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2022 Volume 38 Issue 7
2022, 38(7): 210802
doi: 10.3866/PKU.WHXB202108028
Abstract:
Photocatalytic hydrogen production is an effective strategy for addressing energy shortage and converting solar energy into chemical energy. Exploring effective strategies to improve photocatalytic H2 production is a key challenge in the field of energy conversion. There are numerous oxygen vacancies on the surface of non-stoichiometric W18O49 (WO), which result in suitable light absorption performance, but the hydrogen evolution effect is not ideal because the band potential does not reach the hydrogen evolution potential. A suitable heterojunction is constructed to optimize defects such as high carrier recombination rate and low photocatalytic performance in a semiconductor. Herein, 2D porous carbon nitride (PCN) is synthesized, followed by the in situ growth of 1D WO on the PCN to realize a step-scheme (S-scheme) heterojunction. When WO and PCN are composited, the difference between the Fermi levels of WO and PCN leads to electron migration, which balances the Fermi levels of WO and PCN. Electron transfer leads to the formation of an interfacial electric field and bends the energy bands of WO and PCN, thereby resulting in the recombination of unused electrons and holes while leaving used electrons and holes, which can accelerate the separation and charge transfer at the interface and endow the WO/PCN system with better redox capabilities. In addition, PCN with a porous structure provides more catalytic active sites. The photocatalytic performance of the sample can be investigated using the amount of hydrogen released. Compared to WO and PCN, 20%WO/PCN composite has a higher H2 production rate (1700 μmol·g-1·h-1), which is 56 times greater than that of PCN (30 μmol·g-1·h-1). This study shows the possibility of the application of S-scheme heterojunction in the field of photocatalytic H2 production. ![]()
Photocatalytic hydrogen production is an effective strategy for addressing energy shortage and converting solar energy into chemical energy. Exploring effective strategies to improve photocatalytic H2 production is a key challenge in the field of energy conversion. There are numerous oxygen vacancies on the surface of non-stoichiometric W18O49 (WO), which result in suitable light absorption performance, but the hydrogen evolution effect is not ideal because the band potential does not reach the hydrogen evolution potential. A suitable heterojunction is constructed to optimize defects such as high carrier recombination rate and low photocatalytic performance in a semiconductor. Herein, 2D porous carbon nitride (PCN) is synthesized, followed by the in situ growth of 1D WO on the PCN to realize a step-scheme (S-scheme) heterojunction. When WO and PCN are composited, the difference between the Fermi levels of WO and PCN leads to electron migration, which balances the Fermi levels of WO and PCN. Electron transfer leads to the formation of an interfacial electric field and bends the energy bands of WO and PCN, thereby resulting in the recombination of unused electrons and holes while leaving used electrons and holes, which can accelerate the separation and charge transfer at the interface and endow the WO/PCN system with better redox capabilities. In addition, PCN with a porous structure provides more catalytic active sites. The photocatalytic performance of the sample can be investigated using the amount of hydrogen released. Compared to WO and PCN, 20%WO/PCN composite has a higher H2 production rate (1700 μmol·g-1·h-1), which is 56 times greater than that of PCN (30 μmol·g-1·h-1). This study shows the possibility of the application of S-scheme heterojunction in the field of photocatalytic H2 production.
2022, 38(7): 210902
doi: 10.3866/PKU.WHXB202109023
Abstract:
Constructing an efficient and stable heterojunction photocatalyst system is a promising approach to achieve solar-driven water splitting to produce hydrogen. In this work, a novel Mn0.2Cd0.8S@CoAl LDH (MCCA) S-scheme heterojunction was successfully prepared through the efficient coupling of Mn0.2Cd0.8S nanorods and CoAl LDH nanosheets, employing a physical mixing method. The photoluminescence and photocurrent-time response results demonstrated that the internal electric field of the constructed MCCA S-scheme heterojunction could successfully accelerate charge separation and electron transfer between the Mn0.2Cd0.8S interface and the CoAl LDH. Critically, the introduction of the CoAl LDH effectively inhibited the recombination of photogenerated electrons and holes, thereby improving the photocatalytic hydrogen production activity of Mn0.2Cd0.8S. A maximum H2 production of 1177.9 μmol in 5 h was obtained with MCCA-3. This represents a significant improvement compared to what can be achieved with the pure Mn0.2Cd0.8S nanorods and CoAl LDH nanosheets individually. This work provides a simple and effective approach for the rational design of S-scheme heterojunction photocatalysts for photocatalytic hydrogen production. ![]()
Constructing an efficient and stable heterojunction photocatalyst system is a promising approach to achieve solar-driven water splitting to produce hydrogen. In this work, a novel Mn0.2Cd0.8S@CoAl LDH (MCCA) S-scheme heterojunction was successfully prepared through the efficient coupling of Mn0.2Cd0.8S nanorods and CoAl LDH nanosheets, employing a physical mixing method. The photoluminescence and photocurrent-time response results demonstrated that the internal electric field of the constructed MCCA S-scheme heterojunction could successfully accelerate charge separation and electron transfer between the Mn0.2Cd0.8S interface and the CoAl LDH. Critically, the introduction of the CoAl LDH effectively inhibited the recombination of photogenerated electrons and holes, thereby improving the photocatalytic hydrogen production activity of Mn0.2Cd0.8S. A maximum H2 production of 1177.9 μmol in 5 h was obtained with MCCA-3. This represents a significant improvement compared to what can be achieved with the pure Mn0.2Cd0.8S nanorods and CoAl LDH nanosheets individually. This work provides a simple and effective approach for the rational design of S-scheme heterojunction photocatalysts for photocatalytic hydrogen production.
2022, 38(7): 211001
doi: 10.3866/PKU.WHXB202110014
Abstract:
Throughout the twentieth century, temperatures climbed rapidly as the use of fossil fuels proliferated and greenhouse gas levels soared. Thus, the need to develop environmentally friendly energy sources to replace traditional fossil fuels is urgent. Clean and highly efficient, hydrogen is considered the most promising energy source to replace traditional fossil fuels. The production of hydrogen by photocatalytic water splitting is environmentally friendly, and is considered the most promising method for producing hydrogen energy. Enhancing the separation efficiency of photogenerated electron-hole pairs has been identified as a key milestone for constructing high-efficiency photocatalysts. However, the construction of efficient and stable hydrogen-evolution photocatalysts with highly dispersed cocatalysts remains a challenge. Here, we succeeded, for the first time, in fabricating P-doped CNS (PCNS) with a highly dispersed non-noble trimetallic transition metal phosphide Co0.2Ni1.6Fe0.2P cocatalyst (PCNS-CoNiFeP), by a one-step in situ high-temperature phosphating method. Remarkably, the CoNiFeP in PCNS-CoNiFeP demonstrated no aggregation and high dispersibility compared with CoNiFeP prepared by the traditional hydroxide-precursor phosphating method (PCNS-CoNiFeP-OH). X-ray diffraction, X-ray photoelectron spectroscopy, element mapping images, and high-resolution transmission electron microscopy results demonstrate that PCNS-CoNiFeP was successfully synthesized. The UV-Vis absorption results indicate a slight increase in absorbance for PCNS-CoNiFeP in the 200–800 nm wavelength region compared with that of PCNS. Photoluminescence spectroscopy, electrochemical impedance spectroscopy, and photocurrent results demonstrated that CoNiFeP cocatalysts could effectively promote the separation of photogenerated electron-hole pairs and accelerate the migration of carriers. The linear sweep voltammetry results also demonstrate that the CoNiFeP cocatalyst loading could significantly decrease the overpotential of CNS. Therefore, the maximum hydrogen evolution rate of PCNS-CoNiFeP was 1200 μmol·h-1·g-1, which was approximately four times higher than that of pure CNS-Pt (320 μmol·h-1·g-1) when using TEOA solution as a sacrificial agent. The apparent quantum efficiency of PCNS-CoNiFeP was 1.4% at 420 nm. The PCNS-CoNiFeP also exhibited good stability during the photocatalytic reaction. In addition, the TEM results indicate that CoNiFeP with a size of 6–8 nm are highly dispersed on the PCNS surface. The highly dispersed CoNiFeP demonstrated better charge-separation capacity and higher intrinsic electrocatalytic hydrogen-evolution activity than the aggregated CoNiFeP. Thus, the hydrogen evolution rate of aggregated CoNiFeP-PCNs (300 μmol·h-1·g-1) was much lower than that of PCNS-CoNiFeP. Furthermore, P doping of CNS could improve electric conductivity and charge transport. It is expected that loading highly dispersed CoNiFeP and P doping could be extended to promote photocatalytic hydrogen production using various photocatalysts. ![]()
Throughout the twentieth century, temperatures climbed rapidly as the use of fossil fuels proliferated and greenhouse gas levels soared. Thus, the need to develop environmentally friendly energy sources to replace traditional fossil fuels is urgent. Clean and highly efficient, hydrogen is considered the most promising energy source to replace traditional fossil fuels. The production of hydrogen by photocatalytic water splitting is environmentally friendly, and is considered the most promising method for producing hydrogen energy. Enhancing the separation efficiency of photogenerated electron-hole pairs has been identified as a key milestone for constructing high-efficiency photocatalysts. However, the construction of efficient and stable hydrogen-evolution photocatalysts with highly dispersed cocatalysts remains a challenge. Here, we succeeded, for the first time, in fabricating P-doped CNS (PCNS) with a highly dispersed non-noble trimetallic transition metal phosphide Co0.2Ni1.6Fe0.2P cocatalyst (PCNS-CoNiFeP), by a one-step in situ high-temperature phosphating method. Remarkably, the CoNiFeP in PCNS-CoNiFeP demonstrated no aggregation and high dispersibility compared with CoNiFeP prepared by the traditional hydroxide-precursor phosphating method (PCNS-CoNiFeP-OH). X-ray diffraction, X-ray photoelectron spectroscopy, element mapping images, and high-resolution transmission electron microscopy results demonstrate that PCNS-CoNiFeP was successfully synthesized. The UV-Vis absorption results indicate a slight increase in absorbance for PCNS-CoNiFeP in the 200–800 nm wavelength region compared with that of PCNS. Photoluminescence spectroscopy, electrochemical impedance spectroscopy, and photocurrent results demonstrated that CoNiFeP cocatalysts could effectively promote the separation of photogenerated electron-hole pairs and accelerate the migration of carriers. The linear sweep voltammetry results also demonstrate that the CoNiFeP cocatalyst loading could significantly decrease the overpotential of CNS. Therefore, the maximum hydrogen evolution rate of PCNS-CoNiFeP was 1200 μmol·h-1·g-1, which was approximately four times higher than that of pure CNS-Pt (320 μmol·h-1·g-1) when using TEOA solution as a sacrificial agent. The apparent quantum efficiency of PCNS-CoNiFeP was 1.4% at 420 nm. The PCNS-CoNiFeP also exhibited good stability during the photocatalytic reaction. In addition, the TEM results indicate that CoNiFeP with a size of 6–8 nm are highly dispersed on the PCNS surface. The highly dispersed CoNiFeP demonstrated better charge-separation capacity and higher intrinsic electrocatalytic hydrogen-evolution activity than the aggregated CoNiFeP. Thus, the hydrogen evolution rate of aggregated CoNiFeP-PCNs (300 μmol·h-1·g-1) was much lower than that of PCNS-CoNiFeP. Furthermore, P doping of CNS could improve electric conductivity and charge transport. It is expected that loading highly dispersed CoNiFeP and P doping could be extended to promote photocatalytic hydrogen production using various photocatalysts.
2022, 38(7): 211004
doi: 10.3866/PKU.WHXB202110049
Abstract:
With rapid industrialization, issues pertaining to the environment and energy have become an alarming concern. Photocatalytic water splitting is considered one of the most promising green technologies capable of resolving these issues, as it can convert solar energy into chemical energy and have a positive impact on the realization of "carbon neutrality". Current research focuses on the development of highly efficient catalysts to improve the photocatalytic H2-production activity. Transition metal phosphides and sulfides are often used as photocatalysts owing to their low H2-evolution overpotential and excellent electrical conductivity. Among them, Ni2P and NiS have generally been used independently during photocatalytic H2 production; however, it is necessary to study the synergistic effect when they are combined as a dual cocatalyst. In this work, we successfully prepared a Ni2P-NiS dual cocatalyst for the first time via a simple hydrothermal method using red phosphorus (RP) and thioacetamide (C2H5NS) as the sources of P and S. Ni2P-NiS was then introduced to the surface of g-C3N4 nanosheets through solvent evaporation to create a Ni2P-NiS/g-C3N4 heterojunction. Furthermore, X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), ultraviolet-visible spectrophotometry (UV-Vis), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL), linear sweep voltammetry (LSV), Mott-Schottky (M-S), and electrochemical impedance spectroscopy (EIS) were used to reveal the crystal structures, morphologies, element compositions, and photoelectric characteristics of the samples; thus, it was demonstrated that Ni2P-NiS was successfully deposited on the surface of g-C3N4 and that together they exhibited better activity than their monomers. Moreover, the optimized 15% Ni2P-NiS/g-C3N4 composite exhibits a H2 generation rate of 6892.7 μmol·g-1·h-1, which is about 46.1, 7.5 and 4.4 times higher than that of g-C3N4 (150 μmol·g-1·h-1), 15% NiS/g-C3N4 (914.5 μmol·g-1·h-1), and 15% Ni2P/g-C3N4 (1565.9 μmol·g-1·h-1), respectively. In addition, photoelectric performance tests show that Ni2P-NiS/g-C3N4 has a stronger photocurrent intensity, smaller charge-transfer resistance, more positive H2-evolution overpotential, and better charge-separation ability than the individual components (i.e., Ni2P and NiS), suggesting that the coexistence of Ni2P and NiS can further boost the activity of g-C3N4 during H2 evolution compared with their monomers. This is mainly due to the Schottky barrier effect between Ni2P-NiS nanoparticles and g-C3N4 nanosheets, which can greatly promote charge separation and charge transfer at their interface. Additionally, Ni2P-NiS can reduce the H2-evolution overpotential, leading to the increased surface kinetics of H2 evolution. This work offers a promising approach to obtaining a highly active and stable noble-metal-free dual cocatalyst for photocatalytic H2 production. ![]()
With rapid industrialization, issues pertaining to the environment and energy have become an alarming concern. Photocatalytic water splitting is considered one of the most promising green technologies capable of resolving these issues, as it can convert solar energy into chemical energy and have a positive impact on the realization of "carbon neutrality". Current research focuses on the development of highly efficient catalysts to improve the photocatalytic H2-production activity. Transition metal phosphides and sulfides are often used as photocatalysts owing to their low H2-evolution overpotential and excellent electrical conductivity. Among them, Ni2P and NiS have generally been used independently during photocatalytic H2 production; however, it is necessary to study the synergistic effect when they are combined as a dual cocatalyst. In this work, we successfully prepared a Ni2P-NiS dual cocatalyst for the first time via a simple hydrothermal method using red phosphorus (RP) and thioacetamide (C2H5NS) as the sources of P and S. Ni2P-NiS was then introduced to the surface of g-C3N4 nanosheets through solvent evaporation to create a Ni2P-NiS/g-C3N4 heterojunction. Furthermore, X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), ultraviolet-visible spectrophotometry (UV-Vis), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL), linear sweep voltammetry (LSV), Mott-Schottky (M-S), and electrochemical impedance spectroscopy (EIS) were used to reveal the crystal structures, morphologies, element compositions, and photoelectric characteristics of the samples; thus, it was demonstrated that Ni2P-NiS was successfully deposited on the surface of g-C3N4 and that together they exhibited better activity than their monomers. Moreover, the optimized 15% Ni2P-NiS/g-C3N4 composite exhibits a H2 generation rate of 6892.7 μmol·g-1·h-1, which is about 46.1, 7.5 and 4.4 times higher than that of g-C3N4 (150 μmol·g-1·h-1), 15% NiS/g-C3N4 (914.5 μmol·g-1·h-1), and 15% Ni2P/g-C3N4 (1565.9 μmol·g-1·h-1), respectively. In addition, photoelectric performance tests show that Ni2P-NiS/g-C3N4 has a stronger photocurrent intensity, smaller charge-transfer resistance, more positive H2-evolution overpotential, and better charge-separation ability than the individual components (i.e., Ni2P and NiS), suggesting that the coexistence of Ni2P and NiS can further boost the activity of g-C3N4 during H2 evolution compared with their monomers. This is mainly due to the Schottky barrier effect between Ni2P-NiS nanoparticles and g-C3N4 nanosheets, which can greatly promote charge separation and charge transfer at their interface. Additionally, Ni2P-NiS can reduce the H2-evolution overpotential, leading to the increased surface kinetics of H2 evolution. This work offers a promising approach to obtaining a highly active and stable noble-metal-free dual cocatalyst for photocatalytic H2 production.
2022, 38(7): 211100
doi: 10.3866/PKU.WHXB202111008
Abstract:
Catalytic reduction of CO2 to CO has been considered promising for converting the greenhouse gas into chemical intermediates. Compared to other catalytic methods, photocatalytic CO2 reduction, which uses solar energy as the energy input, has attracted significant attention because it is a clean and inexhaustible resource. Therefore, using high-performance photocatalysts for effective CO2 reduction under mild reaction conditions is an active research hotspot. However, several current photocatalysts suffer from low solar energy conversion efficiency due to the extensive charge recombination and few active sites, leading to low CO2 reduction efficiency. Generally, constructing an S-scheme heterojunction can not only promote charge separation but also help maintain strong redox ability. Therefore, the S-scheme heterojunction is expected to help in achieving high conversion activity and CO2 reduction efficiency. Here, 2D tetragonal BiOBr0.5Cl0.5 nanosheets and hexagonal WO3 nanorods were prepared using a simple hydrothermal synthesis method, and the 2D/1D BiOBr0.5Cl0.5 nanosheets/WO3 nanorods (BiOBr0.5Cl0.5/WO3) S-scheme heterojunction with near infrared (NIR) light (> 780 nm) response were prepared via the electrostatic self-assembly method for the photocatalytic CO2 reduction. Following characterization and analysis, including diffuse reflectance spectra (DRS), Mott-Schottky plots, transient photocurrent response, time-resolution photoluminescence spectrum (TRPL), electrochemical impedance spectroscopy (EIS), linear sweep voltammetry (LSV), and electron spin resonance (ESR) measurements, it can be demonstrated that an S-scheme carrier transfer route was formed between the 2D BiOBr0.5Cl0.5 nanosheets and 1D WO3 nanorods. Driven by the internal electric field, which was formed between the two semiconductors, electron migration was boosted, thus inhibiting the recombination of photogenerated carriers, while the stronger redox ability was maintained, thus providing good reduction efficiency over BiOBr0.5Cl0.5/WO3 composite in CO2 reduction. In addition, the 2D/1D nanosheet/nanorod structure allowed for enhanced interface contact with abundant active sites, which favored charge separation and increased photocatalytic activity. Furthermore, the amount of WO3 nanorods added during the preparation of the composites was altered, which led to the optimal amount of 5% (w, mass fraction) for the photocatalytic CO2 reduction. As a result, the BiOBr0.5Cl0.5/WO3 composite exhibited superior photocatalytic reduction performance with a CO yield of 16.68 μmol·g-1·h-1 in the presence of any precious metal cocatalyst or sacrificial agent, which was 1.7 and 9.8 times that of pure BiOBr0.5Cl0.5 and WO3, respectively. In addition, the BiOBr0.5Cl0.5/WO3 composite provided continuously increased CO yields with excellent selectivity under full-spectrum light irradiation, suggesting good photocatalytic stability. This work describes a novel idea for the construction of 2D/1D S-scheme heterojunction photocatalysts for efficient CO2 reduction. ![]()
Catalytic reduction of CO2 to CO has been considered promising for converting the greenhouse gas into chemical intermediates. Compared to other catalytic methods, photocatalytic CO2 reduction, which uses solar energy as the energy input, has attracted significant attention because it is a clean and inexhaustible resource. Therefore, using high-performance photocatalysts for effective CO2 reduction under mild reaction conditions is an active research hotspot. However, several current photocatalysts suffer from low solar energy conversion efficiency due to the extensive charge recombination and few active sites, leading to low CO2 reduction efficiency. Generally, constructing an S-scheme heterojunction can not only promote charge separation but also help maintain strong redox ability. Therefore, the S-scheme heterojunction is expected to help in achieving high conversion activity and CO2 reduction efficiency. Here, 2D tetragonal BiOBr0.5Cl0.5 nanosheets and hexagonal WO3 nanorods were prepared using a simple hydrothermal synthesis method, and the 2D/1D BiOBr0.5Cl0.5 nanosheets/WO3 nanorods (BiOBr0.5Cl0.5/WO3) S-scheme heterojunction with near infrared (NIR) light (> 780 nm) response were prepared via the electrostatic self-assembly method for the photocatalytic CO2 reduction. Following characterization and analysis, including diffuse reflectance spectra (DRS), Mott-Schottky plots, transient photocurrent response, time-resolution photoluminescence spectrum (TRPL), electrochemical impedance spectroscopy (EIS), linear sweep voltammetry (LSV), and electron spin resonance (ESR) measurements, it can be demonstrated that an S-scheme carrier transfer route was formed between the 2D BiOBr0.5Cl0.5 nanosheets and 1D WO3 nanorods. Driven by the internal electric field, which was formed between the two semiconductors, electron migration was boosted, thus inhibiting the recombination of photogenerated carriers, while the stronger redox ability was maintained, thus providing good reduction efficiency over BiOBr0.5Cl0.5/WO3 composite in CO2 reduction. In addition, the 2D/1D nanosheet/nanorod structure allowed for enhanced interface contact with abundant active sites, which favored charge separation and increased photocatalytic activity. Furthermore, the amount of WO3 nanorods added during the preparation of the composites was altered, which led to the optimal amount of 5% (w, mass fraction) for the photocatalytic CO2 reduction. As a result, the BiOBr0.5Cl0.5/WO3 composite exhibited superior photocatalytic reduction performance with a CO yield of 16.68 μmol·g-1·h-1 in the presence of any precious metal cocatalyst or sacrificial agent, which was 1.7 and 9.8 times that of pure BiOBr0.5Cl0.5 and WO3, respectively. In addition, the BiOBr0.5Cl0.5/WO3 composite provided continuously increased CO yields with excellent selectivity under full-spectrum light irradiation, suggesting good photocatalytic stability. This work describes a novel idea for the construction of 2D/1D S-scheme heterojunction photocatalysts for efficient CO2 reduction.
2022, 38(7): 211102
doi: 10.3866/PKU.WHXB202111021
Abstract:
The rational interface tailoring of nanosheets on hollow spheres is a promising strategy to develop efficient photocatalysts for hydrogen production with solar energy. Among the various photocatalyst materials, metal sulfides have been extensively researched because of their relatively narrow band gap and superior visible-light response. ZnIn2S4 is a layered ternary chalcogenide semiconductor photocatalyst with a tunable band gap energy (approximately 2.4 eV). Among various metal sulfide photocatalysts, ZnIn2S4 has gained considerable attention. However, intrinsic ZnIn2S4 only exhibits a relatively moderate photocatalytic activity, which is mainly owing to the high recombination and low migration rate of photocarriers. Loading cocatalysts onto semiconductor photocatalysts is an effective way to improve the performance of photocatalysts, because it can not only facilitate the separation of electron-hole pairs, but also reduce the activation energy for proton reduction. As a ternary transition metal sulfide, NiCo2S4 features a high electrical conductivity, low electronegativity, excellent redox properties, and outstanding electrocatalytic activity. Such favorable characteristics suggest that NiCo2S4 can expedite charge separation and transfer, thereby promoting photocatalytic H2 production by serving as a cocatalyst. Moreover, both NiCo2S4 and ZnIn2S4 possess the ternary spinel crystal structure, which may facilitate the construction of NiCo2S4/ZnIn2S4 hybrids with tight interfacial contact for an enhanced photocatalytic performance. Herein, ultrathin ZnIn2S4 nanosheets were grown in situ on a non-noble-metal cocatalyst, namely NiCo2S4 hollow spheres, to form hierarchical NiCo2S4@ZnIn2S4 hollow heterostructured photocatalysts with an intimately coupled interface and strong visible light absorption extending to ca. 583 nm. The optimized NiCo2S4@ZnIn2S4 hybrid with a NiCo2S4 content of ca. 3.1% exhibited a high hydrogen evolution rate of 78 μmol·h-1, which was approximately 9 times higher than that of bare ZnIn2S4 and 3 times higher than that of 1% (w, mass fraction) Pt/ZnIn2S4. Additionally, the hybrid photocatalysts displayed good stability in the reaction. Photoluminescence and electrochemical analysis results indicated that NiCo2S4 hollow spheres served as an efficient cocatalyst for facilitating the separation and transport of light-induced charge carriers as well as reducing the hydrogen evolution reaction barrier. Finally, a possible reaction mechanism for the photocatalytic hydrogen evolution was proposed. In the NiCo2S4@ZnIn2S4 composite photocatalyst, the NiCo2S4 cocatalyst with high electrical conductivity favorably accepts the photoinduced electrons transferred from ZnIn2S4 and then employs the electrons to reduce protons for H2 production on the reactive sites. Concurrently, the photogenerated holes are trapped by TEOA that acts as a hole scavenger to accomplish the photoredox cycle. This study provides guidance for the fabrication of hierarchical hollow heterostructures based on nanosheet semiconductor subunits as remarkable photocatalysts for hydrogen production. ![]()
The rational interface tailoring of nanosheets on hollow spheres is a promising strategy to develop efficient photocatalysts for hydrogen production with solar energy. Among the various photocatalyst materials, metal sulfides have been extensively researched because of their relatively narrow band gap and superior visible-light response. ZnIn2S4 is a layered ternary chalcogenide semiconductor photocatalyst with a tunable band gap energy (approximately 2.4 eV). Among various metal sulfide photocatalysts, ZnIn2S4 has gained considerable attention. However, intrinsic ZnIn2S4 only exhibits a relatively moderate photocatalytic activity, which is mainly owing to the high recombination and low migration rate of photocarriers. Loading cocatalysts onto semiconductor photocatalysts is an effective way to improve the performance of photocatalysts, because it can not only facilitate the separation of electron-hole pairs, but also reduce the activation energy for proton reduction. As a ternary transition metal sulfide, NiCo2S4 features a high electrical conductivity, low electronegativity, excellent redox properties, and outstanding electrocatalytic activity. Such favorable characteristics suggest that NiCo2S4 can expedite charge separation and transfer, thereby promoting photocatalytic H2 production by serving as a cocatalyst. Moreover, both NiCo2S4 and ZnIn2S4 possess the ternary spinel crystal structure, which may facilitate the construction of NiCo2S4/ZnIn2S4 hybrids with tight interfacial contact for an enhanced photocatalytic performance. Herein, ultrathin ZnIn2S4 nanosheets were grown in situ on a non-noble-metal cocatalyst, namely NiCo2S4 hollow spheres, to form hierarchical NiCo2S4@ZnIn2S4 hollow heterostructured photocatalysts with an intimately coupled interface and strong visible light absorption extending to ca. 583 nm. The optimized NiCo2S4@ZnIn2S4 hybrid with a NiCo2S4 content of ca. 3.1% exhibited a high hydrogen evolution rate of 78 μmol·h-1, which was approximately 9 times higher than that of bare ZnIn2S4 and 3 times higher than that of 1% (w, mass fraction) Pt/ZnIn2S4. Additionally, the hybrid photocatalysts displayed good stability in the reaction. Photoluminescence and electrochemical analysis results indicated that NiCo2S4 hollow spheres served as an efficient cocatalyst for facilitating the separation and transport of light-induced charge carriers as well as reducing the hydrogen evolution reaction barrier. Finally, a possible reaction mechanism for the photocatalytic hydrogen evolution was proposed. In the NiCo2S4@ZnIn2S4 composite photocatalyst, the NiCo2S4 cocatalyst with high electrical conductivity favorably accepts the photoinduced electrons transferred from ZnIn2S4 and then employs the electrons to reduce protons for H2 production on the reactive sites. Concurrently, the photogenerated holes are trapped by TEOA that acts as a hole scavenger to accomplish the photoredox cycle. This study provides guidance for the fabrication of hierarchical hollow heterostructures based on nanosheet semiconductor subunits as remarkable photocatalysts for hydrogen production.
2022, 38(7): 211202
doi: 10.3866/PKU.WHXB202112027
Abstract:
With the rapid development of industrial technology, a large number of organic pollutants are routinely released into the environment, which has caused serious problems. Semiconductor photocatalysis is an environmentally-friendly and effective method to degrade and remove typical pollutants, and photocatalysts play a key role in the application of this technology. Therefore, various semiconductor materials have been tried and used in the field of pollutant removal. Graphite carbon nitride (g-C3N4) has attracted great interest because of its two-dimensional layered structure and good visible light response range. Owing to a narrow bandgap, adjustable band structure, and high physicochemical stability, g-C3N4 absorbs wavelengths up to 450 nm in the visible spectrum, leading to an opportunity for visible-light photocatalytic performance. Nevertheless, there are still some drawbacks that limit the photocatalytic efficiency of g-C3N4 in the removal of antibiotics and dyes under visible light, such as the rapid recombination of photoinduced charges and the weak oxidation capacity of holes. To advance this promising photocatalytic material, multiple methods have been tried to optimize the electronic band structure of g-C3N4, such as doping with various elements, morphology control, and functional group modification. Recently, a novel type of Step-scheme (S-scheme) heterojunction composed of two n-type semiconductor photocatalysts has been proposed, which can utilize a more positive valance band and a more negative conduction band. It was demonstrated that the formation of S-scheme heterojunctions is a valid way to increase photocatalytic activity of g-C3N4. Herein, novel 0D/2D Bi4V2O11/g-C3N4 S-scheme heterojunctions were prepared by a simple in situ solvothermal growth method. The Bi4V2O11/g-C3N4 composites displayed a high photocatalytic activity through the removal of oxytetracycline (OTC) and Reactive Red 2. In particular, the BVCN-50 composite showed the highest degradation efficiency for OTC of 74.1% and for Reactive Red 2 of 84.2% with ·O2- as the primary active species. This highly improved photocatalytic performance can be ascribed to the generation of S-scheme heterojunctions, which provides for a high redox capacity of the heterojunction system (strong oxidative ability of Bi4V2O11 and strong reductive capacity of g-C3N4) and facilitates the space separation of photo-generated charges. Moreover, the surface plasmon resonance effect of metallic Bi0 broadens the light utilization range of the heterojunction system. In addition, the possible degradation pathway and intermediates throughout the degradation process of OTC based on liquid chromatograph mass spectrometer (LC-MS) analysis were also studied. This work provides a novel tactic for the design and fabrication of g-C3N4-based S-scheme heterojunctions with enhanced photocatalytic performance. ![]()
With the rapid development of industrial technology, a large number of organic pollutants are routinely released into the environment, which has caused serious problems. Semiconductor photocatalysis is an environmentally-friendly and effective method to degrade and remove typical pollutants, and photocatalysts play a key role in the application of this technology. Therefore, various semiconductor materials have been tried and used in the field of pollutant removal. Graphite carbon nitride (g-C3N4) has attracted great interest because of its two-dimensional layered structure and good visible light response range. Owing to a narrow bandgap, adjustable band structure, and high physicochemical stability, g-C3N4 absorbs wavelengths up to 450 nm in the visible spectrum, leading to an opportunity for visible-light photocatalytic performance. Nevertheless, there are still some drawbacks that limit the photocatalytic efficiency of g-C3N4 in the removal of antibiotics and dyes under visible light, such as the rapid recombination of photoinduced charges and the weak oxidation capacity of holes. To advance this promising photocatalytic material, multiple methods have been tried to optimize the electronic band structure of g-C3N4, such as doping with various elements, morphology control, and functional group modification. Recently, a novel type of Step-scheme (S-scheme) heterojunction composed of two n-type semiconductor photocatalysts has been proposed, which can utilize a more positive valance band and a more negative conduction band. It was demonstrated that the formation of S-scheme heterojunctions is a valid way to increase photocatalytic activity of g-C3N4. Herein, novel 0D/2D Bi4V2O11/g-C3N4 S-scheme heterojunctions were prepared by a simple in situ solvothermal growth method. The Bi4V2O11/g-C3N4 composites displayed a high photocatalytic activity through the removal of oxytetracycline (OTC) and Reactive Red 2. In particular, the BVCN-50 composite showed the highest degradation efficiency for OTC of 74.1% and for Reactive Red 2 of 84.2% with ·O2- as the primary active species. This highly improved photocatalytic performance can be ascribed to the generation of S-scheme heterojunctions, which provides for a high redox capacity of the heterojunction system (strong oxidative ability of Bi4V2O11 and strong reductive capacity of g-C3N4) and facilitates the space separation of photo-generated charges. Moreover, the surface plasmon resonance effect of metallic Bi0 broadens the light utilization range of the heterojunction system. In addition, the possible degradation pathway and intermediates throughout the degradation process of OTC based on liquid chromatograph mass spectrometer (LC-MS) analysis were also studied. This work provides a novel tactic for the design and fabrication of g-C3N4-based S-scheme heterojunctions with enhanced photocatalytic performance.
2022, 38(7): 211203
doi: 10.3866/PKU.WHXB202112037
Abstract:
Photocatalytic H2O2 production is a sustainable and inexpensive process that requires water and gaseous O2 as raw materials and sunlight as the energy source. However, the slow kinetics of current photocatalysts limits its practical application. ZnO is commonly used as a photocatalytic material in the solar-to-chemical conversion, owing to its high electron mobility, nontoxicity, and relatively low cost. The adsorption capacity of H2O2 on the ZnO surface is low, which leads to the continuous production of H2O2. However, its photoresponse is limited to the ultraviolet (UV) region due to its wide bandgap (3.2 eV). Polydopamine (PDA) has emerged as an effective surface functionalization material in the field of photocatalysis due to its abundant functional groups. PDA can be strongly anchored onto the surface of a semiconducting photocatalyst through covalent and noncovalent bonds. The superior properties of PDA served as a motivation for this study. Herein, we prepare an inverse opal-structured porous PDA-modified ZnO (ZnO@PDA) photocatalyst by in situ self-polymerization of dopamine hydrochloride. The crystal structure, morphology, valency, stability, and energy band structure of photocatalysts are characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), field-emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), UV-visible diffuse reflectance spectroscopy (UV-Vis DRS), electrochemical impedance spectroscopy (EIS), Mott-Schottky curve (MS), and electron paramagnetic resonance (EPR). The experimental results showed that electrons in PDA are transferred to ZnO upon contact, which results in an electric field at their interface in the direction from PDA to ZnO. The photoexcited electrons in the ZnO conduction bands flow into PDA, driven by the electric field and bent bands, and are recombined with the holes of the highest occupied molecular orbital of PDA, thereby exhibiting an S-scheme charge transfer. This unique S-scheme mechanism ensures effective electron/hole separation and preserves the strong redox ability of used photocarriers. In addition, the inverse opal structure of ZnO@PDA promotes light-harvesting due to the supposed "slow photon" effect, as well as Bragg diffraction and scattering. Moreover, the enhanced surface area provides a high adsorption capacity and increased active sites for photocatalytic reactions. Therefore, the resulting ZnO@PDA (0.03% (atomic fraction) PDA) exhibits the optimal H2O2 production performance (1011.4 μmol·L-1·h-1), which is 4.4 and 8.9 times higher than pristine ZnO and PDA, respectively. The enhanced performance is ascribed to the improved light absorption, efficient charge separation, and strong redox capability of photocarriers in the S-scheme heterojunction. Therefore, this study provides a novel strategy for the design of inorganic/organic S-scheme heterojunctions for efficient photocatalytic H2O2 production. ![]()
Photocatalytic H2O2 production is a sustainable and inexpensive process that requires water and gaseous O2 as raw materials and sunlight as the energy source. However, the slow kinetics of current photocatalysts limits its practical application. ZnO is commonly used as a photocatalytic material in the solar-to-chemical conversion, owing to its high electron mobility, nontoxicity, and relatively low cost. The adsorption capacity of H2O2 on the ZnO surface is low, which leads to the continuous production of H2O2. However, its photoresponse is limited to the ultraviolet (UV) region due to its wide bandgap (3.2 eV). Polydopamine (PDA) has emerged as an effective surface functionalization material in the field of photocatalysis due to its abundant functional groups. PDA can be strongly anchored onto the surface of a semiconducting photocatalyst through covalent and noncovalent bonds. The superior properties of PDA served as a motivation for this study. Herein, we prepare an inverse opal-structured porous PDA-modified ZnO (ZnO@PDA) photocatalyst by in situ self-polymerization of dopamine hydrochloride. The crystal structure, morphology, valency, stability, and energy band structure of photocatalysts are characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), field-emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), UV-visible diffuse reflectance spectroscopy (UV-Vis DRS), electrochemical impedance spectroscopy (EIS), Mott-Schottky curve (MS), and electron paramagnetic resonance (EPR). The experimental results showed that electrons in PDA are transferred to ZnO upon contact, which results in an electric field at their interface in the direction from PDA to ZnO. The photoexcited electrons in the ZnO conduction bands flow into PDA, driven by the electric field and bent bands, and are recombined with the holes of the highest occupied molecular orbital of PDA, thereby exhibiting an S-scheme charge transfer. This unique S-scheme mechanism ensures effective electron/hole separation and preserves the strong redox ability of used photocarriers. In addition, the inverse opal structure of ZnO@PDA promotes light-harvesting due to the supposed "slow photon" effect, as well as Bragg diffraction and scattering. Moreover, the enhanced surface area provides a high adsorption capacity and increased active sites for photocatalytic reactions. Therefore, the resulting ZnO@PDA (0.03% (atomic fraction) PDA) exhibits the optimal H2O2 production performance (1011.4 μmol·L-1·h-1), which is 4.4 and 8.9 times higher than pristine ZnO and PDA, respectively. The enhanced performance is ascribed to the improved light absorption, efficient charge separation, and strong redox capability of photocarriers in the S-scheme heterojunction. Therefore, this study provides a novel strategy for the design of inorganic/organic S-scheme heterojunctions for efficient photocatalytic H2O2 production.
2022, 38(7): 220100
doi: 10.3866/PKU.WHXB202201008
Abstract:
Graphitic carbon nitride (g-C3N4) has been widely used as a potential photocatalytic material for the removal of tetracycline from water. However, the poor visible light absorption ability and high recombination rate of the photogenerated charge significantly inhibit the catalytic activity of g-C3N4. Therefore, facile methods to improve the photocatalytic efficiency of g-C3N4 need to be developed. Hematite (α-Fe2O3), which has a good visible light absorption and corrosion resistance, is often used for photocatalysis and photo-Fenton reactions. Therefore, a two-dimension/two-dimension (2D/2D) S-scheme heterojunction constructed of g-C3N4 and α-Fe2O3 nanosheets could be expected to improve the degradation efficiency of tetracycline. In this study, 2D/2D S-scheme α-Fe2O3/g-C3N4 photo-Fenton catalysts were prepared using a hydrothermal strategy. The photo-Fenton catalytic activity of α-Fe2O3/g-C3N4 (α-Fe2O3 50% (w)) was significantly improved by the addition of a small amount of H2O2, removing 78% of tetracycline within 20 min, which was approximately 3.5 and 5.8 times the removal achieved using α-Fe2O3 and g-C3N4, respectively. The high catalytic activity was attributed to the synergy between the photocatalysis and Fenton reaction promoted by the continuous Fe3+/Fe2+ conversion over the 2D/2D S-scheme heterojunction. The 2D/2D S-scheme heterojunction was crucial in the fabrication of the α-Fe2O3/g-C3N4 photocatalyst with a large surface area, adequate active sites, and strong oxidation-reduction capability. Furthermore, the photo-Fenton reaction provided additional hydroxyl radicals for the degradation of tetracycline with the aid of H2O2. The excess reaction product (Fe3+) was reduced to Fe2+ by the photogenerated electrons from the conduction band of α-Fe2O3. The resulting Fe2+ could participate in the photo-Fenton reaction. The morphological structures of α-Fe2O3/g-C3N4 were analyzed using transmission electron microscopy to demonstrate the formation of a 2D/2D structure with face-to-face contact, and the optical properties of the composites were measured using ultraviolet-visible diffuse reflectance spectroscopy. α-Fe2O3/g-C3N4 possessed a significantly improved visible light absorption compared to g-C3N4. Five sequential cyclic degradation tests and X-ray diffraction (XRD) patterns obtained before and after the reaction showed that the α-Fe2O3/g-C3N4 composites possessed stable photo-Fenton catalytic activity and crystal structures. Transient photocurrent responses of α-Fe2O3/g-C3N4 demonstrated that the prepared composites exhibited a higher charge transfer efficiency compared to that of single α-Fe2O3 and g-C3N4. In addition, according to the photoluminescence analysis and active species trapping experiments, a possible S-scheme heterojunction charge transfer process in the photo-Fenton catalytic reaction was proposed. This study provided a promising method for the construction of a high-performance photo-Fenton catalytic system to remove antibiotics from wastewater. ![]()
Graphitic carbon nitride (g-C3N4) has been widely used as a potential photocatalytic material for the removal of tetracycline from water. However, the poor visible light absorption ability and high recombination rate of the photogenerated charge significantly inhibit the catalytic activity of g-C3N4. Therefore, facile methods to improve the photocatalytic efficiency of g-C3N4 need to be developed. Hematite (α-Fe2O3), which has a good visible light absorption and corrosion resistance, is often used for photocatalysis and photo-Fenton reactions. Therefore, a two-dimension/two-dimension (2D/2D) S-scheme heterojunction constructed of g-C3N4 and α-Fe2O3 nanosheets could be expected to improve the degradation efficiency of tetracycline. In this study, 2D/2D S-scheme α-Fe2O3/g-C3N4 photo-Fenton catalysts were prepared using a hydrothermal strategy. The photo-Fenton catalytic activity of α-Fe2O3/g-C3N4 (α-Fe2O3 50% (w)) was significantly improved by the addition of a small amount of H2O2, removing 78% of tetracycline within 20 min, which was approximately 3.5 and 5.8 times the removal achieved using α-Fe2O3 and g-C3N4, respectively. The high catalytic activity was attributed to the synergy between the photocatalysis and Fenton reaction promoted by the continuous Fe3+/Fe2+ conversion over the 2D/2D S-scheme heterojunction. The 2D/2D S-scheme heterojunction was crucial in the fabrication of the α-Fe2O3/g-C3N4 photocatalyst with a large surface area, adequate active sites, and strong oxidation-reduction capability. Furthermore, the photo-Fenton reaction provided additional hydroxyl radicals for the degradation of tetracycline with the aid of H2O2. The excess reaction product (Fe3+) was reduced to Fe2+ by the photogenerated electrons from the conduction band of α-Fe2O3. The resulting Fe2+ could participate in the photo-Fenton reaction. The morphological structures of α-Fe2O3/g-C3N4 were analyzed using transmission electron microscopy to demonstrate the formation of a 2D/2D structure with face-to-face contact, and the optical properties of the composites were measured using ultraviolet-visible diffuse reflectance spectroscopy. α-Fe2O3/g-C3N4 possessed a significantly improved visible light absorption compared to g-C3N4. Five sequential cyclic degradation tests and X-ray diffraction (XRD) patterns obtained before and after the reaction showed that the α-Fe2O3/g-C3N4 composites possessed stable photo-Fenton catalytic activity and crystal structures. Transient photocurrent responses of α-Fe2O3/g-C3N4 demonstrated that the prepared composites exhibited a higher charge transfer efficiency compared to that of single α-Fe2O3 and g-C3N4. In addition, according to the photoluminescence analysis and active species trapping experiments, a possible S-scheme heterojunction charge transfer process in the photo-Fenton catalytic reaction was proposed. This study provided a promising method for the construction of a high-performance photo-Fenton catalytic system to remove antibiotics from wastewater.
2022, 38(7): 211100
doi: 10.3866/PKU.WHXB202111009
Abstract:
2022, 38(7): 220200
doi: 10.3866/PKU.WHXB202202001
Abstract:
