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2020 Volume 36 Issue 3
2020, 36(3): 180301
doi: 10.3866/PKU.WHXB201803014
Abstract:
Photocatalysis based on visible light is an efficient and promising strategy to convert solar energy into chemical energy and solve the global issues of environmental pollution and energy shortages. CdS, as a visible light responsive semiconductor material, is widely used in photocatalysis and photoluminescence because of its simple synthesis, abundant raw materials, and appropriate bandgap structure. The inverse opal (IO) structure belonging to photonic crystal structure with unique three-dimensionally ordered macro-mesopore, which can tune the propagation direction of incident light and improve photocatalytic performance. Therefore, IO has attracted extensive attention for photocatalysis applications. Herein, CdS IO photonic crystal films were prepared by co-assembly using CdS nanocrystals and poly(styrene-methyl methacrylate-3-sulfopropyl methacrylate, potassium salt) (P(St-MMA-SPMAP)) emulsion. This method is widely used because it is simple and can rapidly prepare large photonic crystal films. The pore size of the IO structure was regulated by changing the diameter of the polymer. The IO structure was characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), ultraviolet-visible absorption spectroscopy (UV-Vis), and reflectance spectroscopy. The photocatalysis performance of three samples was evaluated via photocatalytic water splitting under visible light irradiation (λ ≥ 420 nm). The photocatalytic hydrogen production rate of the CdS IO film fabricated using a 310 nm P(St-MMA-SPMAP) template (CdS-310) was twice that of CdS nanoparticles (CdS-NPs) under visible light irradiation. This photocatalytic performance enhancement was ascribed to the hierarchically porous structure of the IO photonic crystal. On the one hand, the IO structure increased the propagation of photons in the photocatalytic material and improved sunlight utilization. On the other hand, the structure is conductive to transport and adsorption of molecules. In addition, the IO structure was composed of nanoparticles, providing more active sites for the photocatalytic reaction. ![]()
Photocatalysis based on visible light is an efficient and promising strategy to convert solar energy into chemical energy and solve the global issues of environmental pollution and energy shortages. CdS, as a visible light responsive semiconductor material, is widely used in photocatalysis and photoluminescence because of its simple synthesis, abundant raw materials, and appropriate bandgap structure. The inverse opal (IO) structure belonging to photonic crystal structure with unique three-dimensionally ordered macro-mesopore, which can tune the propagation direction of incident light and improve photocatalytic performance. Therefore, IO has attracted extensive attention for photocatalysis applications. Herein, CdS IO photonic crystal films were prepared by co-assembly using CdS nanocrystals and poly(styrene-methyl methacrylate-3-sulfopropyl methacrylate, potassium salt) (P(St-MMA-SPMAP)) emulsion. This method is widely used because it is simple and can rapidly prepare large photonic crystal films. The pore size of the IO structure was regulated by changing the diameter of the polymer. The IO structure was characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), ultraviolet-visible absorption spectroscopy (UV-Vis), and reflectance spectroscopy. The photocatalysis performance of three samples was evaluated via photocatalytic water splitting under visible light irradiation (λ ≥ 420 nm). The photocatalytic hydrogen production rate of the CdS IO film fabricated using a 310 nm P(St-MMA-SPMAP) template (CdS-310) was twice that of CdS nanoparticles (CdS-NPs) under visible light irradiation. This photocatalytic performance enhancement was ascribed to the hierarchically porous structure of the IO photonic crystal. On the one hand, the IO structure increased the propagation of photons in the photocatalytic material and improved sunlight utilization. On the other hand, the structure is conductive to transport and adsorption of molecules. In addition, the IO structure was composed of nanoparticles, providing more active sites for the photocatalytic reaction.
2020, 36(3): 190105
doi: 10.3866/PKU.WHXB201901051
Abstract:
In this study, a novel silicon carbide/platinum/cadmium sulfide (SiC/Pt/CdS) Z-scheme heterojunction nanorod is constructed using a simple chemical reduction-assisted hydrothermal method, in which Pt nanoparticles are anchored at the interface of SiC nanorods and CdS nanoparticles to induce an electron-hole pair transfer along the Z-scheme transport path. Multiple characterization techniques are used to analyze the structure, morphology, and properties of these materials. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) results show that the SiC/Pt/CdS materials with good crystal structure are successfully synthesized. Transmission electron microscopy reveals that Pt nanoparticles grow between the interfaces of SiC nanorods and CdS nanoparticles. UV-Vis diffuse reflectance spectroscopy shows that the as-prepared Z-scheme heterojunction samples have a wider light absorption range in comparison with pristine CdS materials. Photoluminescence spectroscopy and the transient photocurrent response further demonstrate that the SiC/Pt/CdS nanorod sample with an optimal molar ratio possesses the highest electron-hole pair separation efficiency. The loading amount of CdS on the surface of SiC/Pt nanorods is effectively adjusted by controlling the molar ratio of SiC and CdS to achieve the optimal performance of the SiC/Pt/CdS nanorod photocatalysts. The optimal H2 evolution capacity is achieved at SiC : CdS = 5 : 1 (molar ratio) and the maximum H2 evolution rate reaches a high value of 122.3 µmol·h−1. In addition, scanning electron microscopy, XRD, and XPS analyses show that the morphology and crystal structure of the SiC/Pt/CdS photocatalyst remain unchanged after three cycles of activity testing, indicating that the SiC/Pt/CdS nanocomposite has a stable structure for H2 evolution under visible light. To prove the Z-scheme transfer mechanism of electron-hole pairs, selective photo-deposition technology is used to simultaneously carry out the photo-reduction deposition of Au nanoparticles and photo-oxidation deposition of Mn3O4 nanoparticles in the photoreaction. The experimental results indicate that during photocatalysis, the electrons in the conduction band of CdS participate mainly in the reduction reaction, and the holes in the valence band of SiC are more likely to undergo the oxidation reaction. The electrons in the conduction band of SiC combine with the holes in the valence band of CdS to form a Z-scheme transport path. Therefore, a possible Z-scheme charge migration path in SiC/Pt/CdS nanorods during photocatalytic H2 production is proposed to explain the enhancement in the activity. This study provides a new strategy for synthesizing a Z-scheme photocatalytic system based on SiC nanorods. Based on the characterization results, it is determined that SiC/Pt/CdS nanocomposites are highly efficient, inexpensive, easy to prepare, and are stable structures for H2 evolution under visible light with outstanding commercial application prospects. ![]()
In this study, a novel silicon carbide/platinum/cadmium sulfide (SiC/Pt/CdS) Z-scheme heterojunction nanorod is constructed using a simple chemical reduction-assisted hydrothermal method, in which Pt nanoparticles are anchored at the interface of SiC nanorods and CdS nanoparticles to induce an electron-hole pair transfer along the Z-scheme transport path. Multiple characterization techniques are used to analyze the structure, morphology, and properties of these materials. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) results show that the SiC/Pt/CdS materials with good crystal structure are successfully synthesized. Transmission electron microscopy reveals that Pt nanoparticles grow between the interfaces of SiC nanorods and CdS nanoparticles. UV-Vis diffuse reflectance spectroscopy shows that the as-prepared Z-scheme heterojunction samples have a wider light absorption range in comparison with pristine CdS materials. Photoluminescence spectroscopy and the transient photocurrent response further demonstrate that the SiC/Pt/CdS nanorod sample with an optimal molar ratio possesses the highest electron-hole pair separation efficiency. The loading amount of CdS on the surface of SiC/Pt nanorods is effectively adjusted by controlling the molar ratio of SiC and CdS to achieve the optimal performance of the SiC/Pt/CdS nanorod photocatalysts. The optimal H2 evolution capacity is achieved at SiC : CdS = 5 : 1 (molar ratio) and the maximum H2 evolution rate reaches a high value of 122.3 µmol·h−1. In addition, scanning electron microscopy, XRD, and XPS analyses show that the morphology and crystal structure of the SiC/Pt/CdS photocatalyst remain unchanged after three cycles of activity testing, indicating that the SiC/Pt/CdS nanocomposite has a stable structure for H2 evolution under visible light. To prove the Z-scheme transfer mechanism of electron-hole pairs, selective photo-deposition technology is used to simultaneously carry out the photo-reduction deposition of Au nanoparticles and photo-oxidation deposition of Mn3O4 nanoparticles in the photoreaction. The experimental results indicate that during photocatalysis, the electrons in the conduction band of CdS participate mainly in the reduction reaction, and the holes in the valence band of SiC are more likely to undergo the oxidation reaction. The electrons in the conduction band of SiC combine with the holes in the valence band of CdS to form a Z-scheme transport path. Therefore, a possible Z-scheme charge migration path in SiC/Pt/CdS nanorods during photocatalytic H2 production is proposed to explain the enhancement in the activity. This study provides a new strategy for synthesizing a Z-scheme photocatalytic system based on SiC nanorods. Based on the characterization results, it is determined that SiC/Pt/CdS nanocomposites are highly efficient, inexpensive, easy to prepare, and are stable structures for H2 evolution under visible light with outstanding commercial application prospects.
2020, 36(3): 190200
doi: 10.3866/PKU.WHXB201902001
Abstract:
Photocatalytic technology can effectively solve the problem of increasingly serious water pollution, the core of which is the design and synthesis of highly efficient photocatalytic materials. Semiconductor photocatalysts are currently the most widely used photocatalysts. Among these is graphitic carbon nitride (g-C3N4), which has great potential in environment management and the development of new energy owing to its low cost, easy availability, unique band structure, and good thermal stability. However, the photocatalytic activity of g-C3N4 remains low because of problems such as wide bandgap, weakly absorb visible light, and the high recombination rate of photogenerated carriers. Among various modification strategies, doping modification is an effective and simple method used to improve the photocatalytic performance of materials. In this work, Cu/g-C3N4 photocatalysts were successfully prepared by incorporating Cu2+ into g-C3N4 to further optimize photocatalytic performance. At the same time, the structure, morphology, and optical and photoelectric properties of Cu/g-C3N4 photocatalysts were analyzed by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy, UV-Vis diffuse reflectance spectroscopy (DRS), and photoelectric tests. XRD and XPS were used to ensure that the prepared photocatalysts were Cu/g-C3N4 and the valence state of Cu was in the form of Cu2+. Under visible light irradiation, the photocatalytic activity of Cu/g-C3N4 and pure g-C3N4 photocatalysts were investigated in terms of the degradation of RhB and CIP by comparing the amount of introduced copper ions. The experimental results showed that the degradation ability of Cu/g-C3N4 photocatalysts was stronger than that of pure g-C3N4. The N2 adsorption-desorption isotherms of g-C3N4 and Cu/g-C3N4 demonstrated that the introduction of copper had little effect on the microstructure of g-C3N4. The small difference in specific surface area indicates that the enhanced photocatalytic activity may be attributed to the effective separation of photogenerated carriers. Therefore, the enhanced photocatalytic degradation of RhB and CIP over Cu/g-C3N4 may be due to the reduction of carrier recombination rate by copper. The photoelectric test showed that the incorporation of Cu2+ into g-C3N4 could reduce the electron-hole recombination rate of g-C3N4 and accelerate the separation of electron-hole pairs, thus enhancing the photocatalytic activity of Cu/g-C3N4. Free radical trapping experiments and electron spin resonance indicated that the synergistic effect of superoxide radicals (O2•−), hydroxyl radicals (•OH) and holes could increase the photocatalytic activity of Cu/g-C3N4 materials. ![]()
Photocatalytic technology can effectively solve the problem of increasingly serious water pollution, the core of which is the design and synthesis of highly efficient photocatalytic materials. Semiconductor photocatalysts are currently the most widely used photocatalysts. Among these is graphitic carbon nitride (g-C3N4), which has great potential in environment management and the development of new energy owing to its low cost, easy availability, unique band structure, and good thermal stability. However, the photocatalytic activity of g-C3N4 remains low because of problems such as wide bandgap, weakly absorb visible light, and the high recombination rate of photogenerated carriers. Among various modification strategies, doping modification is an effective and simple method used to improve the photocatalytic performance of materials. In this work, Cu/g-C3N4 photocatalysts were successfully prepared by incorporating Cu2+ into g-C3N4 to further optimize photocatalytic performance. At the same time, the structure, morphology, and optical and photoelectric properties of Cu/g-C3N4 photocatalysts were analyzed by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy, UV-Vis diffuse reflectance spectroscopy (DRS), and photoelectric tests. XRD and XPS were used to ensure that the prepared photocatalysts were Cu/g-C3N4 and the valence state of Cu was in the form of Cu2+. Under visible light irradiation, the photocatalytic activity of Cu/g-C3N4 and pure g-C3N4 photocatalysts were investigated in terms of the degradation of RhB and CIP by comparing the amount of introduced copper ions. The experimental results showed that the degradation ability of Cu/g-C3N4 photocatalysts was stronger than that of pure g-C3N4. The N2 adsorption-desorption isotherms of g-C3N4 and Cu/g-C3N4 demonstrated that the introduction of copper had little effect on the microstructure of g-C3N4. The small difference in specific surface area indicates that the enhanced photocatalytic activity may be attributed to the effective separation of photogenerated carriers. Therefore, the enhanced photocatalytic degradation of RhB and CIP over Cu/g-C3N4 may be due to the reduction of carrier recombination rate by copper. The photoelectric test showed that the incorporation of Cu2+ into g-C3N4 could reduce the electron-hole recombination rate of g-C3N4 and accelerate the separation of electron-hole pairs, thus enhancing the photocatalytic activity of Cu/g-C3N4. Free radical trapping experiments and electron spin resonance indicated that the synergistic effect of superoxide radicals (O2•−), hydroxyl radicals (•OH) and holes could increase the photocatalytic activity of Cu/g-C3N4 materials.
Controlling Self-Assembly of 3D In2O3 Nanostructures for Boosting Photocatalytic Hydrogen Production
2020, 36(3): 190304
doi: 10.3866/PKU.WHXB201903047
Abstract:
Exploring economical and efficient photocatalysts for hydrogen production is of great significance for alleviating the energy and environmental crisis. In this study, 3D In2O3 nanostructures with appropriate self-assembly degrees were obtained using a facile hydrothermal strategy. To study the significance of 3D In2O3 nanostructures with appropriate self-assembly degrees in photocatalytic hydrogen production, the photocatalytic performances of samples were evaluated based on the amount of hydrogen gas release under visible-light irradiation (λ > 400 nm) and simulated solar light illumination. Interestingly, the 3D In2O3-150 nanostructured photocatalyst (hydrothermal temperature was 150 ℃, denoted as In2O3-150) exhibited extremely superior photocatalytic hydrogen evolution activity, which may have been caused by their unique structure to improve light reflection and gas evolution. The special structure can enhance light harvesting and induce more carriers to participate in photocatalytic hydrogen production. Despite possessing similar 3D nanostructures, the In2O3-180 photocatalyst exhibited poor photocatalytic activity. This may have been caused by the high self-assembly degree, which can hinder light irradiation and isolate a portion of the water. In addition, the 3D nanostructures could effectively make uniform the carrier migration direction, which is from the interior to the rod end. However, the direction of carrier migration of the In2O3-110 photocatalyst could transfer in various directions, whereas the In2O3-130 photocatalyst could transfer to both ends of the rod. This might cause partial migration to counteract each other. The compact cluster rod-like structure of In2O3-180 might prevent the light from exciting the carrier effectively. Through a photocatalytic recycling test, the 3D In2O3-150 nanostructured photocatalyst exhibited outstanding photochemical stability. This work highlights the importance of controlling the self-assembly degree of 3D In2O3 nanostructures and explores the performances of 3D In2O3 nanostructured photocatalysts in hydrogen production under visible light and simulated solar light. ![]()
Exploring economical and efficient photocatalysts for hydrogen production is of great significance for alleviating the energy and environmental crisis. In this study, 3D In2O3 nanostructures with appropriate self-assembly degrees were obtained using a facile hydrothermal strategy. To study the significance of 3D In2O3 nanostructures with appropriate self-assembly degrees in photocatalytic hydrogen production, the photocatalytic performances of samples were evaluated based on the amount of hydrogen gas release under visible-light irradiation (λ > 400 nm) and simulated solar light illumination. Interestingly, the 3D In2O3-150 nanostructured photocatalyst (hydrothermal temperature was 150 ℃, denoted as In2O3-150) exhibited extremely superior photocatalytic hydrogen evolution activity, which may have been caused by their unique structure to improve light reflection and gas evolution. The special structure can enhance light harvesting and induce more carriers to participate in photocatalytic hydrogen production. Despite possessing similar 3D nanostructures, the In2O3-180 photocatalyst exhibited poor photocatalytic activity. This may have been caused by the high self-assembly degree, which can hinder light irradiation and isolate a portion of the water. In addition, the 3D nanostructures could effectively make uniform the carrier migration direction, which is from the interior to the rod end. However, the direction of carrier migration of the In2O3-110 photocatalyst could transfer in various directions, whereas the In2O3-130 photocatalyst could transfer to both ends of the rod. This might cause partial migration to counteract each other. The compact cluster rod-like structure of In2O3-180 might prevent the light from exciting the carrier effectively. Through a photocatalytic recycling test, the 3D In2O3-150 nanostructured photocatalyst exhibited outstanding photochemical stability. This work highlights the importance of controlling the self-assembly degree of 3D In2O3 nanostructures and explores the performances of 3D In2O3 nanostructured photocatalysts in hydrogen production under visible light and simulated solar light.
2020, 36(3): 190508
doi: 10.3866/PKU.WHXB201905086
Abstract:
The photocatalytic reduction of CO2 has attracted considerable attention owing to the dual suppression of environmental pollution and energy shortage. The technology uses solar energy to convert carbon dioxide into hydrocarbon fuel, which is of great significance for achieving the carbon cycle. The development of low-cost photocatalytic materials is critical to achieving efficient solar energy to fuels conversion. One of the most commonly employed photocatalysts is TiO2. However, it suffers from broad band gap as well as the recombination of photo-excited holes and electron. Hence, in this work, we report the photochemical reduction of CO2 using rod-like PCN-222(Cu)/TiO2 composites as photocatalyst through a simple hydrothermal method, in which TiO2 nanoparticles are anchored at the interface of the SiC rod PCN-222(Cu). Multiple characterization techniques were used to analyze the structure, morphology, and properties of the PCN-222(Cu)/TiO2 composite. A series of characterizations including X-ray diffraction (XRD), scanning electron microscopy (SEM), diffuse reflectance spectroscopy (DRS), Fourier-transform infrared spectroscopy, photo-electrochemical, and photoluminescence (PL) confirm the successful preparation of PCN-222(Cu)/TiO2 composites. SEM reveals that the TiO2 nanoparticles are uniformly distributed on the surface of the rod-shaped PCN-222(Cu)/TiO2. XRD results show that PCN-222(Cu) and PCN-222(Cu)/TiO2 composite photocatalysts with good crystal structure were successfully synthesized. According to the DRS results, the prepared PCN-222(Cu)/TiO2 composite samples exhibit characteristic absorption peaks of metalloporphyrins in the visible region. PL spectroscopy, transient photocurrent response, and electrochemical impedance spectroscopy further confirm that the rod-like PCN-222(Cu)/TiO2 samples have high electron-hole pair separation efficiency. By controlling the mass ratio of PCN-222(Cu) and TiO2, the photocatalytic CO2 reduction performance test shows that the 10% PCN-222(Cu)/TiO2 composite achieves optimal catalytic performance, yielding 13.24 μmol·g−1·h−1 CO and 1.73 μmol·g−1·h−1 CH4, respectively. All the rod-like PCN-222(Cu)/TiO2 composites exhibit better photocatalytic CO2 activity than that of TiO2 nanoparticles or PCN-222(Cu) under the illumination of xenon lamps, which is attributed to charge transport and electron-hole separation capabilities. After three test cycles, the catalytic activity of PCN-222(Cu)/TiO2 photocatalyst was virtually unchanged. The reduction yield of the catalyst increased for 8 h under continuous illumination, indicating that PCN-222(Cu)/TiO2 composites have acceptable stability. The estimation of the band gap curve and the Mote-Schottky curve test show that the lowest unoccupied molecular orbital position of PCN-222(Cu) is more negative than the TiO2 of the conduction band; hence, a possible photocatalytic reaction mechanism of the PCN-222(Cu)/TiO2 composite is proposed. This study provides a new strategy for the integration of metal-organic frameworks and oxide semiconductors to construct efficient photocatalytic systems. ![]()
The photocatalytic reduction of CO2 has attracted considerable attention owing to the dual suppression of environmental pollution and energy shortage. The technology uses solar energy to convert carbon dioxide into hydrocarbon fuel, which is of great significance for achieving the carbon cycle. The development of low-cost photocatalytic materials is critical to achieving efficient solar energy to fuels conversion. One of the most commonly employed photocatalysts is TiO2. However, it suffers from broad band gap as well as the recombination of photo-excited holes and electron. Hence, in this work, we report the photochemical reduction of CO2 using rod-like PCN-222(Cu)/TiO2 composites as photocatalyst through a simple hydrothermal method, in which TiO2 nanoparticles are anchored at the interface of the SiC rod PCN-222(Cu). Multiple characterization techniques were used to analyze the structure, morphology, and properties of the PCN-222(Cu)/TiO2 composite. A series of characterizations including X-ray diffraction (XRD), scanning electron microscopy (SEM), diffuse reflectance spectroscopy (DRS), Fourier-transform infrared spectroscopy, photo-electrochemical, and photoluminescence (PL) confirm the successful preparation of PCN-222(Cu)/TiO2 composites. SEM reveals that the TiO2 nanoparticles are uniformly distributed on the surface of the rod-shaped PCN-222(Cu)/TiO2. XRD results show that PCN-222(Cu) and PCN-222(Cu)/TiO2 composite photocatalysts with good crystal structure were successfully synthesized. According to the DRS results, the prepared PCN-222(Cu)/TiO2 composite samples exhibit characteristic absorption peaks of metalloporphyrins in the visible region. PL spectroscopy, transient photocurrent response, and electrochemical impedance spectroscopy further confirm that the rod-like PCN-222(Cu)/TiO2 samples have high electron-hole pair separation efficiency. By controlling the mass ratio of PCN-222(Cu) and TiO2, the photocatalytic CO2 reduction performance test shows that the 10% PCN-222(Cu)/TiO2 composite achieves optimal catalytic performance, yielding 13.24 μmol·g−1·h−1 CO and 1.73 μmol·g−1·h−1 CH4, respectively. All the rod-like PCN-222(Cu)/TiO2 composites exhibit better photocatalytic CO2 activity than that of TiO2 nanoparticles or PCN-222(Cu) under the illumination of xenon lamps, which is attributed to charge transport and electron-hole separation capabilities. After three test cycles, the catalytic activity of PCN-222(Cu)/TiO2 photocatalyst was virtually unchanged. The reduction yield of the catalyst increased for 8 h under continuous illumination, indicating that PCN-222(Cu)/TiO2 composites have acceptable stability. The estimation of the band gap curve and the Mote-Schottky curve test show that the lowest unoccupied molecular orbital position of PCN-222(Cu) is more negative than the TiO2 of the conduction band; hence, a possible photocatalytic reaction mechanism of the PCN-222(Cu)/TiO2 composite is proposed. This study provides a new strategy for the integration of metal-organic frameworks and oxide semiconductors to construct efficient photocatalytic systems.
2020, 36(3): 190500
doi: 10.3866/PKU.WHXB201905007
Abstract:
Converting solar light into chemical energy is currently a hot topic for addressing the worldwide energy and environmental crises. However, the utilization of solar energy greatly suffers from its low energy flow density and discontinuous space-time distribution, which are essential for a reasonable energy conversion strategy toward effective storage and utilization. To this end, photocatalytic water splitting is a promising method for utilizing solar light to produce environmentally friendly hydrogen energy; yet, the efficiency needs to be improved. Generally, such processes can be divided into three elementary steps: light absorption, charge separation and migration, and surface redox reaction. The overall performance is determined by the cumulative efficiencies of the above three steps. The construction of cocatalysts is among the extensive efforts taken to improve the solar conversion efficiency. First, the cocatalysts possess higher work function than the semiconductors, and the photogenerated electrons migrate from semiconductor to cocatalysts, thereby promoting the charge separation. Second, cocatalysts usually lower the activation energy and provide abundant surface reactive sites. Particularly, the addition of cocatalysts can remarkably accelerate the four-electron transfer O2 evolution kinetics, which usually requires much higher overpotential and is often considered as the bottleneck for water splitting. Third, cocatalysts can timely remove the photogenerated charges from the surface of the semiconductor and subsequently inhibit the photocorrosion and improve the stability of the photocatalysts. Moreover, the cocatalysts also retard the backward recombination of H2 and O2. In general, cocatalysts for water splitting can be classified into three categories: H2 evolution cocatalysts, O2 evolution cocatalysts, and dual cocatalysts. The H2 evolution cocatalysts mainly contain noble metals such as Pt, Au, and other transition metals such as Co, Ni, and Cu and their phosphides or sulfides, which are capable of trapping electrons and promoting proton reduction. The O2 evolution cocatalysts are often noble metal oxides and transition metal (hydro)oxides and corresponding phosphates, which are always efficient in adsorbing and dissociating water molecules. To realize the overall water splitting, H2 evolution cocatalysts and O2 evolution cocatalysts are often integrated on one photocatalyst, which results in the so-called dual cocatalyst system. Furthermore, the performance of cocatalysts can be improved by modulating the loading amount, morphology, particle size, etc. In addition, composites such as Pt/Ni(OH)2 cocatalyst can not only provide both H2 and O2 evolution sites but also accelerate the intrinsic surface redox kinetics by promoting H2O activation, thus being much more active than the conventional dual cocatalyst system. This review summarizes the important role and design principle of cocatalysts in photocatalytic systems. The construction and functional mechanism of H2 evolution cocatalyst, O2 evolution cocatalyst, and dual cocatalysts in overall water splitting photocatalysts are discussed in detail, and the design strategy of new cocatalysts toward water activation is proposed. ![]()
Converting solar light into chemical energy is currently a hot topic for addressing the worldwide energy and environmental crises. However, the utilization of solar energy greatly suffers from its low energy flow density and discontinuous space-time distribution, which are essential for a reasonable energy conversion strategy toward effective storage and utilization. To this end, photocatalytic water splitting is a promising method for utilizing solar light to produce environmentally friendly hydrogen energy; yet, the efficiency needs to be improved. Generally, such processes can be divided into three elementary steps: light absorption, charge separation and migration, and surface redox reaction. The overall performance is determined by the cumulative efficiencies of the above three steps. The construction of cocatalysts is among the extensive efforts taken to improve the solar conversion efficiency. First, the cocatalysts possess higher work function than the semiconductors, and the photogenerated electrons migrate from semiconductor to cocatalysts, thereby promoting the charge separation. Second, cocatalysts usually lower the activation energy and provide abundant surface reactive sites. Particularly, the addition of cocatalysts can remarkably accelerate the four-electron transfer O2 evolution kinetics, which usually requires much higher overpotential and is often considered as the bottleneck for water splitting. Third, cocatalysts can timely remove the photogenerated charges from the surface of the semiconductor and subsequently inhibit the photocorrosion and improve the stability of the photocatalysts. Moreover, the cocatalysts also retard the backward recombination of H2 and O2. In general, cocatalysts for water splitting can be classified into three categories: H2 evolution cocatalysts, O2 evolution cocatalysts, and dual cocatalysts. The H2 evolution cocatalysts mainly contain noble metals such as Pt, Au, and other transition metals such as Co, Ni, and Cu and their phosphides or sulfides, which are capable of trapping electrons and promoting proton reduction. The O2 evolution cocatalysts are often noble metal oxides and transition metal (hydro)oxides and corresponding phosphates, which are always efficient in adsorbing and dissociating water molecules. To realize the overall water splitting, H2 evolution cocatalysts and O2 evolution cocatalysts are often integrated on one photocatalyst, which results in the so-called dual cocatalyst system. Furthermore, the performance of cocatalysts can be improved by modulating the loading amount, morphology, particle size, etc. In addition, composites such as Pt/Ni(OH)2 cocatalyst can not only provide both H2 and O2 evolution sites but also accelerate the intrinsic surface redox kinetics by promoting H2O activation, thus being much more active than the conventional dual cocatalyst system. This review summarizes the important role and design principle of cocatalysts in photocatalytic systems. The construction and functional mechanism of H2 evolution cocatalyst, O2 evolution cocatalyst, and dual cocatalysts in overall water splitting photocatalysts are discussed in detail, and the design strategy of new cocatalysts toward water activation is proposed.
2020, 36(3): 190502
doi: 10.3866/PKU.WHXB201905025
Abstract:
Increasing climate change and environmental pollution caused by the excessive use of fossil fuels have prompted intensive research into clean and efficient renewable sources as a substitute for traditional fossil fuels. A very promising approach is to mimic the water splitting process that occurs in plants during photosynthesis, in order to convert solar energy into chemical energy. A successful water splitting reaction, which comprises two half reactions (water oxidation and the reduction of protons), can generate H2 and O2 from water. Hydrogen is a promising renewable energy carrier because of its clean combustion and high calorific value. Light-driven water splitting is considered to be a feasible way to transform water and solar energy into hydrogen energy. However, water oxidation is considered to be the bottleneck process of water splitting because it advances in a thermodynamically uphill manner with the involvement of 4e− and 4H+. Inspired by the nature of Mn4CaO5 in photosystem Ⅱ (PS Ⅱ), the comprehensive understanding of its key features for use in active molecular water oxidation catalysts (WOCs) remains challenging. Extensive effort has been devoted to researching and manufacturing the structure and biomimicking the catalytic activity of Mn4CaO5 clusters that contain the Mn3CaO4 cubane structure, for the construction of low-cost and robust WOCs. WOCs can be divided into heterogeneous and homogeneous catalysts. Although heterogeneous WOCs are convenient for recycling and are easily prepared on a large scale, homogeneous WOCs, especially complexes based on organic ligands or polyoxometalates (POMs), have more advantages owing to their catalytic efficiency, structural modifications, and mechanistic understanding. Thus, recently, some molecules with an M4O4 (M = transition metals, mainly Mn, Co, Ni, and Cu) cubic structure have been reportedly used as photocatalytic WOCs. In this review, we present an overview of the most important and recent advances based on M4O4 cubic WOCs that contain first-row transition metal cubanes for visible light-driven water oxidation. Our main focus is on the structure of cubane catalysts, including metal complexes, POMs, and a system containing BiVO4 or polymeric carbon nitride (PCN) as a photosensitizer, and cubic complexes as WOCs. Results have shown that the activity and stability of the catalyst can be tuned by the ligand stability, metal center, coordination environment, and other factors. This review will be helpful for designing new cubane catalysts for photocatalytic water oxidation that are highly efficient and stable. ![]()
Increasing climate change and environmental pollution caused by the excessive use of fossil fuels have prompted intensive research into clean and efficient renewable sources as a substitute for traditional fossil fuels. A very promising approach is to mimic the water splitting process that occurs in plants during photosynthesis, in order to convert solar energy into chemical energy. A successful water splitting reaction, which comprises two half reactions (water oxidation and the reduction of protons), can generate H2 and O2 from water. Hydrogen is a promising renewable energy carrier because of its clean combustion and high calorific value. Light-driven water splitting is considered to be a feasible way to transform water and solar energy into hydrogen energy. However, water oxidation is considered to be the bottleneck process of water splitting because it advances in a thermodynamically uphill manner with the involvement of 4e− and 4H+. Inspired by the nature of Mn4CaO5 in photosystem Ⅱ (PS Ⅱ), the comprehensive understanding of its key features for use in active molecular water oxidation catalysts (WOCs) remains challenging. Extensive effort has been devoted to researching and manufacturing the structure and biomimicking the catalytic activity of Mn4CaO5 clusters that contain the Mn3CaO4 cubane structure, for the construction of low-cost and robust WOCs. WOCs can be divided into heterogeneous and homogeneous catalysts. Although heterogeneous WOCs are convenient for recycling and are easily prepared on a large scale, homogeneous WOCs, especially complexes based on organic ligands or polyoxometalates (POMs), have more advantages owing to their catalytic efficiency, structural modifications, and mechanistic understanding. Thus, recently, some molecules with an M4O4 (M = transition metals, mainly Mn, Co, Ni, and Cu) cubic structure have been reportedly used as photocatalytic WOCs. In this review, we present an overview of the most important and recent advances based on M4O4 cubic WOCs that contain first-row transition metal cubanes for visible light-driven water oxidation. Our main focus is on the structure of cubane catalysts, including metal complexes, POMs, and a system containing BiVO4 or polymeric carbon nitride (PCN) as a photosensitizer, and cubic complexes as WOCs. Results have shown that the activity and stability of the catalyst can be tuned by the ligand stability, metal center, coordination environment, and other factors. This review will be helpful for designing new cubane catalysts for photocatalytic water oxidation that are highly efficient and stable.
2020, 36(3): 190604
doi: 10.3866/PKU.WHXB201906048
Abstract:
Carbon dioxide is the most common compound. As a potential source of carbon, it can be used to prepare a variety of high value-added chemicals, such as carbon monoxide, methane, methanol, and formic acid. The traditional method of thermal catalytic conversion of CO2 requires high energy consumption and harsh reaction conditions. Therefore, the efficient conversion of CO2 to value-added chemicals under mild conditions has long been an area of great interest in the field of catalysis. Photocatalysis usually takes place under mild reaction conditions and is environmentally friendly. However, pure photocatalytic reactions generally have a limited solar energy utilization efficiency and low separation efficiency of photogenerated charge carriers. In view of the above problems, the introduction of electrocatalysis on the basis of photocatalysis can improve the charge separation efficiency. At a lower overpotential, multi-electrons and protons can be transferred to CO2, thus improving the catalytic reaction efficiency. Photoelectrochemical catalysis combines the advantages of photocatalysis and electrocatalysis to improve the efficiency of the catalytic reduction of CO2, offering a new method for the clean utilization of CO2. According to the principle of photocatalysis, the absorption capacity of a semiconductor is governed by its band structure. Optimization of the band structure is a major strategy to enhance the absorptivity of photocatalysts. In addition, the loading of light-absorbent materials on photocatalysts is an effective way to enhance the photocatalytic absorption of a photocatalytic system. During photoelectrocatalytic CO2 reduction, numerous photogenerated charge carriers recombine in bulk and on the surface of the catalyst, greatly reducing the efficiency of the catalytic reaction. Therefore, increasing the separation efficiency of charge carriers is an important means to improve the photoelectrocatalytic efficiency. In photoelectrocatalytic CO2 reduction, heterojunction construction and electric field formation often lead to the efficient separation of charge carriers. The interfacial reaction is a crucial step in the photoelectrocatalytic process. After generation, the photogenerated charge carriers need to migrate to the surface of the catalyst to participate in the redox reaction. In photoelectrocatalytic CO2 reduction, electrons participate in the reduction of CO2, while holes participate in the oxidation of water. Studies show that acceleration of the interfacial reaction process is of paramount importance for improving the efficiency of the photoelectrocatalytic reduction of CO2. This review summarizes the basic enhancement strategies of photoelectrocatalytic CO2 reduction from three aspects: light absorption, charge separation, and surface reaction, based on the basic mechanism of the reduction. The future prospects and research areas are also proposed. ![]()
Carbon dioxide is the most common compound. As a potential source of carbon, it can be used to prepare a variety of high value-added chemicals, such as carbon monoxide, methane, methanol, and formic acid. The traditional method of thermal catalytic conversion of CO2 requires high energy consumption and harsh reaction conditions. Therefore, the efficient conversion of CO2 to value-added chemicals under mild conditions has long been an area of great interest in the field of catalysis. Photocatalysis usually takes place under mild reaction conditions and is environmentally friendly. However, pure photocatalytic reactions generally have a limited solar energy utilization efficiency and low separation efficiency of photogenerated charge carriers. In view of the above problems, the introduction of electrocatalysis on the basis of photocatalysis can improve the charge separation efficiency. At a lower overpotential, multi-electrons and protons can be transferred to CO2, thus improving the catalytic reaction efficiency. Photoelectrochemical catalysis combines the advantages of photocatalysis and electrocatalysis to improve the efficiency of the catalytic reduction of CO2, offering a new method for the clean utilization of CO2. According to the principle of photocatalysis, the absorption capacity of a semiconductor is governed by its band structure. Optimization of the band structure is a major strategy to enhance the absorptivity of photocatalysts. In addition, the loading of light-absorbent materials on photocatalysts is an effective way to enhance the photocatalytic absorption of a photocatalytic system. During photoelectrocatalytic CO2 reduction, numerous photogenerated charge carriers recombine in bulk and on the surface of the catalyst, greatly reducing the efficiency of the catalytic reaction. Therefore, increasing the separation efficiency of charge carriers is an important means to improve the photoelectrocatalytic efficiency. In photoelectrocatalytic CO2 reduction, heterojunction construction and electric field formation often lead to the efficient separation of charge carriers. The interfacial reaction is a crucial step in the photoelectrocatalytic process. After generation, the photogenerated charge carriers need to migrate to the surface of the catalyst to participate in the redox reaction. In photoelectrocatalytic CO2 reduction, electrons participate in the reduction of CO2, while holes participate in the oxidation of water. Studies show that acceleration of the interfacial reaction process is of paramount importance for improving the efficiency of the photoelectrocatalytic reduction of CO2. This review summarizes the basic enhancement strategies of photoelectrocatalytic CO2 reduction from three aspects: light absorption, charge separation, and surface reaction, based on the basic mechanism of the reduction. The future prospects and research areas are also proposed.
2020, 36(3): 190701
doi: 10.3866/PKU.WHXB201907013
Abstract:
Methane is a promising energy source with vast reserves, and is considered one of the promising alternatives to nonrenewable petroleum resources because it can be converted into valuable hydrocarbon feedstocks and hydrogen through appropriate reactions. Recently, the conversion of CH4 into other high-value-added products has received increasing attention because of their sustainability for energy and the environment. However, methane has a tetrahedral geometry with four equivalent C―H bonds due to the sp3 hybridization of the central carbon atom, with a C―H bond length of 0.1087 nm and an H-C―H bond angle of 109.5°. The absence of a dipole moment and the small polarizability (2.84 × 10−40 C2·m2·J−1) imply that methane requires a high local electric field for polarization and for nucleophilic or electrophilic attack. Nevertheless, it is believed that an effective method to activate CH4 would be available, so that not only methanol, formaldehyde, and ethylene but also other industrially valuable raw materials can be obtained. On the other hand, the conversion of this combustible gas into the corresponding liquid fossil fuel proceeds via secondary chemical conversion, and it can greatly reduce transportation costs. From the economic viewpoint, this can still provide considerable benefits. Homogeneous catalysts have been reported to catalyze methane, but most of them operate at high pressures (2–7 MPa), or in strongly acidic media and at high temperatures (up to 500 K). Heterogeneous catalysts reported in the literature are also active only at high temperatures. Therefore, finding an efficient method to active methane has become a hot research topic. Photocatalysis technology is recognized as the optimal solution for the conversion of CH4 since solar energy is by far the largest exploitable resource of energy. In the past years, much effort has been undertaken for the conversion of CH4 under light at low temperature. In this regard, several photocatalysts, including silica-alumina-titania, silica-supported oxides, and ceria- and zeolite-based materials, have been developed. In photocatalytic methane conversion, the C―H bond can be selectively activated by adjusting the wavelength and intensity of the incident light and the oxidation capacity of the photocatalysts, thereby avoiding the formation of byproducts. This review summarizes a series of photocatalytic direct methane conversion systems developed in recent years, including methane oxidation and coupling processes. The effects of the catalyst composition and structure, oxidant, and electron transfer on the activation of the C―H bond of methane are detailed. Finally, future perspectives and challenges for the photocatalytic conversion of methane are discussed. ![]()
Methane is a promising energy source with vast reserves, and is considered one of the promising alternatives to nonrenewable petroleum resources because it can be converted into valuable hydrocarbon feedstocks and hydrogen through appropriate reactions. Recently, the conversion of CH4 into other high-value-added products has received increasing attention because of their sustainability for energy and the environment. However, methane has a tetrahedral geometry with four equivalent C―H bonds due to the sp3 hybridization of the central carbon atom, with a C―H bond length of 0.1087 nm and an H-C―H bond angle of 109.5°. The absence of a dipole moment and the small polarizability (2.84 × 10−40 C2·m2·J−1) imply that methane requires a high local electric field for polarization and for nucleophilic or electrophilic attack. Nevertheless, it is believed that an effective method to activate CH4 would be available, so that not only methanol, formaldehyde, and ethylene but also other industrially valuable raw materials can be obtained. On the other hand, the conversion of this combustible gas into the corresponding liquid fossil fuel proceeds via secondary chemical conversion, and it can greatly reduce transportation costs. From the economic viewpoint, this can still provide considerable benefits. Homogeneous catalysts have been reported to catalyze methane, but most of them operate at high pressures (2–7 MPa), or in strongly acidic media and at high temperatures (up to 500 K). Heterogeneous catalysts reported in the literature are also active only at high temperatures. Therefore, finding an efficient method to active methane has become a hot research topic. Photocatalysis technology is recognized as the optimal solution for the conversion of CH4 since solar energy is by far the largest exploitable resource of energy. In the past years, much effort has been undertaken for the conversion of CH4 under light at low temperature. In this regard, several photocatalysts, including silica-alumina-titania, silica-supported oxides, and ceria- and zeolite-based materials, have been developed. In photocatalytic methane conversion, the C―H bond can be selectively activated by adjusting the wavelength and intensity of the incident light and the oxidation capacity of the photocatalysts, thereby avoiding the formation of byproducts. This review summarizes a series of photocatalytic direct methane conversion systems developed in recent years, including methane oxidation and coupling processes. The effects of the catalyst composition and structure, oxidant, and electron transfer on the activation of the C―H bond of methane are detailed. Finally, future perspectives and challenges for the photocatalytic conversion of methane are discussed.
2020, 36(3): 190506
doi: 10.3866/PKU.WHXB201905068
Abstract:
The photocatalytic hydrogen evolution reaction (PHER) has gained much attention as a promising strategy for the generation of clean energy. As opposed to conventional hydrogen evolution strategies (steam methane reforming, electrocatalytic hydrogen evolution, etc.), the PHER is an environmentally friendly and sustainable method for converting solar energy into H2 energy. However, the PHER remains unsuitable for industrial applications because of efficiency losses in three critical steps: light absorption, carrier separation, and surface reaction. In the past four decades, the processes responsible for these efficiency losses have been extensively studied. First, light absorption is the principal factor deciding the performance of most photocatalysts, and it is closely related to band-gap structure of photocatalysts. However, most of the existing photocatalysts have a wide bandgap, indicating a narrow light absorption range, which restricts the photocatalytic efficiency. Therefore, searching for novel semiconductors with a narrow bandgap and broadening the light absorption range of known photocatalysts is an important research direction. Second, only the photogenerated electrons and holes that migrate to the photocatalyst surface can participate in the reaction with H2O, whereas most of the photogenerated electrons and holes readily recombine with one another in the bulk phase of the photocatalysts. Hence, tremendous effort has been undertaken to shorten the charge transfer distance and enhance the electric conductivity of photocatalysts for improving the separation and transfer efficiency of photogenerated carriers. Third, the surface redox reaction is also an important process. Because water oxidation is a four-electron process, sluggish O2 evolution is the bottleneck in photocatalytic water splitting. The unreacted holes can easily recombine with electrons. Sacrificial agents are widely used in most catalytic systems to suppress charge carrier recombination by scavenging the photogenerated holes. Moreover, the low H2 evolution efficiency of most photocatalysts has encouraged researchers to introduce highly active sites on the photocatalyst surface. Based on the abovementioned three steps, multifarious strategies have been applied to modulate the physicochemical properties of semiconductor photocatalysts with the aim of improving the light absorption efficiency, suppressing carrier recombination, and accelerating the kinetics of surface reactions. The strategies include defect generation, localized surface plasmon resonance (LSPR), element doping, heterojunction fabrication, and cocatalyst loading. An in-depth study of these strategies provides guidance for the design of efficient photocatalysts. In this review, we focus on the mechanism and application of these strategies for optimizing light absorption, carrier separation and transport, and surface reactions. Furthermore, we provide a critical view on the promising trends toward the construction of advanced catalysts for H2 evolution. ![]()
The photocatalytic hydrogen evolution reaction (PHER) has gained much attention as a promising strategy for the generation of clean energy. As opposed to conventional hydrogen evolution strategies (steam methane reforming, electrocatalytic hydrogen evolution, etc.), the PHER is an environmentally friendly and sustainable method for converting solar energy into H2 energy. However, the PHER remains unsuitable for industrial applications because of efficiency losses in three critical steps: light absorption, carrier separation, and surface reaction. In the past four decades, the processes responsible for these efficiency losses have been extensively studied. First, light absorption is the principal factor deciding the performance of most photocatalysts, and it is closely related to band-gap structure of photocatalysts. However, most of the existing photocatalysts have a wide bandgap, indicating a narrow light absorption range, which restricts the photocatalytic efficiency. Therefore, searching for novel semiconductors with a narrow bandgap and broadening the light absorption range of known photocatalysts is an important research direction. Second, only the photogenerated electrons and holes that migrate to the photocatalyst surface can participate in the reaction with H2O, whereas most of the photogenerated electrons and holes readily recombine with one another in the bulk phase of the photocatalysts. Hence, tremendous effort has been undertaken to shorten the charge transfer distance and enhance the electric conductivity of photocatalysts for improving the separation and transfer efficiency of photogenerated carriers. Third, the surface redox reaction is also an important process. Because water oxidation is a four-electron process, sluggish O2 evolution is the bottleneck in photocatalytic water splitting. The unreacted holes can easily recombine with electrons. Sacrificial agents are widely used in most catalytic systems to suppress charge carrier recombination by scavenging the photogenerated holes. Moreover, the low H2 evolution efficiency of most photocatalysts has encouraged researchers to introduce highly active sites on the photocatalyst surface. Based on the abovementioned three steps, multifarious strategies have been applied to modulate the physicochemical properties of semiconductor photocatalysts with the aim of improving the light absorption efficiency, suppressing carrier recombination, and accelerating the kinetics of surface reactions. The strategies include defect generation, localized surface plasmon resonance (LSPR), element doping, heterojunction fabrication, and cocatalyst loading. An in-depth study of these strategies provides guidance for the design of efficient photocatalysts. In this review, we focus on the mechanism and application of these strategies for optimizing light absorption, carrier separation and transport, and surface reactions. Furthermore, we provide a critical view on the promising trends toward the construction of advanced catalysts for H2 evolution.
2020, 36(3): 190508
doi: 10.3866/PKU.WHXB201905080
Abstract:
Since Fujishima and Honda demonstrated the photoelectrochemical water splitting on TiO2 photoanode and Pt counter electrode, photocatalysis has been considered as one of the most promising technologies for solving both the problems of environmental pollution and energy shortage. This process can effectively use solar energy, the most abundant energy resource on the earth, to drive various catalytic reactions, such as water splitting, CO2 reduction, organic pollutant degradation, and organic synthesis, for energy generation and environmental purification. Except for the various metal-based semiconductors, such as metal oxides, metal sulfides, and metal oxynitrides, developed for photocatalysis, graphitic carbon nitride (g-C3N4) has attracted significant attention in the recent years because of its earth abundancy, non-toxicity, good stability, and relatively narrow band gap (2.7 eV) for visible light response. However, g-C3N4 suffers from insufficient absorption of visible light in the solar spectrum and rapid recombination of photogenerated electrons and holes, thus resulting in low photocatalytic activity. Until now, various strategies have been developed to enhance the photocatalytic activity of g-C3N4, including element doping, nanostructure and heterostructure design, and co-catalyst decoration. Among these methods, element doping has been found to be very effective for adjusting the unique electronic and molecular structures of g-C3N4, which could significantly expand the range of photoresponse under visible light and improve the charge separation. Especially, non-metal doping has been well investigated frequently to improve the photocatalytic activity of g-C3N4. The non-metal dopants commonly used for the doping of g-C3N4 include oxygen (O), phosphorus (P), sulfur (S), boron (B), and halogen (F, Cl, Br, I) and also carbon (C) and nitrogen (N) (for self-doping), as they are easily accessible and can be introduced into the g-C3N4 framework through different physical and chemical synthetic methods. In this review article, the structural and optical properties of g-C3N4 is introduced first, followed by a brief introduction to the modification of g-C3N4 as photocatalysts. Then, the progress in the non-metal doped g-C3N4 with improved photocatalytic activity is reviewed in detail, with the photocatalytic mechanisms presented for easy understanding of the fundamentals of photocatalysis and for guiding in the design of novel g-C3N4 photocatalysts. Finally, the prospects of the modification of g-C3N4 for further advances in photocatalysis is presented. ![]()
Since Fujishima and Honda demonstrated the photoelectrochemical water splitting on TiO2 photoanode and Pt counter electrode, photocatalysis has been considered as one of the most promising technologies for solving both the problems of environmental pollution and energy shortage. This process can effectively use solar energy, the most abundant energy resource on the earth, to drive various catalytic reactions, such as water splitting, CO2 reduction, organic pollutant degradation, and organic synthesis, for energy generation and environmental purification. Except for the various metal-based semiconductors, such as metal oxides, metal sulfides, and metal oxynitrides, developed for photocatalysis, graphitic carbon nitride (g-C3N4) has attracted significant attention in the recent years because of its earth abundancy, non-toxicity, good stability, and relatively narrow band gap (2.7 eV) for visible light response. However, g-C3N4 suffers from insufficient absorption of visible light in the solar spectrum and rapid recombination of photogenerated electrons and holes, thus resulting in low photocatalytic activity. Until now, various strategies have been developed to enhance the photocatalytic activity of g-C3N4, including element doping, nanostructure and heterostructure design, and co-catalyst decoration. Among these methods, element doping has been found to be very effective for adjusting the unique electronic and molecular structures of g-C3N4, which could significantly expand the range of photoresponse under visible light and improve the charge separation. Especially, non-metal doping has been well investigated frequently to improve the photocatalytic activity of g-C3N4. The non-metal dopants commonly used for the doping of g-C3N4 include oxygen (O), phosphorus (P), sulfur (S), boron (B), and halogen (F, Cl, Br, I) and also carbon (C) and nitrogen (N) (for self-doping), as they are easily accessible and can be introduced into the g-C3N4 framework through different physical and chemical synthetic methods. In this review article, the structural and optical properties of g-C3N4 is introduced first, followed by a brief introduction to the modification of g-C3N4 as photocatalysts. Then, the progress in the non-metal doped g-C3N4 with improved photocatalytic activity is reviewed in detail, with the photocatalytic mechanisms presented for easy understanding of the fundamentals of photocatalysis and for guiding in the design of novel g-C3N4 photocatalysts. Finally, the prospects of the modification of g-C3N4 for further advances in photocatalysis is presented.
2020, 36(3): 190905
doi: 10.3866/PKU.WHXB201909054
Abstract:
2020, 36(3): 191003
doi: 10.3866/PKU.WHXB201910034
Abstract:
2020, 36(3): 191100
doi: 10.3866/PKU.WHXB201911007
Abstract:
2020, 36(3): 191002
doi: 10.3866/PKU.WHXB201910023
Abstract:
