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2018 Volume 39 Issue 3
2018, 39(3):
Abstract:
2018, 39(3): 367-368
doi: 10.1016/S1872-2067(18)63041-4
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2018, 39(3): 369-378
doi: 10.1016/S1872-2067(17)62998-X
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Photoelectrochemical (PEC) water splitting process is regarded as a promising route to generate hydrogen by solar energy and at the heart of PEC is efficient electrode design. Great progress has been achieved in the aspects of material design, cocatalyst study, and electrode fabrication over the past decades. However, some key challenges remain unsolved, including the most demanded conversion efficiency issue. As three critical steps, i.e. light harvesting, charge transfer and surface reaction of the PEC process, occur in a huge range of time scale (from 10-12 s to 100 s), how to manage these subsequent steps to facilitate the seamless cooperation between each step to realize efficient PEC process is essentially important. This review focuses on an integral consideration of the three key criteria based on the recent progress on high efficient and stable photoelectrode design in PEC. The basic principles and potential strategies are summarized. Moreover, the challenge and perspective are also discussed.
Photoelectrochemical (PEC) water splitting process is regarded as a promising route to generate hydrogen by solar energy and at the heart of PEC is efficient electrode design. Great progress has been achieved in the aspects of material design, cocatalyst study, and electrode fabrication over the past decades. However, some key challenges remain unsolved, including the most demanded conversion efficiency issue. As three critical steps, i.e. light harvesting, charge transfer and surface reaction of the PEC process, occur in a huge range of time scale (from 10-12 s to 100 s), how to manage these subsequent steps to facilitate the seamless cooperation between each step to realize efficient PEC process is essentially important. This review focuses on an integral consideration of the three key criteria based on the recent progress on high efficient and stable photoelectrode design in PEC. The basic principles and potential strategies are summarized. Moreover, the challenge and perspective are also discussed.
2018, 39(3): 379-389
doi: 10.1016/S1872-2067(17)62930-9
Abstract:
Converting solar energy into hydrogen and hydrocarbon fuels through photocatalytic H2 production and CO2 photoreduction is a highly promising approach to address growing demand for clean and renewable energy resources. However, solar-to-fuel conversion efficiencies of current photocatalysts are not sufficient to meet commercial requirements. The narrow window of solar energy that can be used has been identified as a key reason behind such low photocatalytic reaction efficiencies. The use of photonic crystals, formed from multiple material components, has been demonstrated to be an effective way of improving light harvesting. Within these nanostructures, the slow-photon effect, a manifestation of light-propagation control, considerably enhances the interaction between light and the semiconductor components. This article reviews recent developments in the applications of photonic crystals to photocatalytic H2 production and CO2 reduction based on slow photons. These advances show great promise for improving light harvesting in solar-energy conversion technologies.
Converting solar energy into hydrogen and hydrocarbon fuels through photocatalytic H2 production and CO2 photoreduction is a highly promising approach to address growing demand for clean and renewable energy resources. However, solar-to-fuel conversion efficiencies of current photocatalysts are not sufficient to meet commercial requirements. The narrow window of solar energy that can be used has been identified as a key reason behind such low photocatalytic reaction efficiencies. The use of photonic crystals, formed from multiple material components, has been demonstrated to be an effective way of improving light harvesting. Within these nanostructures, the slow-photon effect, a manifestation of light-propagation control, considerably enhances the interaction between light and the semiconductor components. This article reviews recent developments in the applications of photonic crystals to photocatalytic H2 production and CO2 reduction based on slow photons. These advances show great promise for improving light harvesting in solar-energy conversion technologies.
2018, 39(3): 390-394
doi: 10.1016/S1872-2067(17)62949-8
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As an energy storage medium, hydrogen has drawn the attention of research institutions and industry over the past decade, motivated in part by developments in renewable energy, which have led to unused surplus wind and photovoltaic power. Hydrogen production from water electrolysis is a good option to make full use of the surplus renewable energy. Among various technologies for producing hydrogen, water electrolysis using electricity from renewable power sources shows great promise. To investigate the prospects of water electrolysis for hydrogen production, this review compares different water electrolysis processes, i.e., alkaline water electrolysis, proton exchange membrane water electrolysis, solid oxide water electrolysis, and alkaline anion exchange membrane water electrolysis. The ion transfer mechanisms, operating characteristics, energy consumption, and industrial products of different water electrolysis apparatus are introduced in this review. Prospects for new water electrolysis technologies are discussed.
As an energy storage medium, hydrogen has drawn the attention of research institutions and industry over the past decade, motivated in part by developments in renewable energy, which have led to unused surplus wind and photovoltaic power. Hydrogen production from water electrolysis is a good option to make full use of the surplus renewable energy. Among various technologies for producing hydrogen, water electrolysis using electricity from renewable power sources shows great promise. To investigate the prospects of water electrolysis for hydrogen production, this review compares different water electrolysis processes, i.e., alkaline water electrolysis, proton exchange membrane water electrolysis, solid oxide water electrolysis, and alkaline anion exchange membrane water electrolysis. The ion transfer mechanisms, operating characteristics, energy consumption, and industrial products of different water electrolysis apparatus are introduced in this review. Prospects for new water electrolysis technologies are discussed.
2018, 39(3): 395-400
doi: 10.1016/S1872-2067(17)62963-2
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We use a two-step hydrothermal method to successfully synthesize Sn2Nb2O7 nanocrystals with an average size of approximately 20 nm. The as-obtained samples are characterized by powder X-ray diffraction, ultraviolet-visible diffuse reflectance spectroscopy, Brunauer-Emmett-Teller analysis, scanning electron microscopy, and transmission electron microscopy. The photocatalytic activity of the Sn2Nb2O7 nanocrystals is evaluated by photocatalytic water splitting under visible light irradiation. The Sn2Nb2O7 nanocrystals with a large surface area of 52.2 m2/g show an enhanced visible-light-driven photocatalytic H2 production activity, approximately 5.5 times higher than that of bulk Sn2Nb2O7 powder. The higher photocatalytic activity of Sn2Nb2O7 nanocrystals is mainly attributed to its relatively high dispersity of nanosized particles and larger specific surface area when compared with the bulk powder.
We use a two-step hydrothermal method to successfully synthesize Sn2Nb2O7 nanocrystals with an average size of approximately 20 nm. The as-obtained samples are characterized by powder X-ray diffraction, ultraviolet-visible diffuse reflectance spectroscopy, Brunauer-Emmett-Teller analysis, scanning electron microscopy, and transmission electron microscopy. The photocatalytic activity of the Sn2Nb2O7 nanocrystals is evaluated by photocatalytic water splitting under visible light irradiation. The Sn2Nb2O7 nanocrystals with a large surface area of 52.2 m2/g show an enhanced visible-light-driven photocatalytic H2 production activity, approximately 5.5 times higher than that of bulk Sn2Nb2O7 powder. The higher photocatalytic activity of Sn2Nb2O7 nanocrystals is mainly attributed to its relatively high dispersity of nanosized particles and larger specific surface area when compared with the bulk powder.
2018, 39(3): 401-406
doi: 10.1016/S1872-2067(17)62945-0
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Molybdenum sulfides are promising electrocatalysts for the hydrogen evolution reaction (HER). S-and Mo-related species have been proposed as the active site for forming adsorbed hydrogen to initiate the HER; however, the nature of the interaction between Mo centers and S ligands is unclear. Further, the development of cost-effective water-splitting systems using neutral water as a proton source for H2 evolution is highly desirable, whereas the mechanism of the HER at neutral pH is rarely discussed. Here, the structural change in the Mo-Mo and S-S species in a synthesized molybdenum sulfide was monitored at neutral pH using in situ electrochemical Raman spectroscopy. Analysis of the potential dependent Raman spectra revealed that the band assigned to a terminal S-S species emerged along with synchronized changes in the frequency of the Mo-Mo, Mo3-μ3S, and Mo-S vibrational bands. This indicates that Mo-Mo bonds and terminal S-S ligands play synergistic roles in facilitating hydrogen evolution, likely via the internal reorganization of trinuclear Mo3-thio species. The nature and role of metal-ligand interactions in the HER revealed in this study demonstrated a mechanism that is distinct from those reported previously in which the S or Mo sites function independently.
Molybdenum sulfides are promising electrocatalysts for the hydrogen evolution reaction (HER). S-and Mo-related species have been proposed as the active site for forming adsorbed hydrogen to initiate the HER; however, the nature of the interaction between Mo centers and S ligands is unclear. Further, the development of cost-effective water-splitting systems using neutral water as a proton source for H2 evolution is highly desirable, whereas the mechanism of the HER at neutral pH is rarely discussed. Here, the structural change in the Mo-Mo and S-S species in a synthesized molybdenum sulfide was monitored at neutral pH using in situ electrochemical Raman spectroscopy. Analysis of the potential dependent Raman spectra revealed that the band assigned to a terminal S-S species emerged along with synchronized changes in the frequency of the Mo-Mo, Mo3-μ3S, and Mo-S vibrational bands. This indicates that Mo-Mo bonds and terminal S-S ligands play synergistic roles in facilitating hydrogen evolution, likely via the internal reorganization of trinuclear Mo3-thio species. The nature and role of metal-ligand interactions in the HER revealed in this study demonstrated a mechanism that is distinct from those reported previously in which the S or Mo sites function independently.
2018, 39(3): 407-412
doi: 10.1016/S1872-2067(17)62970-X
Abstract:
Charge separation is a crucial problem in photocatalysis. We used a wet-chemical method to synthesize asymmetrically tipped PdS-CdSe-seeded CdS (CdSe@CdS)-Au nanorod (NR) heterostructures (HCs). In these HCs, electrons and holes are rapidly separated and transported to opposite ends of the NRs by internal electric fields. Their ultraviolet-visible absorption spectra showed strong electronic coupling between both tips and the CdS body. PdS-CdSe@CdS-Au achieved a H2 production rate of ca. 1100 μmol in 5 h; this is two orders of magnitude greater than the rate achieved with Au-CdSe@CdS NRs with only one tip. PdS-CdSe@CdS-Au NRs can withstand 4 h of photoirradiation, compared to 1.5 h for CdSe@CdS NRs, indicating that the photostability of PdS-CdSe@CdS-Au is much better than that of CdS. The greatly improved photocatalytic activity and stability are attributed to efficient charge separation and rapid charge transport in the PdS-CdSe@CdS-Au HCs.
Charge separation is a crucial problem in photocatalysis. We used a wet-chemical method to synthesize asymmetrically tipped PdS-CdSe-seeded CdS (CdSe@CdS)-Au nanorod (NR) heterostructures (HCs). In these HCs, electrons and holes are rapidly separated and transported to opposite ends of the NRs by internal electric fields. Their ultraviolet-visible absorption spectra showed strong electronic coupling between both tips and the CdS body. PdS-CdSe@CdS-Au achieved a H2 production rate of ca. 1100 μmol in 5 h; this is two orders of magnitude greater than the rate achieved with Au-CdSe@CdS NRs with only one tip. PdS-CdSe@CdS-Au NRs can withstand 4 h of photoirradiation, compared to 1.5 h for CdSe@CdS NRs, indicating that the photostability of PdS-CdSe@CdS-Au is much better than that of CdS. The greatly improved photocatalytic activity and stability are attributed to efficient charge separation and rapid charge transport in the PdS-CdSe@CdS-Au HCs.
2018, 39(3): 413-420
doi: 10.1016/S1872-2067(17)62993-0
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Two homogeneous photoelectrocatalytic systems composed of simple polypyridyl Co complexes[Co(tpy)2](PF6)2 and[Co(bpy)3](PF6)2 as electrocatalysts and a Si wafer as the photoelectrode were used for combined photoelectrochemical reduction of CO2 to CO. A high photocurrent density of 1.4 mA/cm2 was observed for the system with the[Co(tpy)2](PF6)2 catalyst and a photovoltage of 400 mV was generated. Faradaic efficiencies of CO were optimized to 83% and 94% for the[Co(tpy)2](PF6)2 and[Co(bpy)3](PF6)2 complexes, respectively, in acetonitrile solution with 10% methanol (volume fraction, same below) as a protic additive. Addition of 2% water volume fraction induced a large amount of non-specific H2 evolution by the Si photoelectrode.
Two homogeneous photoelectrocatalytic systems composed of simple polypyridyl Co complexes[Co(tpy)2](PF6)2 and[Co(bpy)3](PF6)2 as electrocatalysts and a Si wafer as the photoelectrode were used for combined photoelectrochemical reduction of CO2 to CO. A high photocurrent density of 1.4 mA/cm2 was observed for the system with the[Co(tpy)2](PF6)2 catalyst and a photovoltage of 400 mV was generated. Faradaic efficiencies of CO were optimized to 83% and 94% for the[Co(tpy)2](PF6)2 and[Co(bpy)3](PF6)2 complexes, respectively, in acetonitrile solution with 10% methanol (volume fraction, same below) as a protic additive. Addition of 2% water volume fraction induced a large amount of non-specific H2 evolution by the Si photoelectrode.
2018, 39(3): 421-430
doi: 10.1016/S1872-2067(18)63027-X
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The photocatalytic activity of a semiconductor-based photocatalyst largely depends on the semiconductor's intrinsic crystal and electronic properties. We have prepared two types of La and Cr co-doped SrTiO3 photocatalysts (SrTiO3(La,Cr)) using the polymerized complex method (PCM) and sol-gel hydrothermal method (SHM). Under λ > 420-nm visible light irradiation, only the Pt-loaded SrTiO3(La,Cr) prepared by the SHM showed efficient photocatalytic activities for both H2 evolution in the presence of an I- sacrificial reagent and for Z-scheme overall water splitting when it was coupled with the Pt-loaded WO3in the presence of I- and IO3- as the shuttle redox mediator. The superior photocatalytic activity of SrTiO3(La,Cr) prepared by the SHM has been ascribed to its more negative conduction-band position, higher carrier concentration, and higher carrier mobility, demonstrating that the design and synthesis of an H2-evolution photocatalyst with appropriate electronic properties is crucial for achieving Z-scheme overall water splitting.
The photocatalytic activity of a semiconductor-based photocatalyst largely depends on the semiconductor's intrinsic crystal and electronic properties. We have prepared two types of La and Cr co-doped SrTiO3 photocatalysts (SrTiO3(La,Cr)) using the polymerized complex method (PCM) and sol-gel hydrothermal method (SHM). Under λ > 420-nm visible light irradiation, only the Pt-loaded SrTiO3(La,Cr) prepared by the SHM showed efficient photocatalytic activities for both H2 evolution in the presence of an I- sacrificial reagent and for Z-scheme overall water splitting when it was coupled with the Pt-loaded WO3in the presence of I- and IO3- as the shuttle redox mediator. The superior photocatalytic activity of SrTiO3(La,Cr) prepared by the SHM has been ascribed to its more negative conduction-band position, higher carrier concentration, and higher carrier mobility, demonstrating that the design and synthesis of an H2-evolution photocatalyst with appropriate electronic properties is crucial for achieving Z-scheme overall water splitting.
2018, 39(3): 431-437
doi: 10.1016/S1872-2067(18)63043-8
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Photocatalytic water oxidation for O2 evolution is known as a bottle neck in water splitting. Various strategies have been conducted to keep the energetics of photogenerated holes or to create more holes in the bulk to reach the surface for efficient photocatalytic water oxidation. Our previous study demonstrated the effectiveness of interstitial boron doping in improving photocatalytic water oxidation by lowering the valence band maximum of TiO2 with a concentration gradient of boron. In this study, homogeneous doping of interstitial boron was realized in a TiO2 shell with mixed anatase/rutile phases that was produced by the gaseous hydrolysis of the surface layer of TiB2 crystals in a moist argon atmosphere. Consequently, the homogeneous doping and lowered valence band maximum improved the energetics of holes for efficient photocatalytic water oxidation.
Photocatalytic water oxidation for O2 evolution is known as a bottle neck in water splitting. Various strategies have been conducted to keep the energetics of photogenerated holes or to create more holes in the bulk to reach the surface for efficient photocatalytic water oxidation. Our previous study demonstrated the effectiveness of interstitial boron doping in improving photocatalytic water oxidation by lowering the valence band maximum of TiO2 with a concentration gradient of boron. In this study, homogeneous doping of interstitial boron was realized in a TiO2 shell with mixed anatase/rutile phases that was produced by the gaseous hydrolysis of the surface layer of TiB2 crystals in a moist argon atmosphere. Consequently, the homogeneous doping and lowered valence band maximum improved the energetics of holes for efficient photocatalytic water oxidation.
2018, 39(3): 438-445
doi: 10.1016/S1872-2067(18)63037-2
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Type-Ⅱ-heterojunction TiO2 nanorod arrays (NAs) are achieved by a combination of reduced and pristine TiO2 NAs through a simple electrochemical reduction. The heterojunc-tion-structured TiO2 NAs exhibit an enhanced photo-efficiency, with respect to those of pristine TiO2 NAs and completely reduced black TiO2. The improved efficiency can be attributed to a synergistic effect of two contributions of the partially reduced TiO2 NAs. The light absorption is significantly increased, from the UV to the visible spectrum. Moreover, the type Ⅱ structure leads to enhanced separation and transport of the electrons and charges. The proposed electro-chemical approach could be applied to various semiconductors for a control of the band struc-ture and improved photoelectrochemical performance.
Type-Ⅱ-heterojunction TiO2 nanorod arrays (NAs) are achieved by a combination of reduced and pristine TiO2 NAs through a simple electrochemical reduction. The heterojunc-tion-structured TiO2 NAs exhibit an enhanced photo-efficiency, with respect to those of pristine TiO2 NAs and completely reduced black TiO2. The improved efficiency can be attributed to a synergistic effect of two contributions of the partially reduced TiO2 NAs. The light absorption is significantly increased, from the UV to the visible spectrum. Moreover, the type Ⅱ structure leads to enhanced separation and transport of the electrons and charges. The proposed electro-chemical approach could be applied to various semiconductors for a control of the band struc-ture and improved photoelectrochemical performance.
2018, 39(3): 446-452
doi: 10.1016/S1872-2067(18)63024-4
Abstract:
Binuclear ruthenium complexes bearing the 2,2'-bipyridine-6,6'-dicarboxylate (bda) ligand have been demonstrated to be highly active catalysts towards water oxidation with CeIV as an oxidant. However, the catalytic properties of ruthenium dimers have not yet been explored for visible light-driven water oxidation. Herein, the photocatalytic performance of a dipyridyl propane-bridged ruthenium dimer 2 was investigated in comparison with its monomeric precursor,[Ru(bda)(pic)2] (1), in CH3CN/phosphate buffer mixed solvent in a three-component system including a photosensitizer and a sacrificial electron acceptor. Experimental results showed that the activity of each catalyst was strongly dependent on the content of CH3CN in the phosphate buffer, which not only affected the driving force for water oxidation, but also altered the kinetics of the reaction, probably through different mechanisms associated with the O-O bond formation. As a result, dimer 2 showed significantly higher activity than monomer 1 in the solvent containing a low content of CH3CN, and comparable activities were attained with a high content of CH3CN in the solvent. Under the optimal conditions, complex 2 achieved a turnover number of 638 for photocatalytic O2 evolution.
Binuclear ruthenium complexes bearing the 2,2'-bipyridine-6,6'-dicarboxylate (bda) ligand have been demonstrated to be highly active catalysts towards water oxidation with CeIV as an oxidant. However, the catalytic properties of ruthenium dimers have not yet been explored for visible light-driven water oxidation. Herein, the photocatalytic performance of a dipyridyl propane-bridged ruthenium dimer 2 was investigated in comparison with its monomeric precursor,[Ru(bda)(pic)2] (1), in CH3CN/phosphate buffer mixed solvent in a three-component system including a photosensitizer and a sacrificial electron acceptor. Experimental results showed that the activity of each catalyst was strongly dependent on the content of CH3CN in the phosphate buffer, which not only affected the driving force for water oxidation, but also altered the kinetics of the reaction, probably through different mechanisms associated with the O-O bond formation. As a result, dimer 2 showed significantly higher activity than monomer 1 in the solvent containing a low content of CH3CN, and comparable activities were attained with a high content of CH3CN in the solvent. Under the optimal conditions, complex 2 achieved a turnover number of 638 for photocatalytic O2 evolution.
Steering plasmonic hot electrons to realize enhanced full-spectrum photocatalytic hydrogen evolution
2018, 39(3): 453-462
doi: 10.1016/S1872-2067(17)62938-3
Abstract:
Integration of surface plasmons into photocatalysis is an intriguing approach to extend the light absorption range over the full solar spectrum. However, the low migration rates and uncertain diffusion directions of plasmonic hot electrons make their photocatalytic efficiency fail to meet expectations. It remains a challenging task to steer the migration of hot electrons and take full advantage of the plasmonic effect to achieve the desired high photocatalytic efficiency. Herein, we have developed an efficient strategy to steer the migration of plasmonic hot electrons through a well-designed hybrid structure that synergizes a "surface heterojunction" with a Schottky junction. The hybrid structure was synthesized by modifying titanium dioxide (TiO2) nanosheets (NSs) with gold (Au) nanoparticles (NPs) as a plasmonic metal and platinum (Pt) NPs as a co-catalyst. The "surface heterojunction" formed between two different crystal facets in the TiO2 NSs can induce the injection of plasmonic hot electrons from Au NPs excited by visible light to TiO2. Meanwhile, the Schottky junction formed between the Pt NPs and TiO2 NSs can force the migration of electrons from TiO2 to Pt NPs instead of flowing to Au NPs, attaining the efficient unidirectional transfer of carriers in the Au-TiO2 system. Plasmonic photocatalysts with this design achieved dramatically enhanced activity in full-spectrum photocatalytic hydrogen production. This work opens a new window to rationally design hybrid structures for full-spectrum photocatalysis.
Integration of surface plasmons into photocatalysis is an intriguing approach to extend the light absorption range over the full solar spectrum. However, the low migration rates and uncertain diffusion directions of plasmonic hot electrons make their photocatalytic efficiency fail to meet expectations. It remains a challenging task to steer the migration of hot electrons and take full advantage of the plasmonic effect to achieve the desired high photocatalytic efficiency. Herein, we have developed an efficient strategy to steer the migration of plasmonic hot electrons through a well-designed hybrid structure that synergizes a "surface heterojunction" with a Schottky junction. The hybrid structure was synthesized by modifying titanium dioxide (TiO2) nanosheets (NSs) with gold (Au) nanoparticles (NPs) as a plasmonic metal and platinum (Pt) NPs as a co-catalyst. The "surface heterojunction" formed between two different crystal facets in the TiO2 NSs can induce the injection of plasmonic hot electrons from Au NPs excited by visible light to TiO2. Meanwhile, the Schottky junction formed between the Pt NPs and TiO2 NSs can force the migration of electrons from TiO2 to Pt NPs instead of flowing to Au NPs, attaining the efficient unidirectional transfer of carriers in the Au-TiO2 system. Plasmonic photocatalysts with this design achieved dramatically enhanced activity in full-spectrum photocatalytic hydrogen production. This work opens a new window to rationally design hybrid structures for full-spectrum photocatalysis.
Water oxidation catalytic ability of polypyridine complex containing a μ-OH, μ-O2 dicobalt(iii) core
2018, 39(3): 463-471
doi: 10.1016/S1872-2067(17)62923-1
Abstract:
Two polypyridine complexes containing μ-OH, μ-O2 dicobalt(Ⅲ) cores,[(TPA)CoⅢ(μ-OH)(μ-O2)CoⅢ(TPA)](ClO4)3 and[(BPMEN)CoⅢ(μ-OH)(μ-O2)CoⅢ(BPMEN)](ClO4)3 (TPA=tris(2-pyridylmethyl)amine, BPMEN=N, N'-dimethyl-N, N'-bis(pyridin-2-ylmethyl)ethane-1,2-diamine), have previously been reported as inactive in the light-driven water oxidation reaction (ACS Catal., 2016, 6, 5062-5068). Herein, another dicobalt(Ⅲ) compound, μ-OH, μ-O2-[{(enN4)2Co2}](ClO4)3 (enN4=1,6-bis(2-pyridyl-2,5-diazaocta-2,6-diene), with a similar core structure was synthesized, characterized, and applied to the light-driven water oxidation reaction. Collective experiments showed that the complex itself was also inactive in the light-driven water oxidation, and that the activity observed originated from Co(Ⅱ) impurities. This research establishes that complexes possessing a μ-OH, μ-O2 dicobalt(Ⅲ) core structure are not appropriate choices for true molecular catalysts of water oxidation.
Two polypyridine complexes containing μ-OH, μ-O2 dicobalt(Ⅲ) cores,[(TPA)CoⅢ(μ-OH)(μ-O2)CoⅢ(TPA)](ClO4)3 and[(BPMEN)CoⅢ(μ-OH)(μ-O2)CoⅢ(BPMEN)](ClO4)3 (TPA=tris(2-pyridylmethyl)amine, BPMEN=N, N'-dimethyl-N, N'-bis(pyridin-2-ylmethyl)ethane-1,2-diamine), have previously been reported as inactive in the light-driven water oxidation reaction (ACS Catal., 2016, 6, 5062-5068). Herein, another dicobalt(Ⅲ) compound, μ-OH, μ-O2-[{(enN4)2Co2}](ClO4)3 (enN4=1,6-bis(2-pyridyl-2,5-diazaocta-2,6-diene), with a similar core structure was synthesized, characterized, and applied to the light-driven water oxidation reaction. Collective experiments showed that the complex itself was also inactive in the light-driven water oxidation, and that the activity observed originated from Co(Ⅱ) impurities. This research establishes that complexes possessing a μ-OH, μ-O2 dicobalt(Ⅲ) core structure are not appropriate choices for true molecular catalysts of water oxidation.
2018, 39(3): 472-478
doi: 10.1016/S1872-2067(17)62961-9
Abstract:
A two-step photocatalytic water splitting system, termed a "Z-scheme system", was achieved using Zn-doped g-C3N4 for H2 evolution and BiVO4 for O2 evolution with Fe2+/Fe3+ as a shuttle redox mediator. H2 and O2 were evaluated simultaneously when the doping amount of zinc was 10%. Moreover, Zn-doped (10%) g-C3N4 synthesized by an impregnation method showed superior active ability to form the Z-scheme with BiVO4 than by in-situ synthesis. X-ray diffraction, UV-Vis spectroscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy were used to characterize the samples. It was determined that more Zn-N bonds could be formed on the surface of g-C3N4 by impregnation, which could facilitate charge transfer.
A two-step photocatalytic water splitting system, termed a "Z-scheme system", was achieved using Zn-doped g-C3N4 for H2 evolution and BiVO4 for O2 evolution with Fe2+/Fe3+ as a shuttle redox mediator. H2 and O2 were evaluated simultaneously when the doping amount of zinc was 10%. Moreover, Zn-doped (10%) g-C3N4 synthesized by an impregnation method showed superior active ability to form the Z-scheme with BiVO4 than by in-situ synthesis. X-ray diffraction, UV-Vis spectroscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy were used to characterize the samples. It was determined that more Zn-N bonds could be formed on the surface of g-C3N4 by impregnation, which could facilitate charge transfer.
2018, 39(3): 479-486
doi: 10.1016/S1872-2067(17)62892-4
Abstract:
Developing highly efficient and stable water oxidation catalysts based on abundant metallic elements is a challenge that must be met to fulfill the promise of water splitting for clean energy production. In this work, we developed an oxygen evolution reaction catalyst consisting of a nanostructured film electrodeposited from a phosphate buffer solution (0.2 mol/L, pH=12.0) containing Cu-tricine complex. A Tafel plot showed that the required overpotential for a current density of 1.0 mA/cm2 was only 395 mV and the Tafel slope was 46.7 mV/decade. In addition, the Cu-tricine film maintained a stable current density of 7.5 mA/cm2 for the oxygen evolution reaction in phosphate buffer solution for 10 h, and a Faradaic efficiency of 99% was obtained.
Developing highly efficient and stable water oxidation catalysts based on abundant metallic elements is a challenge that must be met to fulfill the promise of water splitting for clean energy production. In this work, we developed an oxygen evolution reaction catalyst consisting of a nanostructured film electrodeposited from a phosphate buffer solution (0.2 mol/L, pH=12.0) containing Cu-tricine complex. A Tafel plot showed that the required overpotential for a current density of 1.0 mA/cm2 was only 395 mV and the Tafel slope was 46.7 mV/decade. In addition, the Cu-tricine film maintained a stable current density of 7.5 mA/cm2 for the oxygen evolution reaction in phosphate buffer solution for 10 h, and a Faradaic efficiency of 99% was obtained.
2018, 39(3): 487-494
doi: 10.1016/S1872-2067(17)62896-1
Abstract:
Umpolung reactions of C=X bonds (X=O, N) are valuable ways of constructing new C-C bonds, which are sometimes difficult to be constructed using traditional synthetic pathways. Classical polarity inversion of C=X bonds (X=O, N) usually requires air or moisture-sensitive and strong reducing agents, which limit the feasibility of substrate scope. Herein we describe a photo-induced reductive cross-coupling reaction of aldehydes, ketones and imines with electron-deficient arenes (aromatic nitriles) using fac-Ir(ppy)3 as a photocatalyst and diisopropylethylamine (DIPEA) as a terminal reductant under visible light irradiation. Mild conditions and high yields mean that this new polarity inversion strategy can be used with aryl-substituted alcohols and amines. Spectroscopic studies and control experiments have demonstrated the oxidative quenching of Ir(ppy)3* by electron-deficient arenes involved in the key step for the C-C bond formation.
Umpolung reactions of C=X bonds (X=O, N) are valuable ways of constructing new C-C bonds, which are sometimes difficult to be constructed using traditional synthetic pathways. Classical polarity inversion of C=X bonds (X=O, N) usually requires air or moisture-sensitive and strong reducing agents, which limit the feasibility of substrate scope. Herein we describe a photo-induced reductive cross-coupling reaction of aldehydes, ketones and imines with electron-deficient arenes (aromatic nitriles) using fac-Ir(ppy)3 as a photocatalyst and diisopropylethylamine (DIPEA) as a terminal reductant under visible light irradiation. Mild conditions and high yields mean that this new polarity inversion strategy can be used with aryl-substituted alcohols and amines. Spectroscopic studies and control experiments have demonstrated the oxidative quenching of Ir(ppy)3* by electron-deficient arenes involved in the key step for the C-C bond formation.
2018, 39(3): 495-501
doi: 10.1016/S1872-2067(17)62946-2
Abstract:
A series of alloyed Zn-Cd-S solid solutions with a cubic zinc blende structure were fabricated hydrothermally with the assistance of L-cystine under mild conditions. The products were characterized by XRD, TEM, HRTEM, XPS, UV-vis, and BET techniques, and the photocatalytic performance for the reduction of water to H2 on the solid solutions was evaluated in the presence of S2-/SO32- as hole scavengers under visible light illumination. Among all the samples, the highest photocatalytic activity was achieved over Zn0.9Cd0.1S with a rate of 4.4 mmol h-1 g-1, even without a co-catalyst, which far exceeded that of CdS. Moreover, Zn0.9Cd0.1S displayed excellent anti-photocorrosion properties during the photoreduction of water into H2. The enhancement in the photocatalytic performance was mainly attributed to the efficient charge transfer in the Zn0.9Cd0.1 alloyed structure and the high surface area. This work provides a simple, cost-effective and green technique, which can be generalized as a rational preparation route for the large-scale fabrication of metal sulfide photocatalysts.
A series of alloyed Zn-Cd-S solid solutions with a cubic zinc blende structure were fabricated hydrothermally with the assistance of L-cystine under mild conditions. The products were characterized by XRD, TEM, HRTEM, XPS, UV-vis, and BET techniques, and the photocatalytic performance for the reduction of water to H2 on the solid solutions was evaluated in the presence of S2-/SO32- as hole scavengers under visible light illumination. Among all the samples, the highest photocatalytic activity was achieved over Zn0.9Cd0.1S with a rate of 4.4 mmol h-1 g-1, even without a co-catalyst, which far exceeded that of CdS. Moreover, Zn0.9Cd0.1S displayed excellent anti-photocorrosion properties during the photoreduction of water into H2. The enhancement in the photocatalytic performance was mainly attributed to the efficient charge transfer in the Zn0.9Cd0.1 alloyed structure and the high surface area. This work provides a simple, cost-effective and green technique, which can be generalized as a rational preparation route for the large-scale fabrication of metal sulfide photocatalysts.
2018, 39(3): 502-509
doi: 10.1016/S1872-2067(17)62943-7
Abstract:
Photocatalytic water oxidation based on semiconductors usually suffers from poor charge transfer from the bulk to the interface, which is necessary for oxygen generation. Here, we construct a hybrid artificial photosynthesis system for photocatalytic water oxidation. The system consists of BiVO4 as the light harvester, a transitional metal complex (M(dca)2, M=Co, Ni, dca:dicyanamide) as the water oxidation catalyst, and S2O82- as a sacrificial electron acceptor. The system exhibits enhanced oxygen evolution activity when M(dca)2 is introduced. The BiVO4/Co(dca)2 and BiVO4/Ni(dca)2 systems exhibit excellent oxygen evolution rates of 508.1 and 297.7 μmol/(h·g) compared to the pure BiVO4 which shows a photocatalytic oxygen evolution rate of 252.2 μmol/(h·g) during 6 h of photocatalytic reaction. Co(dca)2 is found to be more effective than Ni(dca)2 as a water oxidation catalyst. The enhanced photocatalytic performance is ascribed to the M(dca)2-engineered BiVO4/electrolyte interface energetics, and to the M(dca)2-catalyzed surface water oxidation. These two factors lead to a decrease in the energy barrier for hole transfer from the bulk to the surface of BiVO4, which promotes the water oxidation kinetics.
Photocatalytic water oxidation based on semiconductors usually suffers from poor charge transfer from the bulk to the interface, which is necessary for oxygen generation. Here, we construct a hybrid artificial photosynthesis system for photocatalytic water oxidation. The system consists of BiVO4 as the light harvester, a transitional metal complex (M(dca)2, M=Co, Ni, dca:dicyanamide) as the water oxidation catalyst, and S2O82- as a sacrificial electron acceptor. The system exhibits enhanced oxygen evolution activity when M(dca)2 is introduced. The BiVO4/Co(dca)2 and BiVO4/Ni(dca)2 systems exhibit excellent oxygen evolution rates of 508.1 and 297.7 μmol/(h·g) compared to the pure BiVO4 which shows a photocatalytic oxygen evolution rate of 252.2 μmol/(h·g) during 6 h of photocatalytic reaction. Co(dca)2 is found to be more effective than Ni(dca)2 as a water oxidation catalyst. The enhanced photocatalytic performance is ascribed to the M(dca)2-engineered BiVO4/electrolyte interface energetics, and to the M(dca)2-catalyzed surface water oxidation. These two factors lead to a decrease in the energy barrier for hole transfer from the bulk to the surface of BiVO4, which promotes the water oxidation kinetics.
2018, 39(3): 510-516
doi: 10.1016/S1872-2067(17)62968-1
Abstract:
Ti3+ species and oxygen vacancies were successfully introduced into anatase TiO2 by a simple method. The physicochemical properties of the as-prepared samples were explored by techniques including X-ray diffraction and field emission scanning electron microscopy. The amount of H2O2 precursor and hydrothermal reaction time were found to affect the formation of the TiO2 nanorod-type microstructure. Possible mechanisms for the formation of this microstructure are proposed on the basis of analytical results, including electron paramagnetic resonance, where parameters such as the amount of H2O2 and hydrothermal reaction time were adjusted. More importantly, light absorption in the visible light region was strongly affected by the number of oxygen vacancies within the samples. The oxygen vacancy content featured an optimal level for producing the highest photocatalytic hydrogen production activity.
Ti3+ species and oxygen vacancies were successfully introduced into anatase TiO2 by a simple method. The physicochemical properties of the as-prepared samples were explored by techniques including X-ray diffraction and field emission scanning electron microscopy. The amount of H2O2 precursor and hydrothermal reaction time were found to affect the formation of the TiO2 nanorod-type microstructure. Possible mechanisms for the formation of this microstructure are proposed on the basis of analytical results, including electron paramagnetic resonance, where parameters such as the amount of H2O2 and hydrothermal reaction time were adjusted. More importantly, light absorption in the visible light region was strongly affected by the number of oxygen vacancies within the samples. The oxygen vacancy content featured an optimal level for producing the highest photocatalytic hydrogen production activity.
2018, 39(3): 517-526
doi: 10.1016/S1872-2067(17)62940-1
Abstract:
Increasing interest has been paid to the development of earth-abundant metal complexes as promising surrogates of platinum for the electrocatalytically and photocatalytically driven hydrogen evolution reaction. In this work, we report on molecular H2-evolving catalysts based on two octahedral complexes of cobalt thiosemicarbazide, fac-[Co(Htsc)3]Cl3·3H2O (C1, Htsc=thiosemicarbazide) and mer-[Co(Htsc)3]Cl3·4H2O (C2), which have facial (fac) and meridional (mer) geometry, respectively. Electrochemical studies confirmed that both C1 and C2 are active electrocatalysts in MeOH solution using acetic acid as the proton source, with the same overpotential of~640 mV and TOF of~210 s-1. The complex C1 also exhibits electrocatalytic activity for hydrogen evolution reaction in aqueous media free of organic solvent with a moderate overpotential (560 mV). Visible light-driven hydrogen evolution experiments were carried out in combination with fluorescein as photosensitizer and triethylamine as sacrificial reductant in homogeneous environments. Our studies showed that both C1 and C2 can be used as efficient proton-reduction catalysts in purely aqueous solution and have the same photocatalytic activities. A TOF of 125 h-1 with a TON of 900 for photocatalytic H2 generation using C1 and C2 in water were achieved for the noble-metal-free homogeneous system. It should be noted that this is the first reported study investigating the effect on the catalytic hydrogen production performance of using fac-and mer-isomers of molecular catalysts.
Increasing interest has been paid to the development of earth-abundant metal complexes as promising surrogates of platinum for the electrocatalytically and photocatalytically driven hydrogen evolution reaction. In this work, we report on molecular H2-evolving catalysts based on two octahedral complexes of cobalt thiosemicarbazide, fac-[Co(Htsc)3]Cl3·3H2O (C1, Htsc=thiosemicarbazide) and mer-[Co(Htsc)3]Cl3·4H2O (C2), which have facial (fac) and meridional (mer) geometry, respectively. Electrochemical studies confirmed that both C1 and C2 are active electrocatalysts in MeOH solution using acetic acid as the proton source, with the same overpotential of~640 mV and TOF of~210 s-1. The complex C1 also exhibits electrocatalytic activity for hydrogen evolution reaction in aqueous media free of organic solvent with a moderate overpotential (560 mV). Visible light-driven hydrogen evolution experiments were carried out in combination with fluorescein as photosensitizer and triethylamine as sacrificial reductant in homogeneous environments. Our studies showed that both C1 and C2 can be used as efficient proton-reduction catalysts in purely aqueous solution and have the same photocatalytic activities. A TOF of 125 h-1 with a TON of 900 for photocatalytic H2 generation using C1 and C2 in water were achieved for the noble-metal-free homogeneous system. It should be noted that this is the first reported study investigating the effect on the catalytic hydrogen production performance of using fac-and mer-isomers of molecular catalysts.
2018, 39(3): 527-533
doi: 10.1016/S1872-2067(17)62931-0
Abstract:
Photocatalytic H2 evolution under visible light irradiation is an ideal process for solving energy shortage. The low cost of photocatalysts and high efficiency of hydrogen evolution are the two key factors to realize the industrialization of the process. The substitution of a noble-metal cocatalyst with a non-noble-metal catalyst can significantly reduce the cost of the photocatalyst. The large-scale synthesis and assembly of semiconductors and non-noble-metal cocatalysts to form photocatalysts through a simple method can further decrease the cost of photocatalysis. Here, we report a large-scale and low-cost coprecipitation method to form phosphide/CdS photocatalysts to realize photocatalytic H2 evolution. CoP and MoP cocatalysts significantly enhanced the photocatalytic production of hydrogen. The optimal H2 production rates on CoP/CdS and MoP/CdS were 140and 78 μmol/h, which were 7.0 and 4.0 times higher than those obtained with bare CdS, respectively, and 2.0 times and 1.1 times higher than those obtained with 1.0% Pt/CdS, respectively. This work provides a practical method for the large-scale preparation of low-cost photocatalysts.
Photocatalytic H2 evolution under visible light irradiation is an ideal process for solving energy shortage. The low cost of photocatalysts and high efficiency of hydrogen evolution are the two key factors to realize the industrialization of the process. The substitution of a noble-metal cocatalyst with a non-noble-metal catalyst can significantly reduce the cost of the photocatalyst. The large-scale synthesis and assembly of semiconductors and non-noble-metal cocatalysts to form photocatalysts through a simple method can further decrease the cost of photocatalysis. Here, we report a large-scale and low-cost coprecipitation method to form phosphide/CdS photocatalysts to realize photocatalytic H2 evolution. CoP and MoP cocatalysts significantly enhanced the photocatalytic production of hydrogen. The optimal H2 production rates on CoP/CdS and MoP/CdS were 140and 78 μmol/h, which were 7.0 and 4.0 times higher than those obtained with bare CdS, respectively, and 2.0 times and 1.1 times higher than those obtained with 1.0% Pt/CdS, respectively. This work provides a practical method for the large-scale preparation of low-cost photocatalysts.
2018, 39(3): 534-541
doi: 10.1016/S1872-2067(17)62973-5
Abstract:
A[H3AgI(H2O)PW11O39]3--TiO2/ITO electrode was fabricated by immobilizing a molecular polyoxometalate-based water oxidation catalyst,[H3AgI(H2O)PW11O39]3- (AgPW11), on a TiO2 electrode. The resulting electrode was characterized by X-ray powder diffraction, scanning electron microscopy, and energy dispersive X-ray spectroscopy. Linear sweep voltammetry, chronoamperometry, and electrochemical impedance measurements were performed in aqueous Na2SO4 solution (0.1 mol L-1). We found that a higher applied voltage led to better catalytic performance by AgPW11. The AgPW11-TiO2/ITO electrode gave currents respectively 10 and 2.5 times as high as those of the TiO2/ITO and AgNO3-TiO2/ITO electrodes at an applied voltage of 1.5 V vs Ag/AgCl. This result was attributed to the lower charge transfer resistance at the electrode-electrolyte interface for the AgPW11-TiO2/ITO electrode. Under illumination, the photocurrent was not obviously enhanced although the total anode current increased. The AgPW11-TiO2/ITO electrode was relatively stable. Cyclic voltammetry of AgPW11 was performed in phosphate buffer solution (0.1 mol L-1). We found that oxidation of AgPW11 was a quasi-reversible process related to one-electron and one-proton transfer. We deduced that disproportionation of the oxidized[H2AgⅡ(H2O)PW11O39]3- might have occurred and the resulting[H3AgⅢOPW11O39]3- oxidized water to O2.
A[H3AgI(H2O)PW11O39]3--TiO2/ITO electrode was fabricated by immobilizing a molecular polyoxometalate-based water oxidation catalyst,[H3AgI(H2O)PW11O39]3- (AgPW11), on a TiO2 electrode. The resulting electrode was characterized by X-ray powder diffraction, scanning electron microscopy, and energy dispersive X-ray spectroscopy. Linear sweep voltammetry, chronoamperometry, and electrochemical impedance measurements were performed in aqueous Na2SO4 solution (0.1 mol L-1). We found that a higher applied voltage led to better catalytic performance by AgPW11. The AgPW11-TiO2/ITO electrode gave currents respectively 10 and 2.5 times as high as those of the TiO2/ITO and AgNO3-TiO2/ITO electrodes at an applied voltage of 1.5 V vs Ag/AgCl. This result was attributed to the lower charge transfer resistance at the electrode-electrolyte interface for the AgPW11-TiO2/ITO electrode. Under illumination, the photocurrent was not obviously enhanced although the total anode current increased. The AgPW11-TiO2/ITO electrode was relatively stable. Cyclic voltammetry of AgPW11 was performed in phosphate buffer solution (0.1 mol L-1). We found that oxidation of AgPW11 was a quasi-reversible process related to one-electron and one-proton transfer. We deduced that disproportionation of the oxidized[H2AgⅡ(H2O)PW11O39]3- might have occurred and the resulting[H3AgⅢOPW11O39]3- oxidized water to O2.
2018, 39(3): 542-548
doi: 10.1016/S1872-2067(18)63044-X
Abstract:
Inefficient charge separation and limited light absorption are two critical issues associated with high-efficiency photocatalytic H2 production using TiO2. Surface defects within a certain concentration range in photocatalyst materials are beneficial for photocatalytic activity. In this study, surface defects (oxygen vacancies and metal cation replacement defects) were induced with a facile and effective approach by surface doping with low-cost transition metals (Co, Ni, Cu, and Mn) on ultrafine TiO2. The obtained surface-defective TiO2 exhibited a 3-4-fold improved activity compared to that of the original ultrafine TiO2. In addition, a H2 production rate of 3.4 μmol/h was obtained using visible light (λ > 420 nm) irradiation. The apparent quantum yield (AQY) at 365 nm reached 36.9% over TiO2-Cu, significantly more than the commercial P25 TiO2. The enhancement of photocatalytic H2 production activity can be attributed to improved rapid charge separation efficiency and expanded light absorption window. This hydrothermal treatment with transition metal was proven to be a very facile and effective method for obtaining surface defects.
Inefficient charge separation and limited light absorption are two critical issues associated with high-efficiency photocatalytic H2 production using TiO2. Surface defects within a certain concentration range in photocatalyst materials are beneficial for photocatalytic activity. In this study, surface defects (oxygen vacancies and metal cation replacement defects) were induced with a facile and effective approach by surface doping with low-cost transition metals (Co, Ni, Cu, and Mn) on ultrafine TiO2. The obtained surface-defective TiO2 exhibited a 3-4-fold improved activity compared to that of the original ultrafine TiO2. In addition, a H2 production rate of 3.4 μmol/h was obtained using visible light (λ > 420 nm) irradiation. The apparent quantum yield (AQY) at 365 nm reached 36.9% over TiO2-Cu, significantly more than the commercial P25 TiO2. The enhancement of photocatalytic H2 production activity can be attributed to improved rapid charge separation efficiency and expanded light absorption window. This hydrothermal treatment with transition metal was proven to be a very facile and effective method for obtaining surface defects.
