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2017 Volume 38 Issue 12
2017, 38(12):
Abstract:
2017, 38(12): 1935-1935
doi: 10.1016/S1872-2067(17)63000-6
Abstract:
2017, 38(12): 1936-1955
doi: 10.1016/S1872-2067(17)62962-0
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TiO2-based Z-scheme photocatalysts have attracted considerable attention because of the low recombination rate of their photogenerated electron-hole pairs and their high photocatalytic efficiency. In this review, the reaction mechanism of Z-scheme photocatalysts, recent research progress in the application of TiO2-based Z-scheme photocatalysts, and improved methods for photocatalytic performance enhancement are explored. Their applications, including water splitting, CO2 reduction, decomposition of volatile organic compounds, and degradation of organic pollutants, are also described. The main factors affecting the photocatalytic performance of TiO2-based Z-scheme photocatalysts, such as pH, conductive medium, cocatalyst, architecture, and mass ratio, are discussed. Concluding remarks are presented, and some suggestions for the future development of TiO2-based Z-scheme photocatalysts are highlighted.
TiO2-based Z-scheme photocatalysts have attracted considerable attention because of the low recombination rate of their photogenerated electron-hole pairs and their high photocatalytic efficiency. In this review, the reaction mechanism of Z-scheme photocatalysts, recent research progress in the application of TiO2-based Z-scheme photocatalysts, and improved methods for photocatalytic performance enhancement are explored. Their applications, including water splitting, CO2 reduction, decomposition of volatile organic compounds, and degradation of organic pollutants, are also described. The main factors affecting the photocatalytic performance of TiO2-based Z-scheme photocatalysts, such as pH, conductive medium, cocatalyst, architecture, and mass ratio, are discussed. Concluding remarks are presented, and some suggestions for the future development of TiO2-based Z-scheme photocatalysts are highlighted.
2017, 38(12): 1956-1969
doi: 10.1016/S1872-2067(17)62955-3
Abstract:
Photocatalytic conversion of "greenhouse gas" CO2 is considered to be one of the most effective ways to alleviate current energy and environmental problems without additional energy consumption and pollutant emission. The performance of many traditional semiconductor photocatalysts is not efficient enough to satisfy the requirements of practical applications because of their limited specific surface area and low CO2 adsorption capacity. Therefore, the exploration of photocatalysts with high CO2 uptake is significant in the field of CO2 conversion. Recently the porous materials appeared to be a kind of superior candidate for enriching the CO2 molecules on the surface of photocatalysts for catalytic conversion. This paper first summarizes the advances in the development of nanoporous adsorbents for CO2 capture. Three main classes of porous materials are considered:inorganic porous materials, metal organic frameworks, and microporous organic polymers. Based on systematic research on CO2 uptake, we then highlight the recent progress in these porous-material-based photocatalysts for CO2 conversion. Benefiting from the improved CO2 uptake capacity, the porous-material-based photocatalysts exhibited remarkably enhanced efficiency in the reduction of CO2 to chemical fuels, such as CO, CH4, and CH3OH. Based on reported recent achievements, we predict a trend of development in multifunctional materials with both high adsorption capability and photocatalytic performance for CO2 utilization.
Photocatalytic conversion of "greenhouse gas" CO2 is considered to be one of the most effective ways to alleviate current energy and environmental problems without additional energy consumption and pollutant emission. The performance of many traditional semiconductor photocatalysts is not efficient enough to satisfy the requirements of practical applications because of their limited specific surface area and low CO2 adsorption capacity. Therefore, the exploration of photocatalysts with high CO2 uptake is significant in the field of CO2 conversion. Recently the porous materials appeared to be a kind of superior candidate for enriching the CO2 molecules on the surface of photocatalysts for catalytic conversion. This paper first summarizes the advances in the development of nanoporous adsorbents for CO2 capture. Three main classes of porous materials are considered:inorganic porous materials, metal organic frameworks, and microporous organic polymers. Based on systematic research on CO2 uptake, we then highlight the recent progress in these porous-material-based photocatalysts for CO2 conversion. Benefiting from the improved CO2 uptake capacity, the porous-material-based photocatalysts exhibited remarkably enhanced efficiency in the reduction of CO2 to chemical fuels, such as CO, CH4, and CH3OH. Based on reported recent achievements, we predict a trend of development in multifunctional materials with both high adsorption capability and photocatalytic performance for CO2 utilization.
2017, 38(12): 1970-1980
doi: 10.1016/S1872-2067(17)62965-6
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CdS nanosheets (NSs) photocatalysts modified with dual earth-abundant co-catalysts of metallic carbon black (CB) and NiS2 were synthesized by a two-step solvother-mal/impregnation method. All the experiment results demonstrated that the co-loading of CB and NiS2 could significantly enhance the photocatalytic H2-evolution activity of CdS NSs. The photocatalytic performance of the as-prepared CdS/CB/NiS2 samples was tested under visible light (λ ≥ 420 nm) by using an aqueous solution containing 0.25 mol L-1 Na2S-Na2SO3 as the sacrifice agent. The CdS-0.5% CB-1.0%NiS2 composite photocatalysts exhibited the highest H2-evolution rate of 166.7 μmol h-1, which was approximately 5.16 and 1.87 times higher than those of pure CdS NSs and CdS-1.0%NiS2, respectively. The possible mechanism for the enhanced H2-evolution activity of CdS/CB/NiS2 composite photocatalysts was proposed. The results showed that the enhanced photocatalytic H2-evolution activities could be ascribed to the co-loading of metallic CB and NiS2 as co-catalysts onto the surface of CdS NSs. The excellent synergetic effect between the CB and NiS2 could obviously improve visible light absorption, promote separation of photogenerated electron-hole pairs and boost the H2-evolution kinetics, thus leading to an enhanced activity for H2 evolution. More interestingly, the metallic CB could not only act as a cocatalyst for H2 evolution, but also serve as a conductive electron bridge to promote the charge migration. This work not only demonstrates that loading CB as a co-catalyst is a promising strategy to further boost the photocatalytic activity of CdS/NiS2 composites, but also offers a new mechanistic insight into the construction of highly efficient and stable CdS NSs-based hybrid photocatalysts with dual earth-abundant co-catalysts for photocatalytic applications.
CdS nanosheets (NSs) photocatalysts modified with dual earth-abundant co-catalysts of metallic carbon black (CB) and NiS2 were synthesized by a two-step solvother-mal/impregnation method. All the experiment results demonstrated that the co-loading of CB and NiS2 could significantly enhance the photocatalytic H2-evolution activity of CdS NSs. The photocatalytic performance of the as-prepared CdS/CB/NiS2 samples was tested under visible light (λ ≥ 420 nm) by using an aqueous solution containing 0.25 mol L-1 Na2S-Na2SO3 as the sacrifice agent. The CdS-0.5% CB-1.0%NiS2 composite photocatalysts exhibited the highest H2-evolution rate of 166.7 μmol h-1, which was approximately 5.16 and 1.87 times higher than those of pure CdS NSs and CdS-1.0%NiS2, respectively. The possible mechanism for the enhanced H2-evolution activity of CdS/CB/NiS2 composite photocatalysts was proposed. The results showed that the enhanced photocatalytic H2-evolution activities could be ascribed to the co-loading of metallic CB and NiS2 as co-catalysts onto the surface of CdS NSs. The excellent synergetic effect between the CB and NiS2 could obviously improve visible light absorption, promote separation of photogenerated electron-hole pairs and boost the H2-evolution kinetics, thus leading to an enhanced activity for H2 evolution. More interestingly, the metallic CB could not only act as a cocatalyst for H2 evolution, but also serve as a conductive electron bridge to promote the charge migration. This work not only demonstrates that loading CB as a co-catalyst is a promising strategy to further boost the photocatalytic activity of CdS/NiS2 composites, but also offers a new mechanistic insight into the construction of highly efficient and stable CdS NSs-based hybrid photocatalysts with dual earth-abundant co-catalysts for photocatalytic applications.
2017, 38(12): 1981-1989
doi: 10.1016/S1872-2067(17)62936-X
Abstract:
Photocatalytic hydrogen production based on semiconductor photocatalysts has been considered as one of the most promising strategies to resolve the global energy shortage. Graphitic carbon nitride (g-C3N4) has been a star visible-light photocatalyst in this field due to its various advantages. However, pristine g-C3N4 usually exhibits limited activity. Herein, to enhance the performance of g-C3N4, alkali metal ion (Li+, Na+, or K+)-doped g-C3N4 are prepared via facile high-temperature treatment. The prepared samples are characterized and analyzed using the technique of XRD, ICP-AES, SEM, UV-vis DRS, BET, XPS, PL, TRPL, photoelectrochemical measurements, photocatalytic tests, etc. The resultant doped photocatalysts show enhanced visible-light photocatalytic activities for hydrogen production, benefiting from the increased specific surface areas (which provide more active sites), decreased band gaps for extended visible-light absorption, and improved electronic structures for efficient charge transfer. In particular, because of the optimal tuning of both microstructure and electronic structure, the Na-doped g-C3N4 shows the most effective utilization of photogenerated electrons during the water reduction process. As a result, the highest photocatalytic performance is achieved over the Na-doped g-C3N4 photocatalyst (18.7 μmol/h), 3.7 times that of pristine g-C3N4 (5.0 μmol/h). This work gives a systematic study for the understanding of doping effect of alkali metals in semiconductor photocatalysis.
Photocatalytic hydrogen production based on semiconductor photocatalysts has been considered as one of the most promising strategies to resolve the global energy shortage. Graphitic carbon nitride (g-C3N4) has been a star visible-light photocatalyst in this field due to its various advantages. However, pristine g-C3N4 usually exhibits limited activity. Herein, to enhance the performance of g-C3N4, alkali metal ion (Li+, Na+, or K+)-doped g-C3N4 are prepared via facile high-temperature treatment. The prepared samples are characterized and analyzed using the technique of XRD, ICP-AES, SEM, UV-vis DRS, BET, XPS, PL, TRPL, photoelectrochemical measurements, photocatalytic tests, etc. The resultant doped photocatalysts show enhanced visible-light photocatalytic activities for hydrogen production, benefiting from the increased specific surface areas (which provide more active sites), decreased band gaps for extended visible-light absorption, and improved electronic structures for efficient charge transfer. In particular, because of the optimal tuning of both microstructure and electronic structure, the Na-doped g-C3N4 shows the most effective utilization of photogenerated electrons during the water reduction process. As a result, the highest photocatalytic performance is achieved over the Na-doped g-C3N4 photocatalyst (18.7 μmol/h), 3.7 times that of pristine g-C3N4 (5.0 μmol/h). This work gives a systematic study for the understanding of doping effect of alkali metals in semiconductor photocatalysis.
2017, 38(12): 1990-1998
doi: 10.1016/S1872-2067(17)62971-1
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Silver-modified semiconductor photocatalysts typically exhibit enhanced photocatalytic activity toward the degradation of organic substances. In comparison, their hydrogen-evolution rates are relatively low owing to poor interfacial catalytic reactions to producing hydrogen. In the present study, thiocyanate anions (SCN-) as interfacial catalytic active sites were selectively adsorbed onto the Ag surface of g-C3N4/Ag photocatalyst to promote interfacial H2-evolution reactions. The thiocyanate-modified g-C3N4/Ag (g-C3N4/Ag-SCN) photocatalysts were synthesized via photodeposition of metallic Ag on g-C3N4 and subsequent selective adsorption of SCN- ions on the Ag surface by an impregnation method. The resulting g-C3N4/Ag-SCN photocatalysts exhibited considerably higher photocatalytic H2-evolution activity than the g-C3N4, g-C3N4/Ag, and g-C3N4/SCN photocatalysts. Furthermore, the g-C3N4/Ag-SCN photocatalyst displayed the highest H2-evolution rate (3.9 μmol h-1) when the concentration of the SCN- ions was adjusted to 0.3 mmol L-1. The H2-evolution rate obtained was higher than those of g-C3N4 (0.15 μmol h-1) and g-C3N4/Ag (0.71 μmol h-1). Considering the enhanced performance of g-C3N4/Ag upon minimal addition of SCN- ions, a synergistic effect of metallic Ag and SCN- ions is proposed-the Ag nanoparticles act as an effective electron-transfer mediator for the steady capture and rapid transportation of photogenerated electrons, while the adsorbed SCN- ions serve as an interfacial active site to effectively absorb protons from solution and promote rapid interfacial H2-evolution reactions. Considering the present facile synthesis and its high efficacy, the present work may provide new insights into preparing high-performance photocatalytic materials.
Silver-modified semiconductor photocatalysts typically exhibit enhanced photocatalytic activity toward the degradation of organic substances. In comparison, their hydrogen-evolution rates are relatively low owing to poor interfacial catalytic reactions to producing hydrogen. In the present study, thiocyanate anions (SCN-) as interfacial catalytic active sites were selectively adsorbed onto the Ag surface of g-C3N4/Ag photocatalyst to promote interfacial H2-evolution reactions. The thiocyanate-modified g-C3N4/Ag (g-C3N4/Ag-SCN) photocatalysts were synthesized via photodeposition of metallic Ag on g-C3N4 and subsequent selective adsorption of SCN- ions on the Ag surface by an impregnation method. The resulting g-C3N4/Ag-SCN photocatalysts exhibited considerably higher photocatalytic H2-evolution activity than the g-C3N4, g-C3N4/Ag, and g-C3N4/SCN photocatalysts. Furthermore, the g-C3N4/Ag-SCN photocatalyst displayed the highest H2-evolution rate (3.9 μmol h-1) when the concentration of the SCN- ions was adjusted to 0.3 mmol L-1. The H2-evolution rate obtained was higher than those of g-C3N4 (0.15 μmol h-1) and g-C3N4/Ag (0.71 μmol h-1). Considering the enhanced performance of g-C3N4/Ag upon minimal addition of SCN- ions, a synergistic effect of metallic Ag and SCN- ions is proposed-the Ag nanoparticles act as an effective electron-transfer mediator for the steady capture and rapid transportation of photogenerated electrons, while the adsorbed SCN- ions serve as an interfacial active site to effectively absorb protons from solution and promote rapid interfacial H2-evolution reactions. Considering the present facile synthesis and its high efficacy, the present work may provide new insights into preparing high-performance photocatalytic materials.
2017, 38(12): 1999-2008
doi: 10.1016/S1872-2067(17)62926-7
Abstract:
A series of N-CQDs/Ag2CO3 composite crystals (where N-CQDs=Nitrogen doped carbon quantum dots) were prepared by adding different volumes of a solution of N-CQDs during Ag2CO3 crystal growth. Under irradiation from a 350-W Xe lamp light (with optical filter, λ ≥ 420 nm), the performance of N-CQDs/Ag2CO3 in photocatalytic degradation of phenol was evaluated. The as-prepared samples were analyzed by XRD, SEM, TEM, BET, element mapping, UV-vis DRS, FT-IR, XPS, transient photocurrent response and EIS testing. The results showed that after coupling with trace amounts of N-CQDs, both the photocatalytic activity and stability of Ag2CO3 were greatly boosted. The addition of N-CQDs solution influenced the crystallization of Ag2CO3, resulting in a distinct decrease in Ag2CO3 crystal size and an obvious increase in surface area. Moreover, the charge transfer resistance was greatly reduced, and the separation efficiency of photogenerated electrons and holes was strongly promoted. The presence of NCQDs on the surface of the catalysts facilitates the transfer of photogenerated electrons, slowing the photocorrosion rate of Ag2CO3, and then resulting in higher stability than bare Ag2CO3 in degradation. The synergistic effect of the improvement of morphology and charge transfer rate thus accounted for the superior photocatalytic performance of N-CQDs/Ag2CO3.
A series of N-CQDs/Ag2CO3 composite crystals (where N-CQDs=Nitrogen doped carbon quantum dots) were prepared by adding different volumes of a solution of N-CQDs during Ag2CO3 crystal growth. Under irradiation from a 350-W Xe lamp light (with optical filter, λ ≥ 420 nm), the performance of N-CQDs/Ag2CO3 in photocatalytic degradation of phenol was evaluated. The as-prepared samples were analyzed by XRD, SEM, TEM, BET, element mapping, UV-vis DRS, FT-IR, XPS, transient photocurrent response and EIS testing. The results showed that after coupling with trace amounts of N-CQDs, both the photocatalytic activity and stability of Ag2CO3 were greatly boosted. The addition of N-CQDs solution influenced the crystallization of Ag2CO3, resulting in a distinct decrease in Ag2CO3 crystal size and an obvious increase in surface area. Moreover, the charge transfer resistance was greatly reduced, and the separation efficiency of photogenerated electrons and holes was strongly promoted. The presence of NCQDs on the surface of the catalysts facilitates the transfer of photogenerated electrons, slowing the photocorrosion rate of Ag2CO3, and then resulting in higher stability than bare Ag2CO3 in degradation. The synergistic effect of the improvement of morphology and charge transfer rate thus accounted for the superior photocatalytic performance of N-CQDs/Ag2CO3.
2017, 38(12): 2009-2020
doi: 10.1016/S1872-2067(17)62935-8
Abstract:
In the present study, zinc molybdate (β-ZnMoO4) and graphitic carbon nitride (g-C3N4)-modified β-ZnMoO4 (β-ZnMoO4/g-C3N4) were prepared to decontaminate aqueous solutions from the antibiotic sulfamethazine (SMZ). Our results revealed that the hydrothermal synthesis method greatly influenced the photocatalytic activity of the resultant catalysts. The pristine β-ZnMoO4 samples obtained under more intensive synthesis conditions (24 h at 280℃) showed higher photocatalytic activity than that prepared for 12 h at 180℃ (denoted β-ZnMoO4-180). In the case of in situ hydrothermal synthesis of β-ZnMoO4/g-C3N4, a surface-modified sample was only obtained under the reaction conditions of 180℃ for 12 h. Compared with the sheet-like β-ZnMoO4-180 sample, the β-ZnMoO4-180/g-C3N4 composite showed enhanced photocatalytic activity for the degradation of SMZ. By contrast, the hydrothermal reaction at 280℃ caused the gradual decomposition of g-C3N4. It is believed that the structural incorporation of g-C3N4 into β-ZnMoO4 at 280℃ might disrupt the crystal growth, thereby deteriorating the performance of the composite catalysts formed at this temperature. For the composite catalysts prepared by the ultrasonic method, a remarkable increase in the degradation rate of SMZ was only observed at a high g-C3N4 content of 8 mol%. The photocatalytic degradation of SMZ by β-ZnMoO4-180/g-C3N4 composite catalysts followed pseudo-first-order kinetics. Further study of the photocatalytic mechanism revealed that holes and superoxide radicals were the dominant oxidative species in the photodegradation process. The enhanced photocatalytic performance of the composites was attributed to the higher separation efficiency of the photogenerated electron-hole pairs at heterogeneous junctions. The degradation intermediates of SMZ were detected by liquid chromatography-mass spectrometry, from which plausible reaction pathways for the photodegradation of SMZ were proposed. Our results indicated that the synthesis method for g-C3N4 composites should be care-fully selected to achieve superior photocatalytic performance.
In the present study, zinc molybdate (β-ZnMoO4) and graphitic carbon nitride (g-C3N4)-modified β-ZnMoO4 (β-ZnMoO4/g-C3N4) were prepared to decontaminate aqueous solutions from the antibiotic sulfamethazine (SMZ). Our results revealed that the hydrothermal synthesis method greatly influenced the photocatalytic activity of the resultant catalysts. The pristine β-ZnMoO4 samples obtained under more intensive synthesis conditions (24 h at 280℃) showed higher photocatalytic activity than that prepared for 12 h at 180℃ (denoted β-ZnMoO4-180). In the case of in situ hydrothermal synthesis of β-ZnMoO4/g-C3N4, a surface-modified sample was only obtained under the reaction conditions of 180℃ for 12 h. Compared with the sheet-like β-ZnMoO4-180 sample, the β-ZnMoO4-180/g-C3N4 composite showed enhanced photocatalytic activity for the degradation of SMZ. By contrast, the hydrothermal reaction at 280℃ caused the gradual decomposition of g-C3N4. It is believed that the structural incorporation of g-C3N4 into β-ZnMoO4 at 280℃ might disrupt the crystal growth, thereby deteriorating the performance of the composite catalysts formed at this temperature. For the composite catalysts prepared by the ultrasonic method, a remarkable increase in the degradation rate of SMZ was only observed at a high g-C3N4 content of 8 mol%. The photocatalytic degradation of SMZ by β-ZnMoO4-180/g-C3N4 composite catalysts followed pseudo-first-order kinetics. Further study of the photocatalytic mechanism revealed that holes and superoxide radicals were the dominant oxidative species in the photodegradation process. The enhanced photocatalytic performance of the composites was attributed to the higher separation efficiency of the photogenerated electron-hole pairs at heterogeneous junctions. The degradation intermediates of SMZ were detected by liquid chromatography-mass spectrometry, from which plausible reaction pathways for the photodegradation of SMZ were proposed. Our results indicated that the synthesis method for g-C3N4 composites should be care-fully selected to achieve superior photocatalytic performance.
2017, 38(12): 2021-2029
doi: 10.1016/S1872-2067(17)62942-5
Abstract:
Ag3PO4 has good potential for use in photocatalytic degradation of organic contaminants. However, the activity and stability of Ag3PO4 is hard to sustain because of photocorrosion and the positive potential of the conduction band of Ag3PO4. In this study, A composite consisting of Bi2WO6 nanosheets and Ag3PO4 was developed to curb recombination of charge carriers and enhance the activity and stability of the catalyst. Formation of a Ag3PO4/Bi2WO6 composite was confirmed using X-ray diffraction, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. Photoluminescence spectroscopy provided convincing evidence that compositing Bi2WO6 with Ag3PO4 effectively reduced photocorrosion of Ag3PO4. The Ag3PO4/Bi2WO6 composite gave a high photocatalytic performance in photodegradation of methylene blue. A degradation rate of 0.61 min-1 was achieved; this is 1.3 and 6.0 times higher than those achieved using Ag3PO4 (0.47 min-1) and Bi2WO6 (0.10 min-1), respectively. Reactive species trapping experiments using the Ag3PO4/Bi2WO6 composite showed that holes, ·OH, and ·O2-all played specific roles in the photodegradation process. The photocatalytic mechanism was investigated and a Z-scheme was proposed as a plausible mechanism.
Ag3PO4 has good potential for use in photocatalytic degradation of organic contaminants. However, the activity and stability of Ag3PO4 is hard to sustain because of photocorrosion and the positive potential of the conduction band of Ag3PO4. In this study, A composite consisting of Bi2WO6 nanosheets and Ag3PO4 was developed to curb recombination of charge carriers and enhance the activity and stability of the catalyst. Formation of a Ag3PO4/Bi2WO6 composite was confirmed using X-ray diffraction, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. Photoluminescence spectroscopy provided convincing evidence that compositing Bi2WO6 with Ag3PO4 effectively reduced photocorrosion of Ag3PO4. The Ag3PO4/Bi2WO6 composite gave a high photocatalytic performance in photodegradation of methylene blue. A degradation rate of 0.61 min-1 was achieved; this is 1.3 and 6.0 times higher than those achieved using Ag3PO4 (0.47 min-1) and Bi2WO6 (0.10 min-1), respectively. Reactive species trapping experiments using the Ag3PO4/Bi2WO6 composite showed that holes, ·OH, and ·O2-all played specific roles in the photodegradation process. The photocatalytic mechanism was investigated and a Z-scheme was proposed as a plausible mechanism.
2017, 38(12): 2030-2038
doi: 10.1016/S1872-2067(17)62941-3
Abstract:
Bi12O17Br2 and Bi4O5Br2 visible-light driven photocatalysts, were respectively fabricated by hydrothermal and room-temperature deposition methods with the use of BiBr3 and NaOH as precursors. Both Bi12O17Br2 and Bi4O5Br2 were composed of irregular nanosheets. The Bi4O5Br2 nanosheets exhibited high and stable visible-light photocatalytic efficiency for ppb-level NO removal. The performance of Bi4O5Br2 was markedly higher than that of the Bi12O17Br2 nanosheets. The hydroxyl radical (·OH) was determined to be the main reactive oxygen species for the photo-degradation processes of both Bi12O17Br2 and Bi4O5Br2. However, in situ diffuse reflectance infrared Fourier transform spectroscopy analysis revealed that Bi12O17Br2 and Bi4O5Br2 featured different conversion pathways for visible light driven photocatalytic NO oxidation. The excellent photocatalytic activity of Bi4O5Br2 resulted from a high surface area and large pore volumes, which facilitated the transport of reactants and intermediate products, and provided more active sites for photochemical reaction. Furthermore, the Bi4O5Br2 nanosheets produced more ·OH and presented stronger valence band hole oxidation. In addition, the oxygen atoms of NO could insert into oxygen-vacancies of Bi4O5Br2, which provided more active sites for the reaction. This work gives insight into the photocatalytic pollutant-degradation mechanism of bismuth oxyhalide.
Bi12O17Br2 and Bi4O5Br2 visible-light driven photocatalysts, were respectively fabricated by hydrothermal and room-temperature deposition methods with the use of BiBr3 and NaOH as precursors. Both Bi12O17Br2 and Bi4O5Br2 were composed of irregular nanosheets. The Bi4O5Br2 nanosheets exhibited high and stable visible-light photocatalytic efficiency for ppb-level NO removal. The performance of Bi4O5Br2 was markedly higher than that of the Bi12O17Br2 nanosheets. The hydroxyl radical (·OH) was determined to be the main reactive oxygen species for the photo-degradation processes of both Bi12O17Br2 and Bi4O5Br2. However, in situ diffuse reflectance infrared Fourier transform spectroscopy analysis revealed that Bi12O17Br2 and Bi4O5Br2 featured different conversion pathways for visible light driven photocatalytic NO oxidation. The excellent photocatalytic activity of Bi4O5Br2 resulted from a high surface area and large pore volumes, which facilitated the transport of reactants and intermediate products, and provided more active sites for photochemical reaction. Furthermore, the Bi4O5Br2 nanosheets produced more ·OH and presented stronger valence band hole oxidation. In addition, the oxygen atoms of NO could insert into oxygen-vacancies of Bi4O5Br2, which provided more active sites for the reaction. This work gives insight into the photocatalytic pollutant-degradation mechanism of bismuth oxyhalide.
2017, 38(12): 2039-2047
doi: 10.1016/S1872-2067(17)62953-X
Abstract:
The nickel-based complex Ni-CH3CH2NH2-intercalated niobate layered perovskite Ni-CH3CH2NH2/H1.78Sr0.78Bi0.22Nb2O7 was synthesized via a facile in situ chemical reaction method. Using ultrathin H1.78Sr0.78Bi0.22Nb2O7 nanosheets and nickel acetate as precursors. The composition, structure, photophysical properties, and photocatalytic activity for H2 production of Ni-CH3CH2NH2/H1.78Sr0.78Bi0.22Nb2O7 were studied systematically. The photocatalyst loaded with 0.5 wt% Ni exhibited the highest H2 evolution rate of 372.67 μmo/h. This was 0.54 times higher than the activity of the H1.78Sr0.78Bi0.22Nb2O7 nanosheets. The activity of the optimized Ni-CH3CH2NH2/H1.78Sr0.78Bi0.22Nb2O7 was comparable to that of the Pt-loaded sample under the same reaction conditions. The photocatalytic activity of the Ni-CH3CH2NH2/H1.78Sr0.78Bi0.22Nb2O7 was mainly attributed to the excellent separation of photogenerated carriers, after formation of the intercalated complex Ni-CH3CH2NH2. This study provides a facile strategy to synthesize a non-precious metal-loaded photocatalyst for H2 production.
The nickel-based complex Ni-CH3CH2NH2-intercalated niobate layered perovskite Ni-CH3CH2NH2/H1.78Sr0.78Bi0.22Nb2O7 was synthesized via a facile in situ chemical reaction method. Using ultrathin H1.78Sr0.78Bi0.22Nb2O7 nanosheets and nickel acetate as precursors. The composition, structure, photophysical properties, and photocatalytic activity for H2 production of Ni-CH3CH2NH2/H1.78Sr0.78Bi0.22Nb2O7 were studied systematically. The photocatalyst loaded with 0.5 wt% Ni exhibited the highest H2 evolution rate of 372.67 μmo/h. This was 0.54 times higher than the activity of the H1.78Sr0.78Bi0.22Nb2O7 nanosheets. The activity of the optimized Ni-CH3CH2NH2/H1.78Sr0.78Bi0.22Nb2O7 was comparable to that of the Pt-loaded sample under the same reaction conditions. The photocatalytic activity of the Ni-CH3CH2NH2/H1.78Sr0.78Bi0.22Nb2O7 was mainly attributed to the excellent separation of photogenerated carriers, after formation of the intercalated complex Ni-CH3CH2NH2. This study provides a facile strategy to synthesize a non-precious metal-loaded photocatalyst for H2 production.
2017, 38(12): 2048-2055
doi: 10.1016/S1872-2067(17)62954-1
Abstract:
The present article reports a novel self-standing nanostructured Au-Cu(I)@Na2Ti6O13 plasmonic photocatalytic membrane, which is prepared by a hydrothermal reaction followed by a simple subsequent heat treatment process. The morphological structure, elemental composition, crystalline phases, and optical properties of the membrane were studied in detail by field-emission scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and ultraviolet-visible spectroscopy. Compared with that of a pure Na2Ti6O13 membrane, the Au-Cu(I)@Na2Ti6O13 membrane displayed much higher photocatalytic activity for the decomposition of acetaldehyde, a typical volatile organic compound, under visible light illumination. It was found that the photocatalytic activity of the Au-Cu(I)@Na2Ti6O13 membrane increased as the amount of Au was increased. The membrane loaded with 2.85 wt% Au showed the highest photocatalytic activity in the decomposition of acetaldehyde of the investigated materials. We found that in the photocatalyst membrane, Na2Ti6O13 acted as a support material, Au displayed plasmonic absorption, and Cu(I) behaved as a co-catalyst. The present membrane materials can avoid the self-aggregation typically observed during the course of photocatalytic reactions. As a result, they can be easily separated, recycled, and reactivated after their practical application, making these functional materials attractive for use in air cleaning applications.
The present article reports a novel self-standing nanostructured Au-Cu(I)@Na2Ti6O13 plasmonic photocatalytic membrane, which is prepared by a hydrothermal reaction followed by a simple subsequent heat treatment process. The morphological structure, elemental composition, crystalline phases, and optical properties of the membrane were studied in detail by field-emission scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and ultraviolet-visible spectroscopy. Compared with that of a pure Na2Ti6O13 membrane, the Au-Cu(I)@Na2Ti6O13 membrane displayed much higher photocatalytic activity for the decomposition of acetaldehyde, a typical volatile organic compound, under visible light illumination. It was found that the photocatalytic activity of the Au-Cu(I)@Na2Ti6O13 membrane increased as the amount of Au was increased. The membrane loaded with 2.85 wt% Au showed the highest photocatalytic activity in the decomposition of acetaldehyde of the investigated materials. We found that in the photocatalyst membrane, Na2Ti6O13 acted as a support material, Au displayed plasmonic absorption, and Cu(I) behaved as a co-catalyst. The present membrane materials can avoid the self-aggregation typically observed during the course of photocatalytic reactions. As a result, they can be easily separated, recycled, and reactivated after their practical application, making these functional materials attractive for use in air cleaning applications.
2017, 38(12): 2056-2066
doi: 10.1016/S1872-2067(17)62969-3
Abstract:
An efficient CuO-modified zeolitic imidazolate framework-9 (ZIF-9) photocatalyst is successfully prepared at room temperature under mild conditions. It is observed that the ZIF-9/CuO photocatalyst is effective for H2 generation under visible light with sacrificial agent conditions. When the CuO is introduced, the photocatalytic properties of ZIF-9 are greatly improved and when the content of CuO is 40%, the photocatalytic activity reaches a maximum of 78.74 μmol after 5 h. This results from the 200-300 nm cube structure of ZIF-9 being able to adsorb more dye molecules and the CuO, which connects with ZIF-9, greatly improving the electronic transmission efficiency. Moreover, the interaction between the dye molecule Eosin Y (EY) and the catalyst is also studied by transient fluorescence spectroscopy. A series of characterizations, such as SEM, TEM, XPS, XRD, UV-vis, FTIR, transient fluorescence and photocurrent, are conducted, and the results are in good agreement with the experimental result. In addition, the possible reaction mechanism over EY-sensitized ZIF-9/CuO under visible light irradiation is proposed.
An efficient CuO-modified zeolitic imidazolate framework-9 (ZIF-9) photocatalyst is successfully prepared at room temperature under mild conditions. It is observed that the ZIF-9/CuO photocatalyst is effective for H2 generation under visible light with sacrificial agent conditions. When the CuO is introduced, the photocatalytic properties of ZIF-9 are greatly improved and when the content of CuO is 40%, the photocatalytic activity reaches a maximum of 78.74 μmol after 5 h. This results from the 200-300 nm cube structure of ZIF-9 being able to adsorb more dye molecules and the CuO, which connects with ZIF-9, greatly improving the electronic transmission efficiency. Moreover, the interaction between the dye molecule Eosin Y (EY) and the catalyst is also studied by transient fluorescence spectroscopy. A series of characterizations, such as SEM, TEM, XPS, XRD, UV-vis, FTIR, transient fluorescence and photocurrent, are conducted, and the results are in good agreement with the experimental result. In addition, the possible reaction mechanism over EY-sensitized ZIF-9/CuO under visible light irradiation is proposed.
2017, 38(12): 2067-2075
doi: 10.1016/S1872-2067(17)62981-4
Abstract:
MoS2/ZnIn2S4 composites with MoS2 anchored on the surface of ZnIn2S4 microspheres were synthesized by a two-step hydrothermal process. The obtained samples were characterized by X-ray diffraction, field emission scanning electron microscopy, energy dispersive X-ray spectroscopy, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, ultraviolet-visible diffuse reflectance absorption spectroscopy, nitrogen adsorption-desorption measurements, photoluminescence spectroscopy, and photoelectrochemical tests. The influence of the loading of MoS2 on the photocatalytic H2 evolution activity was investigated using lactic acid as a sacrificial reagent. A H2 evolution rate of 343 μmol/h was achieved under visible light irradiation over the 1 wt% MoS2/ZnIn2S4 composite, corresponding to an apparent quantum efficiency of about 3.85% at 420 nm monochromatic light. The marked improvement of the photocatalytic H2 evolution activity compared with ZnIn2S4 can be ascribed to efficient transfer and separation of photogenerated charge carriers and facilitation of the photocatalytic H2 evolution reaction at the MoS2 active sites.
MoS2/ZnIn2S4 composites with MoS2 anchored on the surface of ZnIn2S4 microspheres were synthesized by a two-step hydrothermal process. The obtained samples were characterized by X-ray diffraction, field emission scanning electron microscopy, energy dispersive X-ray spectroscopy, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, ultraviolet-visible diffuse reflectance absorption spectroscopy, nitrogen adsorption-desorption measurements, photoluminescence spectroscopy, and photoelectrochemical tests. The influence of the loading of MoS2 on the photocatalytic H2 evolution activity was investigated using lactic acid as a sacrificial reagent. A H2 evolution rate of 343 μmol/h was achieved under visible light irradiation over the 1 wt% MoS2/ZnIn2S4 composite, corresponding to an apparent quantum efficiency of about 3.85% at 420 nm monochromatic light. The marked improvement of the photocatalytic H2 evolution activity compared with ZnIn2S4 can be ascribed to efficient transfer and separation of photogenerated charge carriers and facilitation of the photocatalytic H2 evolution reaction at the MoS2 active sites.
2017, 38(12): 2076-2084
doi: 10.1016/S1872-2067(17)62951-6
Abstract:
Although humic acid (HA) can inhibit TiO2 photocatalysis, it can sensitize TiO2 and induce significant visible-light (VL) activity in phenol degradation. This favorable effect of HA was negligible on phosphate-modified TiO2 (P-TiO2), but significantly stronger on Nafion-modified TiO2 (Nf-TiO2). The reaction rate constants for phenol degradation on Nf-TiO2 increased from (0.003±0.001) to (0.025±0.003) min-1 when the reaction was performed in the presence of 20 mg/L HA. The different effects of HA on P-TiO2 and Nf-TiO2 photocatalysis cannot be attributed to adsorption changes, because the adsorption capacities of P-TiO2 and Nf-TiO2 were only slightly lower than that of TiO2 at an initial HA concentration of 20 mg/mL. Scavenger tests, electron paramagnetic resonance spectroscopy, and H2O2 detection were taken to understand the low VL activity of the P-TiO2/HA suspension. The main active species for phenol degradation in the TiO2 and Nf-TiO2 suspensions were superoxide radicals. There were negligible amounts of superoxide radicals in the P-TiO2/HA suspension, possibly because a direct four-electron oxygen reduction reaction occurred. The better VL activity of Nf-TiO2 was rationalized on the basis of Mott-Schottky and electrochemical impedance plots. Nafion modification resulted in cathodic shifts of the energy band positions, increased electron density, and less resistance to electron transfer across the interface between TiO2 and electrolytes. All these factors facilitated electron transfer and improved the production of active species. Phosphate modification therefore did not improve the VL response of HA sensitized TiO2, and low concentrations of HA can facilitate VL photocatalytic degradation of organic pollutants on Nafion surface-modified TiO2.
Although humic acid (HA) can inhibit TiO2 photocatalysis, it can sensitize TiO2 and induce significant visible-light (VL) activity in phenol degradation. This favorable effect of HA was negligible on phosphate-modified TiO2 (P-TiO2), but significantly stronger on Nafion-modified TiO2 (Nf-TiO2). The reaction rate constants for phenol degradation on Nf-TiO2 increased from (0.003±0.001) to (0.025±0.003) min-1 when the reaction was performed in the presence of 20 mg/L HA. The different effects of HA on P-TiO2 and Nf-TiO2 photocatalysis cannot be attributed to adsorption changes, because the adsorption capacities of P-TiO2 and Nf-TiO2 were only slightly lower than that of TiO2 at an initial HA concentration of 20 mg/mL. Scavenger tests, electron paramagnetic resonance spectroscopy, and H2O2 detection were taken to understand the low VL activity of the P-TiO2/HA suspension. The main active species for phenol degradation in the TiO2 and Nf-TiO2 suspensions were superoxide radicals. There were negligible amounts of superoxide radicals in the P-TiO2/HA suspension, possibly because a direct four-electron oxygen reduction reaction occurred. The better VL activity of Nf-TiO2 was rationalized on the basis of Mott-Schottky and electrochemical impedance plots. Nafion modification resulted in cathodic shifts of the energy band positions, increased electron density, and less resistance to electron transfer across the interface between TiO2 and electrolytes. All these factors facilitated electron transfer and improved the production of active species. Phosphate modification therefore did not improve the VL response of HA sensitized TiO2, and low concentrations of HA can facilitate VL photocatalytic degradation of organic pollutants on Nafion surface-modified TiO2.
2017, 38(12): 2085-2093
doi: 10.1016/S1872-2067(17)62952-8
Abstract:
TiO2 hollow microspheres (TiO2-HMSs) have attracted much attention because of their high photoreactivity, low density, and good permeability. However, anatase TiO2-HMSs have poor thermal stability. In this study, surface-fluorinated TiO2-HMSs were assembled from hollow nanoparticles by the hydrothermal reaction of the mixed Ti(SO4)2-NH4HF-H2O2 solution at 180℃. The effect of the calcination temperature on the structure and photoreactivity of the TiO2-HMSs was systematically investigated, which was evaluated by photocatalytic oxidation of acetone in air under ultraviolet irradiation. We found that after calcination at 300℃, the photoreactivity of the TiO2-HMSs decreases from 1.39×10-3 min-1 (TiO2-HMS precursor) to 0.82×10-3 min-1 because of removal of surface-adsorbed fluoride ions. With increasing calcination temperature from 300 to 900℃, the building blocks of the TiO2-HMSs evolve from truncated bipyramidal shaped hollow nanoparticles to round solid nanoparticles, and the photoreactivity of the TiO2-HMSs steady increases from 0.82×10-3 to 2.09×10-3 min-1 because of enhanced crystallization. Further increasing the calcination temperature to 1000 and 1100℃ results in a decrease of the photoreactivity, which is ascribed to a sharp decrease of the Brunau-er-Emmett-Teller surface area and the beginning of the anatase-rutile phase transformation at 1100℃. The effect of surface-adsorbed fluoride ions on the thermal stability of the TiO2-HMSs is also discussed.
TiO2 hollow microspheres (TiO2-HMSs) have attracted much attention because of their high photoreactivity, low density, and good permeability. However, anatase TiO2-HMSs have poor thermal stability. In this study, surface-fluorinated TiO2-HMSs were assembled from hollow nanoparticles by the hydrothermal reaction of the mixed Ti(SO4)2-NH4HF-H2O2 solution at 180℃. The effect of the calcination temperature on the structure and photoreactivity of the TiO2-HMSs was systematically investigated, which was evaluated by photocatalytic oxidation of acetone in air under ultraviolet irradiation. We found that after calcination at 300℃, the photoreactivity of the TiO2-HMSs decreases from 1.39×10-3 min-1 (TiO2-HMS precursor) to 0.82×10-3 min-1 because of removal of surface-adsorbed fluoride ions. With increasing calcination temperature from 300 to 900℃, the building blocks of the TiO2-HMSs evolve from truncated bipyramidal shaped hollow nanoparticles to round solid nanoparticles, and the photoreactivity of the TiO2-HMSs steady increases from 0.82×10-3 to 2.09×10-3 min-1 because of enhanced crystallization. Further increasing the calcination temperature to 1000 and 1100℃ results in a decrease of the photoreactivity, which is ascribed to a sharp decrease of the Brunau-er-Emmett-Teller surface area and the beginning of the anatase-rutile phase transformation at 1100℃. The effect of surface-adsorbed fluoride ions on the thermal stability of the TiO2-HMSs is also discussed.
2017, 38(12): 2094-2101
doi: 10.1016/S1872-2067(17)62960-7
Abstract:
The consecutive two-photon photocatalytic behavior of perylene diimide (PDI) enables it to catalyze photoreduction reactions that are thermodynamically unfavorable via single-photon processes. In this work, we developed a heterogeneous PDI photocatalyst by covalently binding PDI molecules on the surface of nanosilica. This photocatalyst structure overcomes the intrinsic limitation of the low solubility of PDI, but retains its consecutive two-photon photocatalytic property. Detailed characterization of the photocatalyst by techniques such as thermogravimetric analysis, solid-state nuclear magnetic resonance spectroscopy, and Fourier transform infrared spectroscopy indicated that the PDI molecules were anchored covalently on the surface of nanosilica. The obtained photocatalyst reduced aryl halides under visible-light irradiation in polar organic solvent and in water. The present study provides a promising strategy to realize two-photon activity of PDI in common solvents for photocatalytic appli-cations.
The consecutive two-photon photocatalytic behavior of perylene diimide (PDI) enables it to catalyze photoreduction reactions that are thermodynamically unfavorable via single-photon processes. In this work, we developed a heterogeneous PDI photocatalyst by covalently binding PDI molecules on the surface of nanosilica. This photocatalyst structure overcomes the intrinsic limitation of the low solubility of PDI, but retains its consecutive two-photon photocatalytic property. Detailed characterization of the photocatalyst by techniques such as thermogravimetric analysis, solid-state nuclear magnetic resonance spectroscopy, and Fourier transform infrared spectroscopy indicated that the PDI molecules were anchored covalently on the surface of nanosilica. The obtained photocatalyst reduced aryl halides under visible-light irradiation in polar organic solvent and in water. The present study provides a promising strategy to realize two-photon activity of PDI in common solvents for photocatalytic appli-cations.
2017, 38(12): 2102-2109
doi: 10.1016/S1872-2067(17)62956-5
Abstract:
Photocatalytic water splitting is an economical and sustainable pathway to use solar energy for large-scale H2 production. We report a highly efficient noble-metal-free photocatalyst formed by integrating amorphous NiS with a CdS nanorods (NRs)/ZnS heterojunction material for photocatalytic H2 production in water under visible light irradiation (λ > 420 nm). The results show that the photocatalytic H2 production rate reaches an optimal value of up to 574 μmol·h-1 after the loading of NiS, which is more than 38 times higher than the catalytic activity of pure CdS NRs. The average apparent quantum yield is~43.2% during 5 h of irradiation by monochromatic 420 nm light. The present study demonstrates the advantage of integration strategies to form not only semiconductor heterojunctions but also photocatalyst-cocatalyst interfaces to enhance the catalytic activity for photocatalytic H2 production.
Photocatalytic water splitting is an economical and sustainable pathway to use solar energy for large-scale H2 production. We report a highly efficient noble-metal-free photocatalyst formed by integrating amorphous NiS with a CdS nanorods (NRs)/ZnS heterojunction material for photocatalytic H2 production in water under visible light irradiation (λ > 420 nm). The results show that the photocatalytic H2 production rate reaches an optimal value of up to 574 μmol·h-1 after the loading of NiS, which is more than 38 times higher than the catalytic activity of pure CdS NRs. The average apparent quantum yield is~43.2% during 5 h of irradiation by monochromatic 420 nm light. The present study demonstrates the advantage of integration strategies to form not only semiconductor heterojunctions but also photocatalyst-cocatalyst interfaces to enhance the catalytic activity for photocatalytic H2 production.
2017, 38(12): 2110-2119
doi: 10.1016/S1872-2067(17)62957-7
Abstract:
We prepared the Fe3O4/g-C3N4 nanoparticles (NPs) through a simple electrostatic self-assembly method with a 3:97 weight ratio to investigate their Fenton, photo-Fenton and oxidative functionalities besides photocatalytic functionality. We observed an improvement of the Fenton and photo-Fenton activities of the Fe3O4/g-C3N4 nanocomposites. This improvement was attributed to efficient charge transfer between Fe3O4 and g-C3N4 at the heterojunctions, inhibition of electron-hole recombination, a high surface area, and stabilization of Fe3O4 against leaching by the hydrophobic g-C3N4. The obtained NPs showed a higher degradation potential for rhodamine B (RhB) dye than those of Fe3O4 and g-C3N4. As compared to photocatalysis, the efficiency of RhB degradation in the Fenton and photo-Fenton reactions was increased by 20% and 90%, respectively. Additionally, the horseradish peroxidase (HRP) activity of the prepared nanomaterials was studied with 3,3,5,5-tetramethylbenzidinedihydrochloride (TMB) as a substrate. Dopamine oxidation was also examined. Results indicate that Fe3O4/g-C3N4 nanocomposites offers more efficient degradation of RhB dye in a photo-Fenton system compared with regular photocatalytic degradation, which requires a long time. Our study also confirmed that Fe3O4/g-C3N4 nanocomposites can be used as a potential material for mimicking HRP owing to its high affinity for TMB. These findings suggest good potential for applications in biosensing and as a catalyst in oxidation reactions.
We prepared the Fe3O4/g-C3N4 nanoparticles (NPs) through a simple electrostatic self-assembly method with a 3:97 weight ratio to investigate their Fenton, photo-Fenton and oxidative functionalities besides photocatalytic functionality. We observed an improvement of the Fenton and photo-Fenton activities of the Fe3O4/g-C3N4 nanocomposites. This improvement was attributed to efficient charge transfer between Fe3O4 and g-C3N4 at the heterojunctions, inhibition of electron-hole recombination, a high surface area, and stabilization of Fe3O4 against leaching by the hydrophobic g-C3N4. The obtained NPs showed a higher degradation potential for rhodamine B (RhB) dye than those of Fe3O4 and g-C3N4. As compared to photocatalysis, the efficiency of RhB degradation in the Fenton and photo-Fenton reactions was increased by 20% and 90%, respectively. Additionally, the horseradish peroxidase (HRP) activity of the prepared nanomaterials was studied with 3,3,5,5-tetramethylbenzidinedihydrochloride (TMB) as a substrate. Dopamine oxidation was also examined. Results indicate that Fe3O4/g-C3N4 nanocomposites offers more efficient degradation of RhB dye in a photo-Fenton system compared with regular photocatalytic degradation, which requires a long time. Our study also confirmed that Fe3O4/g-C3N4 nanocomposites can be used as a potential material for mimicking HRP owing to its high affinity for TMB. These findings suggest good potential for applications in biosensing and as a catalyst in oxidation reactions.
2017, 38(12): 2120-2131
doi: 10.1016/S1872-2067(17)62959-0
Abstract:
Bandgap engineering by doping and co-catalyst loading are two primary approaches to designing efficient photocatalysts by promoting visible-light absorption and charge separation, respectively. Shifting of the TiO2 conduction band edge is frequently applied to increase visible-light absorption but also lowers the reductive properties of photo-excited electrons. Herein, we report a visible-light-driven photocatalyst based on valance band edge control induced by oxygen excess defects and modification with a CuxO electron transfer co-catalyst. The CuxO grafted oxygen-rich TiO2 microspheres were prepared by ultrasonic spray pyrolysis of the peroxotitanate precursor followed by a wet chemical impregnated treatment. We found that oxygen excess defects in TiO2 shifted the valence band maximum upward and improved the visible-light absorption. The CuxO grafted onto the surface acted as a co-catalyst that efficiently reduced oxygen molecules to active intermediates (i.e., O2·-radial and H2O2), thus consuming the photo-generated electrons. Consequently, the CuxO grafted oxygen-rich TiO2 microspheres achieved a photocatalytic activity respectively 8.6, 13.0 and 11.0 as times high as those of oxygen-rich TiO2, normal TiO2 and CuxO grafted TiO2, for degradation of gaseous acetaldehyde under visible-light irradiation. Our results suggest that high visible-light photocatalytic efficiency can be achieved by combining oxygen excess defects to improve visible-light absorption together with a CuxO electron transfer co-catalyst. These findings provide a new approach to developing efficient heterojunction photocatalysts.
Bandgap engineering by doping and co-catalyst loading are two primary approaches to designing efficient photocatalysts by promoting visible-light absorption and charge separation, respectively. Shifting of the TiO2 conduction band edge is frequently applied to increase visible-light absorption but also lowers the reductive properties of photo-excited electrons. Herein, we report a visible-light-driven photocatalyst based on valance band edge control induced by oxygen excess defects and modification with a CuxO electron transfer co-catalyst. The CuxO grafted oxygen-rich TiO2 microspheres were prepared by ultrasonic spray pyrolysis of the peroxotitanate precursor followed by a wet chemical impregnated treatment. We found that oxygen excess defects in TiO2 shifted the valence band maximum upward and improved the visible-light absorption. The CuxO grafted onto the surface acted as a co-catalyst that efficiently reduced oxygen molecules to active intermediates (i.e., O2·-radial and H2O2), thus consuming the photo-generated electrons. Consequently, the CuxO grafted oxygen-rich TiO2 microspheres achieved a photocatalytic activity respectively 8.6, 13.0 and 11.0 as times high as those of oxygen-rich TiO2, normal TiO2 and CuxO grafted TiO2, for degradation of gaseous acetaldehyde under visible-light irradiation. Our results suggest that high visible-light photocatalytic efficiency can be achieved by combining oxygen excess defects to improve visible-light absorption together with a CuxO electron transfer co-catalyst. These findings provide a new approach to developing efficient heterojunction photocatalysts.
2017, 38(12): 2132-2140
doi: 10.1016/S1872-2067(17)62948-6
Abstract:
Photoactive WO3 is attractive as a photocatalyst for green energy evolution through water splitting. In the present work, an electrochemical anodic oxidation method was used to fabricate a photo-responsive nanotube array-like WO3/W (NA-WO3/W) photoanode from W foil as a precursor. Compared with a reference commercial WO3/W electrode, the NA-WO3/W photoanode exhibited enhanced and stable photoelectrocatalytic (PEC) activity for visible-light-driven water splitting with a typical H2/O2 stoichiometric ratio of 2:1 and quantum efficiency of approximately 5.23% under visible-light irradiation from a light-emitting diode (λ=420 nm, 15 mW/cm2). The greatly enhanced PEC performance of the NA-WO3/Wphotoanode was attributed to its fast electron-hole separation rate, which resulted from the one-dimensional nanotube array-like structure, high crystallinity of monoclinic WO3, and strong interaction between WO3 and W foil. This work paves the way to a facile route to prepare highly active photoelectrodes for solar light transfer to chemical energy.
Photoactive WO3 is attractive as a photocatalyst for green energy evolution through water splitting. In the present work, an electrochemical anodic oxidation method was used to fabricate a photo-responsive nanotube array-like WO3/W (NA-WO3/W) photoanode from W foil as a precursor. Compared with a reference commercial WO3/W electrode, the NA-WO3/W photoanode exhibited enhanced and stable photoelectrocatalytic (PEC) activity for visible-light-driven water splitting with a typical H2/O2 stoichiometric ratio of 2:1 and quantum efficiency of approximately 5.23% under visible-light irradiation from a light-emitting diode (λ=420 nm, 15 mW/cm2). The greatly enhanced PEC performance of the NA-WO3/Wphotoanode was attributed to its fast electron-hole separation rate, which resulted from the one-dimensional nanotube array-like structure, high crystallinity of monoclinic WO3, and strong interaction between WO3 and W foil. This work paves the way to a facile route to prepare highly active photoelectrodes for solar light transfer to chemical energy.
2017, 38(12): 2141-2149
doi: 10.1016/S1872-2067(17)62947-4
Abstract:
A new coordination polymer, Zn(bpy)L (BUC-21), (H2L=cis-1,3-dibenzyl-2-imidazolidone-4,5-dicarboxylic acid, bpy=4,4'-bipyridine), has been synthesized under hydrothermal conditions, and characterized by single-crystal X-ray analysis, Fourier transform infrared spectroscopy, thermogravimetric analyses, CNH elemental analysis and UV-Vis diffuse reflectance spectroscopy. BUC-21 exhibited an excellent performance for photocatalytic Cr(VI) reduction with a conversion efficiency of 96%, better than that of commercial P25 (39%), under UV light irradiation for 30 min. BUC-21 could also be used to conduct photocatalytic degradation of organic dyes including methylene blue, rhodamine B, methyl orange and reactive red X-3B. Also, the photocatalytic activity of BUC-21 remained high across a wide pH range from 2.0 to 12.0. It is interesting to note, however, that BUC-21 was unable to achieve simultaneous reduction of Cr(VI) and degradation of an organic pollutant in a mixed matrix, which can be attributed to the competition between Cr(VI) and the organic dyes for access to the photo-excited electrons.
A new coordination polymer, Zn(bpy)L (BUC-21), (H2L=cis-1,3-dibenzyl-2-imidazolidone-4,5-dicarboxylic acid, bpy=4,4'-bipyridine), has been synthesized under hydrothermal conditions, and characterized by single-crystal X-ray analysis, Fourier transform infrared spectroscopy, thermogravimetric analyses, CNH elemental analysis and UV-Vis diffuse reflectance spectroscopy. BUC-21 exhibited an excellent performance for photocatalytic Cr(VI) reduction with a conversion efficiency of 96%, better than that of commercial P25 (39%), under UV light irradiation for 30 min. BUC-21 could also be used to conduct photocatalytic degradation of organic dyes including methylene blue, rhodamine B, methyl orange and reactive red X-3B. Also, the photocatalytic activity of BUC-21 remained high across a wide pH range from 2.0 to 12.0. It is interesting to note, however, that BUC-21 was unable to achieve simultaneous reduction of Cr(VI) and degradation of an organic pollutant in a mixed matrix, which can be attributed to the competition between Cr(VI) and the organic dyes for access to the photo-excited electrons.
2017, 38(12): 2150-2159
doi: 10.1016/S1872-2067(17)62964-4
Abstract:
CdSe quantum dots (QDs) hybridized with graphene oxide (GO) are synthesized by a facile chemical precipitation method. The absorption of the CdSe/GO nanocomposite is increased with a significant blue shift with respect to CdSe QDs. The specific surface area of the CdSe/GO nanocomposite is 10.4 m2/g, which is higher than that of CdSe QDs (5 m2/g). The PL intensity of the CdSe/GO nanocomposite is lower than that of the CdSe QDs owing to the inhibition of the recombination of electron-hole pairs in the composite. In Raman analysis, the two bands of the CdSe/GO nanocomposite are shifted to higher wavenumbers with respect to graphene oxide, which is attributed to electron injection that is induced by CdSe QDs into graphene oxide. Using the Brilliant Green dye, the photocatalytic reduction efficiency of CdSe QDs and the CdSe/GO nanocomposite under sunlight irradiation for 90 min are approximately 81.9% and 95.5%, respectively. The calculated photodegradation rate constants for CdSe QDs and the CdSe/GO nanocomposite are 0.0190 min-1 and 0.0345 min-1, respectively. The enhanced photocatalytic activity of the CdSe/GO nanocomposite can be attributed to the high specific surface area and the reduction of electron-hole pair recombination because of the introduction of graphene oxide.
CdSe quantum dots (QDs) hybridized with graphene oxide (GO) are synthesized by a facile chemical precipitation method. The absorption of the CdSe/GO nanocomposite is increased with a significant blue shift with respect to CdSe QDs. The specific surface area of the CdSe/GO nanocomposite is 10.4 m2/g, which is higher than that of CdSe QDs (5 m2/g). The PL intensity of the CdSe/GO nanocomposite is lower than that of the CdSe QDs owing to the inhibition of the recombination of electron-hole pairs in the composite. In Raman analysis, the two bands of the CdSe/GO nanocomposite are shifted to higher wavenumbers with respect to graphene oxide, which is attributed to electron injection that is induced by CdSe QDs into graphene oxide. Using the Brilliant Green dye, the photocatalytic reduction efficiency of CdSe QDs and the CdSe/GO nanocomposite under sunlight irradiation for 90 min are approximately 81.9% and 95.5%, respectively. The calculated photodegradation rate constants for CdSe QDs and the CdSe/GO nanocomposite are 0.0190 min-1 and 0.0345 min-1, respectively. The enhanced photocatalytic activity of the CdSe/GO nanocomposite can be attributed to the high specific surface area and the reduction of electron-hole pair recombination because of the introduction of graphene oxide.
2017, 38(12): 2160-2170
doi: 10.1016/S1872-2067(17)62911-5
Abstract:
Graphite-like carbon nitride (g-C3N4)-based compounds have attracted considerable attention because of their excellent photocatalytic performance. In this work, a novel direct Z-scheme system constructed from two-dimensional (2D) g-C3N4 nanoplates and zero-dimensional (0D) MoS2 quantum dots (QDs) was prepared through the combination of a hydrothermal process and microemulsion preparation. The morphologies, structures, and optical properties of the as-prepared photocatalysts were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, atomic force microscopy, transmission electron microscopy, and UV-vis diffuse reflectance spectroscopy. In addition, the photocatalytic performances of the prepared 2D/0D hybrid composites were evaluated based on the photodegradation of rhodamine B under visible-light irradiation. The results demonstrated that the introduction of MoS2 QDs to g-C3N4 greatly enhanced the photocatalytic efficiency. For the optimum 7% MoS2 QD/g-C3N4 photocatalyst, the degradation rate constant was 8.8 times greater than that of pure g-C3N4 under visible-light irradiation. Photocurrent and electrochemical impedance spectroscopy results further demonstrated that the MoS2 QD/g-C3N4 composites exhibited higher photocurrent density and lower chargetransfer resistance than those of the pure g-C3N4 or MoS2 QDs. Active species trapping, terephthalic acid photoluminescence, and nitro blue tetrazolium transformation experiments were performed to investigate the evolution of reactive oxygen species, including hydroxyl radicals and superoxide radicals. The possible enhanced photocatalytic mechanism was attributed to a direct Z-scheme system, which not only can increase the separation efficiency of photogenerated electron-hole pairs but also possesses excellent oxidation and reduction ability for high photocatalytic performances. This work provides an effective synthesis approach and insight to help develop other C3N4-based direct Z-scheme photocatalytic systems for environmental purification and energy conversion.
Graphite-like carbon nitride (g-C3N4)-based compounds have attracted considerable attention because of their excellent photocatalytic performance. In this work, a novel direct Z-scheme system constructed from two-dimensional (2D) g-C3N4 nanoplates and zero-dimensional (0D) MoS2 quantum dots (QDs) was prepared through the combination of a hydrothermal process and microemulsion preparation. The morphologies, structures, and optical properties of the as-prepared photocatalysts were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, atomic force microscopy, transmission electron microscopy, and UV-vis diffuse reflectance spectroscopy. In addition, the photocatalytic performances of the prepared 2D/0D hybrid composites were evaluated based on the photodegradation of rhodamine B under visible-light irradiation. The results demonstrated that the introduction of MoS2 QDs to g-C3N4 greatly enhanced the photocatalytic efficiency. For the optimum 7% MoS2 QD/g-C3N4 photocatalyst, the degradation rate constant was 8.8 times greater than that of pure g-C3N4 under visible-light irradiation. Photocurrent and electrochemical impedance spectroscopy results further demonstrated that the MoS2 QD/g-C3N4 composites exhibited higher photocurrent density and lower chargetransfer resistance than those of the pure g-C3N4 or MoS2 QDs. Active species trapping, terephthalic acid photoluminescence, and nitro blue tetrazolium transformation experiments were performed to investigate the evolution of reactive oxygen species, including hydroxyl radicals and superoxide radicals. The possible enhanced photocatalytic mechanism was attributed to a direct Z-scheme system, which not only can increase the separation efficiency of photogenerated electron-hole pairs but also possesses excellent oxidation and reduction ability for high photocatalytic performances. This work provides an effective synthesis approach and insight to help develop other C3N4-based direct Z-scheme photocatalytic systems for environmental purification and energy conversion.
