van der Waals forces between the layers, g-C3N4 nanosheets can be prepared by exfoliating bulk g-C3N4. Alternatively, g-C3N4 nanosheets can otherwise be obtained through the anisotropic assembly of organic precursors. Nevertheless, some of these methods have various limitations, such as high energy consumption, are time consuming, and have low yield. Accordingly, developing green and cost-effective exfoliation and preparation strategies for g-C3N4 nanosheets is necessary. Herein, the research progress of the exfoliation and preparation strategies (including the thermal oxidation etching process, the ultrasound-assisted route, the chemical exfoliation, the mechanical method, and the template method) for two-dimensional C3N4 nanosheets are introduced. Their features are systematically analyzed and the perspectives and challenges in the preparation of g-C3N4 nanosheets are discussed. This study emphasizes the following: (1) The preparation method of g-C3N4 nanosheets should be properly selected according to the practical application needs. Additionally, various strategies (such as chemical method and ultrasonic method) can be combined to exfoliate nanosheets from bulk g-C3N4; (2) More reasonable nano- or even subnanostructured g-C3N4 nanosheets should be continuously explored; (3) Novel modification strategies, such as defective engineering, heterojunction construction, and surface functional group regulation, should be introduced to improve the reactivity and selectivity of the g-C3N4 nanosheets; (4) The application of in situ characterization techniques (such as in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), electron spin resonance (ESR) spectroscopy, and Raman spectroscopy) should also be strengthened to monitor the detailed catalytic process and investigate the g-C3N4 nanosheet structure-efficiency relationship. (5) To gain a deeper understanding of the relationship between the macroscopic properties and the microscopic structure, the combination of theoretical calculations and experimental results should be strengthened, which will be beneficial for exploiting high-quality g-C3N4 nanosheets. ![]()
Layered graphitic carbon nitride (g-C3N4) is a typical polymeric semiconductor with an sp2 π-conjugated system having great potential in energy conversion, environmental purification, materials science, etc., owing to its unique physicochemical and electrical properties. However, bulk g-C3N4 obtained by calcination suffers from a low specific surface area, rapid charge carrier recombination, and poor dispersion in aqueous solutions, which limit its practical applications. Controlling the size of g-C3N4 (e.g., preparing g-C3N4 nanosheets) can effectively solve the above problems. Compared with the bulk material, g-C3N4 nanosheets have a larger specific surface area, richer active sites, and a larger band gap due to the quantum confinement effect. As g-C3N4 has a layered structure with strong in-plane C-N covalent bonds and weak van der Waals forces between the layers, g-C3N4 nanosheets can be prepared by exfoliating bulk g-C3N4. Alternatively, g-C3N4 nanosheets can otherwise be obtained through the anisotropic assembly of organic precursors. Nevertheless, some of these methods have various limitations, such as high energy consumption, are time consuming, and have low yield. Accordingly, developing green and cost-effective exfoliation and preparation strategies for g-C3N4 nanosheets is necessary. Herein, the research progress of the exfoliation and preparation strategies (including the thermal oxidation etching process, the ultrasound-assisted route, the chemical exfoliation, the mechanical method, and the template method) for two-dimensional C3N4 nanosheets are introduced. Their features are systematically analyzed and the perspectives and challenges in the preparation of g-C3N4 nanosheets are discussed. This study emphasizes the following: (1) The preparation method of g-C3N4 nanosheets should be properly selected according to the practical application needs. Additionally, various strategies (such as chemical method and ultrasonic method) can be combined to exfoliate nanosheets from bulk g-C3N4; (2) More reasonable nano- or even subnanostructured g-C3N4 nanosheets should be continuously explored; (3) Novel modification strategies, such as defective engineering, heterojunction construction, and surface functional group regulation, should be introduced to improve the reactivity and selectivity of the g-C3N4 nanosheets; (4) The application of in situ characterization techniques (such as in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), electron spin resonance (ESR) spectroscopy, and Raman spectroscopy) should also be strengthened to monitor the detailed catalytic process and investigate the g-C3N4 nanosheet structure-efficiency relationship. (5) To gain a deeper understanding of the relationship between the macroscopic properties and the microscopic structure, the combination of theoretical calculations and experimental results should be strengthened, which will be beneficial for exploiting high-quality g-C3N4 nanosheets.
2021, 37(8): 201001
doi: 10.3866/PKU.WHXB202010010
Abstract:
Two-dimensional photocatalytic materials have potential applications in the fields of environmental purification and energy conversion owing to their rich surface active sites, unique geometric structures, adjustable electronic structures, and good photocatalytic activities. At present, the main two-dimensional photocatalytic materials include metal oxides, metal composite oxides, metal hydroxides, metal sulfides, bismuth-based materials, and non-metallic photocatalytic materials. The absorption of photons in bulk materials or nanoparticles is often limited by the transmittance and reflection at the grain boundary, while the two-dimensional structure can provide a large specific surface area and abundant surface low-coordination atoms to obtain more UV visible light. In addition, the smaller atomic thickness of two-dimensional photocatalytic materials can shorten the carrier migration distance. Thus, in two-dimensional photocatalytic materials, the carriers generated in the interior migrate to the surface faster than that in the bulk materials, which can reduce the recombination of photogenerated carriers and facilitate the photocatalytic reaction. For the surface redox reaction, the two-dimensional structure can provide more abundant surface-active sites to accelerate the reaction process. Additionally, when the thickness is reduced to the atomic scale, the escape energy of atoms is relatively small, thereby increasing the surface defects, which is helpful for the adsorption and activation of target molecules. Thus, the synthesis methods and performance enhancement strategies of two-dimensional photocatalytic materials have been developed rapidly. The former strategies mainly focus on the adjustment of morphology and geometric structure characteristics, which cannot fully meet the design requirements of efficient and stable photocatalysts. The photocatalytic performance and stability can be improved by surface design to construct abundant active sites and adjust the electronic structure. Research on the reaction mechanism of photocatalysis can help us understand the demand for photocatalytic structure characteristics in different reactions, thereby guiding the design of photocatalysts. In this paper, the advances in surface design and electronic structure regulation strategies of two-dimensional photocatalytic materials are reviewed from three aspects: light absorption; charge separation; and active sites, including element doping, heterojunction design, defect construction, single atom modification, and plasmonic metal loading. The effects on the reaction mechanism for typical air pollutant purification by regulating the electronic structure of two-dimensional photocatalytic materials are summarized. Finally, the problems and challenges associated with the development of two-dimensional photocatalytic materials are analyzed and discussed. ![]()
Two-dimensional photocatalytic materials have potential applications in the fields of environmental purification and energy conversion owing to their rich surface active sites, unique geometric structures, adjustable electronic structures, and good photocatalytic activities. At present, the main two-dimensional photocatalytic materials include metal oxides, metal composite oxides, metal hydroxides, metal sulfides, bismuth-based materials, and non-metallic photocatalytic materials. The absorption of photons in bulk materials or nanoparticles is often limited by the transmittance and reflection at the grain boundary, while the two-dimensional structure can provide a large specific surface area and abundant surface low-coordination atoms to obtain more UV visible light. In addition, the smaller atomic thickness of two-dimensional photocatalytic materials can shorten the carrier migration distance. Thus, in two-dimensional photocatalytic materials, the carriers generated in the interior migrate to the surface faster than that in the bulk materials, which can reduce the recombination of photogenerated carriers and facilitate the photocatalytic reaction. For the surface redox reaction, the two-dimensional structure can provide more abundant surface-active sites to accelerate the reaction process. Additionally, when the thickness is reduced to the atomic scale, the escape energy of atoms is relatively small, thereby increasing the surface defects, which is helpful for the adsorption and activation of target molecules. Thus, the synthesis methods and performance enhancement strategies of two-dimensional photocatalytic materials have been developed rapidly. The former strategies mainly focus on the adjustment of morphology and geometric structure characteristics, which cannot fully meet the design requirements of efficient and stable photocatalysts. The photocatalytic performance and stability can be improved by surface design to construct abundant active sites and adjust the electronic structure. Research on the reaction mechanism of photocatalysis can help us understand the demand for photocatalytic structure characteristics in different reactions, thereby guiding the design of photocatalysts. In this paper, the advances in surface design and electronic structure regulation strategies of two-dimensional photocatalytic materials are reviewed from three aspects: light absorption; charge separation; and active sites, including element doping, heterojunction design, defect construction, single atom modification, and plasmonic metal loading. The effects on the reaction mechanism for typical air pollutant purification by regulating the electronic structure of two-dimensional photocatalytic materials are summarized. Finally, the problems and challenges associated with the development of two-dimensional photocatalytic materials are analyzed and discussed.
2021, 37(8): 200902
doi: 10.3866/PKU.WHXB202009022
Abstract:
The use of fossil fuels has caused serious environmental problems such as air pollution and the greenhouse effect. Moreover, because fossil fuels are a non-renewable energy source, they cannot meet the continuously increasing demand for energy. Therefore, the development of clean and renewable energy sources is necessitated. Hydrogen energy is a clean, non-polluting renewable energy source that can ease the energy pressure of the whole society. The sunlight received by the Earth is 1.7× 1014 J in 1 s, which far exceeds the total energy consumption of humans in one year. Therefore, conversion of solar energy to valuable hydrogen energy is of significance for reducing the dependence on fossil fuels. Since Fujishima and Honda first reported on TiO2 in 1972, it has been discovered that semiconductors can generate clean, pollution-free hydrogen through water splitting driven by electricity or light. Hydrogen generated through this approach can not only replace fossil fuels but also provide environmentally friendly renewable hydrogen energy, which has attracted considerable attention. Photoelectrochemical (PEC) water splitting can use solar energy to produce clean, sustainable hydrogen energy. Because the oxygen evolution reaction (OER) over a photoanode is sluggish, the overall energy conversion efficiency is considerably low, limiting the practical application of PEC water splitting. A cocatalyst is, thus, necessary to improve PEC water splitting performance. So far, the synthesis of first-row transition-metal-based (e.g., Fe, Co, Ni, and Mn) cocatalysts has been intensively studied. Iron is earth-abundant and less toxic than other transition metals, making it a good cocatalyst. In addition, iron-based compounds exhibit the properties of a semiconductor/metal and have unique electronic structures, which can improve electrical conductivity and water adsorption. Various iron-based catalysts with high activity have been designed to improve the efficiency of PEC water oxidation. This article briefly summarizes the research progress related to the structure, synthesis, and application of iron oxyhydroxides, iron-based layered double hydroxides, and iron-based perovskites and discusses the evaluation of the performance of these cocatalysts toward photoelectrochemical water oxidation. ![]()
The use of fossil fuels has caused serious environmental problems such as air pollution and the greenhouse effect. Moreover, because fossil fuels are a non-renewable energy source, they cannot meet the continuously increasing demand for energy. Therefore, the development of clean and renewable energy sources is necessitated. Hydrogen energy is a clean, non-polluting renewable energy source that can ease the energy pressure of the whole society. The sunlight received by the Earth is 1.7× 1014 J in 1 s, which far exceeds the total energy consumption of humans in one year. Therefore, conversion of solar energy to valuable hydrogen energy is of significance for reducing the dependence on fossil fuels. Since Fujishima and Honda first reported on TiO2 in 1972, it has been discovered that semiconductors can generate clean, pollution-free hydrogen through water splitting driven by electricity or light. Hydrogen generated through this approach can not only replace fossil fuels but also provide environmentally friendly renewable hydrogen energy, which has attracted considerable attention. Photoelectrochemical (PEC) water splitting can use solar energy to produce clean, sustainable hydrogen energy. Because the oxygen evolution reaction (OER) over a photoanode is sluggish, the overall energy conversion efficiency is considerably low, limiting the practical application of PEC water splitting. A cocatalyst is, thus, necessary to improve PEC water splitting performance. So far, the synthesis of first-row transition-metal-based (e.g., Fe, Co, Ni, and Mn) cocatalysts has been intensively studied. Iron is earth-abundant and less toxic than other transition metals, making it a good cocatalyst. In addition, iron-based compounds exhibit the properties of a semiconductor/metal and have unique electronic structures, which can improve electrical conductivity and water adsorption. Various iron-based catalysts with high activity have been designed to improve the efficiency of PEC water oxidation. This article briefly summarizes the research progress related to the structure, synthesis, and application of iron oxyhydroxides, iron-based layered double hydroxides, and iron-based perovskites and discusses the evaluation of the performance of these cocatalysts toward photoelectrochemical water oxidation.
2021, 37(8): 201107
doi: 10.3866/PKU.WHXB202011073
Abstract:
Since the pioneering work on polychlorinated biphenyl photodegradation by Carey in 1976, photocatalytic technology has emerged as a promising and sustainable strategy to overcome the significant challenges posed by energy crisis and environmental pollution. In photocatalysis, sunlight, which is an inexhaustible source of energy, is utilized to generate strongly active species on the surface of the photocatalyst for triggering photo-redox reactions toward the successful removal of environmental pollutants, or for water splitting. The photocatalytic performance is related to the photoabsorption, photoinduced carrier separation, and redox ability of the semiconductor employed as the photocatalyst. Apart from traditional and noble metal oxide semiconductors such as P25, bismuth-based compounds, and Pt-based compounds, 2D g-C3N4 is now identified to have enormous potential in photocatalysis owing to the special π-π conjugated bond in its structure. However, some inherent drawbacks of the conventional g-C3N4, including the insufficient visible-light absorption ability, fast recombination of photogenerated electron-hole pairs, and low quantum efficiency, decrease its photocatalytic activity and limit its application. To date, various strategies such as heterojunction fabrication, special morphology design, and element doping have been adopted to tune the physicochemical properties of g-C3N4. Recent studies have highlighted the potential of defect engineering for boosting the light harvesting, charge separation, and adsorption efficiency of g-C3N4 by tailoring the local surface microstructure, electronic structure, and carrier concentration. In this review, we summarize cutting-edge achievements related to g-C3N4 modified with classified non-external-caused defects (carbon vacancies, nitrogen vacancies, etc.) and external-caused defects (doping and functionalization) for optimizing the photocatalytic performance in water splitting, removal of contaminants in the gas phase and wastewater, nitrogen fixation, etc. The distinctive roles of various defects in the g-C3N4 skeleton in the photocatalytic process are also summarized. Moreover, the practical application of 2D g-C3N4 in air pollution control is highlighted. Finally, the ongoing challenges and perspectives of defective g-C3N4 are presented. The overarching aim of this article is to provide a useful scaffold for future research and application studies on defect-modulated g-C3N4. ![]()
Since the pioneering work on polychlorinated biphenyl photodegradation by Carey in 1976, photocatalytic technology has emerged as a promising and sustainable strategy to overcome the significant challenges posed by energy crisis and environmental pollution. In photocatalysis, sunlight, which is an inexhaustible source of energy, is utilized to generate strongly active species on the surface of the photocatalyst for triggering photo-redox reactions toward the successful removal of environmental pollutants, or for water splitting. The photocatalytic performance is related to the photoabsorption, photoinduced carrier separation, and redox ability of the semiconductor employed as the photocatalyst. Apart from traditional and noble metal oxide semiconductors such as P25, bismuth-based compounds, and Pt-based compounds, 2D g-C3N4 is now identified to have enormous potential in photocatalysis owing to the special π-π conjugated bond in its structure. However, some inherent drawbacks of the conventional g-C3N4, including the insufficient visible-light absorption ability, fast recombination of photogenerated electron-hole pairs, and low quantum efficiency, decrease its photocatalytic activity and limit its application. To date, various strategies such as heterojunction fabrication, special morphology design, and element doping have been adopted to tune the physicochemical properties of g-C3N4. Recent studies have highlighted the potential of defect engineering for boosting the light harvesting, charge separation, and adsorption efficiency of g-C3N4 by tailoring the local surface microstructure, electronic structure, and carrier concentration. In this review, we summarize cutting-edge achievements related to g-C3N4 modified with classified non-external-caused defects (carbon vacancies, nitrogen vacancies, etc.) and external-caused defects (doping and functionalization) for optimizing the photocatalytic performance in water splitting, removal of contaminants in the gas phase and wastewater, nitrogen fixation, etc. The distinctive roles of various defects in the g-C3N4 skeleton in the photocatalytic process are also summarized. Moreover, the practical application of 2D g-C3N4 in air pollution control is highlighted. Finally, the ongoing challenges and perspectives of defective g-C3N4 are presented. The overarching aim of this article is to provide a useful scaffold for future research and application studies on defect-modulated g-C3N4.
2021, 37(8): 200906
doi: 10.3866/PKU.WHXB202009063
Abstract:
Photocatalytic oxidation is a promising technology for governing emission of environmental pollutants and managing energy crisis. Typically, the photocatalytic performance of photocatalysts is highly dependent on the type of exposed crystal surfaces. As a semiconductor oxide photocatalyst, the different exposed crystal surfaces of bismuth oxyiodide (BiOI) exhibit different photocatalytic oxidation performances. In this study, we chose BiOI as the model material and provided a novel method to improve the photocatalytic oxidation performance by regulating the main exposed crystal facets. Using boron nitride (BN) nanosheets as the templates, two-dimensional/two-dimensional (2D/2D) BiOI/BN nanocompounds were fabricated via an in situ growth method. Owing to the electrostatic interaction, the positively charged BiOI {001} facets prefer to contact the negatively charged BN {001} facet, thus inducing the exposure of BiOI {110} facets. This was identified via X-ray diffraction and transmission electron microscopy analyses. Compared with BiOI {001} facets, there were more lattice oxygen atoms in the BiOI {110} facets. Thus, the exposure of BiOI {110} facets would promote more surface lattice oxygen atoms exposed on the surface of BiOI, which was confirmed by X-ray photoelectron spectroscopy and density functional theory calculations. To evaluate the photocatalytic oxidation performance of BiOI/BN, the photocatalytic NO oxidation reaction was tested under visible light irradiation (λ > 420 nm). Among all the nanocompounds, the BiOI/BN-1.0:1.4 nanocompound exhibited the best NO oxidation ratio of 44.2%, which was almost 30 times higher than that of pristine BiOI (1.4%). The enhanced photocatalytic activity could be attributed to the following two aspects. One, the successful combination of BN effectively promoted the separation of photogenerated carriers, which was identified by steady-state and time-resolved fluorescence spectra, transient photocurrent responses, and electrochemical impedance spectra. Two, benefiting from the introduction of BN nanosheets, BiOI tends to mainly expose the oxygen-rich {110} facets. As a result, the content of O on the BiOI surface increased from 38.3% to 46.6%. Thus, NO preferred to adsorb on the {110} facets of BiOI nanosheets, which was confirmed by theoretical and experimental results. More importantly, the adsorbed NO spontaneously combined with the lattice oxygen atom of the BiOI (110) surface to form nitrogen dioxide (NO2). These findings can provide a novel strategy to tune exposed oxygen-rich facets by constructing 2D/2D photocatalysts for ensuring efficient photocatalytic oxidation performance. ![]()
Photocatalytic oxidation is a promising technology for governing emission of environmental pollutants and managing energy crisis. Typically, the photocatalytic performance of photocatalysts is highly dependent on the type of exposed crystal surfaces. As a semiconductor oxide photocatalyst, the different exposed crystal surfaces of bismuth oxyiodide (BiOI) exhibit different photocatalytic oxidation performances. In this study, we chose BiOI as the model material and provided a novel method to improve the photocatalytic oxidation performance by regulating the main exposed crystal facets. Using boron nitride (BN) nanosheets as the templates, two-dimensional/two-dimensional (2D/2D) BiOI/BN nanocompounds were fabricated via an in situ growth method. Owing to the electrostatic interaction, the positively charged BiOI {001} facets prefer to contact the negatively charged BN {001} facet, thus inducing the exposure of BiOI {110} facets. This was identified via X-ray diffraction and transmission electron microscopy analyses. Compared with BiOI {001} facets, there were more lattice oxygen atoms in the BiOI {110} facets. Thus, the exposure of BiOI {110} facets would promote more surface lattice oxygen atoms exposed on the surface of BiOI, which was confirmed by X-ray photoelectron spectroscopy and density functional theory calculations. To evaluate the photocatalytic oxidation performance of BiOI/BN, the photocatalytic NO oxidation reaction was tested under visible light irradiation (λ > 420 nm). Among all the nanocompounds, the BiOI/BN-1.0:1.4 nanocompound exhibited the best NO oxidation ratio of 44.2%, which was almost 30 times higher than that of pristine BiOI (1.4%). The enhanced photocatalytic activity could be attributed to the following two aspects. One, the successful combination of BN effectively promoted the separation of photogenerated carriers, which was identified by steady-state and time-resolved fluorescence spectra, transient photocurrent responses, and electrochemical impedance spectra. Two, benefiting from the introduction of BN nanosheets, BiOI tends to mainly expose the oxygen-rich {110} facets. As a result, the content of O on the BiOI surface increased from 38.3% to 46.6%. Thus, NO preferred to adsorb on the {110} facets of BiOI nanosheets, which was confirmed by theoretical and experimental results. More importantly, the adsorbed NO spontaneously combined with the lattice oxygen atom of the BiOI (110) surface to form nitrogen dioxide (NO2). These findings can provide a novel strategy to tune exposed oxygen-rich facets by constructing 2D/2D photocatalysts for ensuring efficient photocatalytic oxidation performance.
2021, 37(8): 200910
doi: 10.3866/PKU.WHXB202009100
Abstract:
Photocatalytic oxidation has been widely acknowledged as an economical and effective technology for the treatment of low-concentration NO. Three-dimensional (3D) BiOI microspheres, which are typical visible-light responsive semiconductor photocatalysts, often suffer from quick recombination of photogenerated carriers and unsatisfactory electrical conductivity when applied in NO photocatalytic oxidation reactions. However, owing to their micro-sized structures, they are usually difficult to couple with other semiconductors and co-catalysts because of their incompact interfaces that provide insufficient contact. In this study, a rare-earth metal (La) doping strategy was first adopted to modify BiOI microspheres via a simple one-step solvothermal method; subsequently, the photocatalytic NO oxidation performance under visible light illumination was systematically investigated. Further, the La precursors and doping contents were optimized. It was found that La(NO3)2 was the best precursor when compared to LaCl3 and La(AC)3. Moreover, 0.3%La/BiOI exhibited the best NO photocatalytic conversion efficiency of up to 74%, which was significantly higher than that of the pure BiOI benchmark (44%). It also exhibited excellent stability during the continuous 5-cycle experiments. Analysis of the physicochemical properties revealed that La doping facilitated the crystallization of BiOI without altering its morphology and structure. La3+ may enter the BiOI lattice by substituting Bi3+ or forming La2O3 nanoclusters that homogeneously scatter in the mesopores of BiOI microspheres. The analysis of the underlying mechanism further revealed that La doping not only enhanced the light harvesting properties by decreasing the bandgaps of BiOI and accelerating the charge separation and transfer dynamics, but also introduced more oxygen vacancies and facilitated the formation of more OH radicals by dissociating the water molecules. All these factors co-contributed to the promotion of NO photocatalytic oxidation activities. Furthermore, NO was mainly oxidized to NO2 over La/BiOI, and the formed NO2 tended to desorb from the catalyst surface, which not only maintained the intactness of active sites and facilitated the sustainable occurrence of NO photocatalytic oxidation reactions, but also prevented the photocatalysts from frequent washing-regeneration; therefore, these factors account for the superior photocatalytic stability of La/BiOI and its long-term operation. The formed NO2 could be easily and totally absorbed by the tail alkaline liquid, thereby effectively avoiding secondary pollution. Therefore, this study elucidates that doping is indeed a feasible and effective approach for the modification of 3D BiOI microspheres, while providing inspiration for the rational design and modification of other 3D semiconductor materials for various photocatalytic applications. ![]()
Photocatalytic oxidation has been widely acknowledged as an economical and effective technology for the treatment of low-concentration NO. Three-dimensional (3D) BiOI microspheres, which are typical visible-light responsive semiconductor photocatalysts, often suffer from quick recombination of photogenerated carriers and unsatisfactory electrical conductivity when applied in NO photocatalytic oxidation reactions. However, owing to their micro-sized structures, they are usually difficult to couple with other semiconductors and co-catalysts because of their incompact interfaces that provide insufficient contact. In this study, a rare-earth metal (La) doping strategy was first adopted to modify BiOI microspheres via a simple one-step solvothermal method; subsequently, the photocatalytic NO oxidation performance under visible light illumination was systematically investigated. Further, the La precursors and doping contents were optimized. It was found that La(NO3)2 was the best precursor when compared to LaCl3 and La(AC)3. Moreover, 0.3%La/BiOI exhibited the best NO photocatalytic conversion efficiency of up to 74%, which was significantly higher than that of the pure BiOI benchmark (44%). It also exhibited excellent stability during the continuous 5-cycle experiments. Analysis of the physicochemical properties revealed that La doping facilitated the crystallization of BiOI without altering its morphology and structure. La3+ may enter the BiOI lattice by substituting Bi3+ or forming La2O3 nanoclusters that homogeneously scatter in the mesopores of BiOI microspheres. The analysis of the underlying mechanism further revealed that La doping not only enhanced the light harvesting properties by decreasing the bandgaps of BiOI and accelerating the charge separation and transfer dynamics, but also introduced more oxygen vacancies and facilitated the formation of more OH radicals by dissociating the water molecules. All these factors co-contributed to the promotion of NO photocatalytic oxidation activities. Furthermore, NO was mainly oxidized to NO2 over La/BiOI, and the formed NO2 tended to desorb from the catalyst surface, which not only maintained the intactness of active sites and facilitated the sustainable occurrence of NO photocatalytic oxidation reactions, but also prevented the photocatalysts from frequent washing-regeneration; therefore, these factors account for the superior photocatalytic stability of La/BiOI and its long-term operation. The formed NO2 could be easily and totally absorbed by the tail alkaline liquid, thereby effectively avoiding secondary pollution. Therefore, this study elucidates that doping is indeed a feasible and effective approach for the modification of 3D BiOI microspheres, while providing inspiration for the rational design and modification of other 3D semiconductor materials for various photocatalytic applications.
2021, 37(8): 200910
doi: 10.3866/PKU.WHXB202009102
Abstract:
Energy crisis has become a serious global issue due to the increasing depletion of fossil fuels; therefore, it is crucial to develop environmentally friendly and renewable energy resources, such as hydrogen (H2), to replace fossil fuels. From this viewpoint, photocatalytic H2 production is considered as one of the most promising technologies. Noble metal platinum (Pt) can be applied as an efficient cocatalyst for improving the H2 production performance of photocatalytic systems; however, its high cost limits its further application. Thus, the development of novel, high-activity, and low-cost cocatalysts for replacing noble metal cocatalysts is of great significance for use in photocatalytic H2 evolution techniques. Herein, we successfully synthesized a Ni2P/graphite-like carbonitride photocatalyst (Ni2P/CN) using a conjugated polymer (SCN)n as precursor for enhanced photocatalytic H2 production under visible light illumination. Various characterization techniques, including optical and photoelectronic chemical tests, were used to investigate the structural composition, morphology, and light adsorption ability of these materials. X-ray diffraction, Fourier transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy results showed that Ni2P/CN nanocomposites with good crystal structure were obtained. Scanning electron microscopy and transmission electron microscopy results revealed that the Ni2P/CN samples had a typical two-dimensional layered structure, and the Ni2P nanoparticles were uniformly loaded on the surface of the CN to form a non-noble metal promoter. UV-Vis diffuse reflectance spectra results demonstrated that the loading of Ni2P nanoparticles effectively enhances the adsorption capacity of CN to visible light. Photoluminescence spectroscopy and photocurrent (PL) results suggested that Ni2P loading to CN is beneficial for promoting the migration and separation efficiency of photogenerated carriers. Photocatalytic H2 production was conducted under visible light irradiation with triethanolamine as a sacrificial agent. The results suggest that the Ni2P/CN composite photocatalysts exhibit excellent photocatalytic reduction performance. In particular, the H2 evolution rate of the optimal Ni2P/CN nanocomposite is 623.77 μmol·h-1·g-1, which is higher than that of CN modified by noble metal Pt, i.e., 524.63 μmol·h-1·g-1. In conclusion, Ni2P nanoparticles are homogeneously attached to the surface of CN, and a strong interfacial effect exists between them, thereby forming an electron transfer tunnel that greatly inhibits the recombination of photoinduced carriers and promotes the migration of electrons from CN to Ni2P. In addition, a possible photocatalytic mechanism is proposed based on the experiments and characterizations. This work has profound significance for developing non-noble metal cocatalysts for the substitution of noble metal cocatalysts for high-efficiency photocatalytic H2 evolution. ![]()
Energy crisis has become a serious global issue due to the increasing depletion of fossil fuels; therefore, it is crucial to develop environmentally friendly and renewable energy resources, such as hydrogen (H2), to replace fossil fuels. From this viewpoint, photocatalytic H2 production is considered as one of the most promising technologies. Noble metal platinum (Pt) can be applied as an efficient cocatalyst for improving the H2 production performance of photocatalytic systems; however, its high cost limits its further application. Thus, the development of novel, high-activity, and low-cost cocatalysts for replacing noble metal cocatalysts is of great significance for use in photocatalytic H2 evolution techniques. Herein, we successfully synthesized a Ni2P/graphite-like carbonitride photocatalyst (Ni2P/CN) using a conjugated polymer (SCN)n as precursor for enhanced photocatalytic H2 production under visible light illumination. Various characterization techniques, including optical and photoelectronic chemical tests, were used to investigate the structural composition, morphology, and light adsorption ability of these materials. X-ray diffraction, Fourier transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy results showed that Ni2P/CN nanocomposites with good crystal structure were obtained. Scanning electron microscopy and transmission electron microscopy results revealed that the Ni2P/CN samples had a typical two-dimensional layered structure, and the Ni2P nanoparticles were uniformly loaded on the surface of the CN to form a non-noble metal promoter. UV-Vis diffuse reflectance spectra results demonstrated that the loading of Ni2P nanoparticles effectively enhances the adsorption capacity of CN to visible light. Photoluminescence spectroscopy and photocurrent (PL) results suggested that Ni2P loading to CN is beneficial for promoting the migration and separation efficiency of photogenerated carriers. Photocatalytic H2 production was conducted under visible light irradiation with triethanolamine as a sacrificial agent. The results suggest that the Ni2P/CN composite photocatalysts exhibit excellent photocatalytic reduction performance. In particular, the H2 evolution rate of the optimal Ni2P/CN nanocomposite is 623.77 μmol·h-1·g-1, which is higher than that of CN modified by noble metal Pt, i.e., 524.63 μmol·h-1·g-1. In conclusion, Ni2P nanoparticles are homogeneously attached to the surface of CN, and a strong interfacial effect exists between them, thereby forming an electron transfer tunnel that greatly inhibits the recombination of photoinduced carriers and promotes the migration of electrons from CN to Ni2P. In addition, a possible photocatalytic mechanism is proposed based on the experiments and characterizations. This work has profound significance for developing non-noble metal cocatalysts for the substitution of noble metal cocatalysts for high-efficiency photocatalytic H2 evolution.
2021, 37(8): 201007
doi: 10.3866/PKU.WHXB202010073
Abstract:
Photocatalytic reduction of carbon dioxide into chemical fuels is a promising route to generate renewable energy and curtail the greenhouse effect. Therefore, various photocatalysts have been intensively studied for this purpose. Among them, g-C3N4, a 2D metal-free semiconductor, has been a promising photocatalyst because of its unique properties, such as high chemical stability, suitable electronic structure, and facile preparation. However, pristine g-C3N4 suffers from low solar energy conversion efficiency, owing to its small specific surface area and extensive charge recombination. Therefore, designing g-C3N4 (CN) nanosheets with a large specific surface area is an effective strategy for enhancing the CO2 reduction performance. Unfortunately, the performance of CN nanosheets remains moderate due to the aforementioned charge recombination. To counter this issue, loading a cocatalyst (especially a two-dimensional (2D) one) can enable effective electron migration and suppress electron-hole recombination during photo-irradiation. Herein, CN nanosheets with a large specific surface area (97 m2·g-1) were synthesized by a two-step calcination method, using urea as the precursor. Following this, a 2D/2D FeNi-LDH/g-C3N4 hybrid photocatalyst was obtained by loading a FeNi layered double hydroxide (FeNi-LDH) cocatalyst onto CN nanosheets by a simple hydrothermal method. It was found that the production rate of methanol from photocatalytic CO2 reduction over the FeNi-LDH/g-C3N4 composite is significantly higher than that of pristine CN. Following a series of characterization and analysis, it was demonstrated that the FeNi-LDH/g-C3N4 composite photocatalyst exhibited enhanced photo-absorption, which was ascribed to the excellent light absorption ability of FeNi-LDH. The CO2 adsorption capacity of the FeNi-LDH/g-C3N4 hybrid photocatalyst improved, owing to the large specific surface area and alkaline nature of FeNi-LDH. More importantly, the introduction of FeNi-LDH on the CN nanosheet surface led to the formation of a 2D/2D heterojunction with a large contact area at the interface, which could promote the interfacial separation of charge carriers and effectively inhibit the recombination of the photogenerated electrons and holes. This subsequently resulted in the enhancement of the CO2 photo-reduction activity. In addition, by altering the loading amount of FeNi-LDH for photocatalytic performance evaluation, it was found that the optimal loading amount was 4% (w, mass fraction), with a methanol production rate of 1.64 μmol·h-1·g-1 (approximately 6 times that of pure CN). This study provides an effective strategy to improve the photocatalytic CO2 reduction activity of g-C3N4 by employing 2D layered double hydroxide as the cocatalyst. It also proposes a protocol for the successful design of 2D/2D photocatalysts for solar energy conversion. ![]()
Photocatalytic reduction of carbon dioxide into chemical fuels is a promising route to generate renewable energy and curtail the greenhouse effect. Therefore, various photocatalysts have been intensively studied for this purpose. Among them, g-C3N4, a 2D metal-free semiconductor, has been a promising photocatalyst because of its unique properties, such as high chemical stability, suitable electronic structure, and facile preparation. However, pristine g-C3N4 suffers from low solar energy conversion efficiency, owing to its small specific surface area and extensive charge recombination. Therefore, designing g-C3N4 (CN) nanosheets with a large specific surface area is an effective strategy for enhancing the CO2 reduction performance. Unfortunately, the performance of CN nanosheets remains moderate due to the aforementioned charge recombination. To counter this issue, loading a cocatalyst (especially a two-dimensional (2D) one) can enable effective electron migration and suppress electron-hole recombination during photo-irradiation. Herein, CN nanosheets with a large specific surface area (97 m2·g-1) were synthesized by a two-step calcination method, using urea as the precursor. Following this, a 2D/2D FeNi-LDH/g-C3N4 hybrid photocatalyst was obtained by loading a FeNi layered double hydroxide (FeNi-LDH) cocatalyst onto CN nanosheets by a simple hydrothermal method. It was found that the production rate of methanol from photocatalytic CO2 reduction over the FeNi-LDH/g-C3N4 composite is significantly higher than that of pristine CN. Following a series of characterization and analysis, it was demonstrated that the FeNi-LDH/g-C3N4 composite photocatalyst exhibited enhanced photo-absorption, which was ascribed to the excellent light absorption ability of FeNi-LDH. The CO2 adsorption capacity of the FeNi-LDH/g-C3N4 hybrid photocatalyst improved, owing to the large specific surface area and alkaline nature of FeNi-LDH. More importantly, the introduction of FeNi-LDH on the CN nanosheet surface led to the formation of a 2D/2D heterojunction with a large contact area at the interface, which could promote the interfacial separation of charge carriers and effectively inhibit the recombination of the photogenerated electrons and holes. This subsequently resulted in the enhancement of the CO2 photo-reduction activity. In addition, by altering the loading amount of FeNi-LDH for photocatalytic performance evaluation, it was found that the optimal loading amount was 4% (w, mass fraction), with a methanol production rate of 1.64 μmol·h-1·g-1 (approximately 6 times that of pure CN). This study provides an effective strategy to improve the photocatalytic CO2 reduction activity of g-C3N4 by employing 2D layered double hydroxide as the cocatalyst. It also proposes a protocol for the successful design of 2D/2D photocatalysts for solar energy conversion.
2021, 37(8): 201007
doi: 10.3866/PKU.WHXB202010074
Abstract:
2021, 37(8): 201100
doi: 10.3866/PKU.WHXB202011005
Abstract:
2021, 37(8): 201103
doi: 10.3866/PKU.WHXB202011039
Abstract:
2021, 37(8): 201201
doi: 10.3866/PKU.WHXB202012010
Abstract:
2021, 37(8): 201102
doi: 10.3866/PKU.WHXB202011021
Abstract:
2021, 37(8): 210100
doi: 10.3866/PKU.WHXB202101002
Abstract:
