Regulating Lewis Acidic Sites of 1T-2H MoS2 Catalysts for Solar-Driven Photothermal Catalytic H2 Production from Lignocellulosic Biomass.

Solar-driven photothermal catalytic H2 production from lignocellulosic biomass was achieved by using 1T-2H MoS2 with tunable Lewis acidic sites as catalysts in an alkaline aqueous solution, in which the number of Lewis acidic sites derived from the exposed Mo edges of MoS2 was successfully regulated by both the formation of an edge-terminated 1T-2H phase structure and tunable layer number. Owing to the abundant Lewis acidic sites for the oxygenolysis of lignocellulosic biomass, the 1T-2H MoS2 catalyst shows high photothermal catalytic lignocellulosic biomass-to-H2 transformation performance in polar wood chips, bamboo, rice straw corncobs, and rice hull aqueous solutions, and the highest H2 generation rate and solar-to-H2 (STH) efficiency respectively achieves 3661 µmol·h-1·g-1 and 0.18% in the polar wood chip system under 300 W Xe lamp illumination. This study provides a sustainable and cost-effective method for the direct transformation of renewable lignocellulosic biomass to H2 fuel driven by solar energy.

Photocatalytic Nitrogen Fixation Coupled with the Generation of Value-Added Chemicals from N2 and Cellulose over MoO3 Nanosheets.

Photocatalytic nitrogen fixation from N2 provides an alternative strategy for ammonia (NH3) production, but it was limited by the consumption of a sacrificial electron donor for the currently reported half-reaction system. Here, we use naturally abundant and renewable cellulose as the sacrificial reagent for photocatalytic nitrogen fixation over oxygen-vacancy-modified MoO3 nanosheets as the photocatalyst. In this smartly designed photocatalytic system, the photooxidation of cellulose not only generates value-added chemicals but also provides electrons for the N2 reduction reaction and results in the production of NH3 with a maximum rate of 68 µmol·h-1·g-1. Also, the oxygen vacancies provide efficient active sites for both cellulose oxygenolysis and nitrogen fixation reactions. This work represents useful inspiration for realizing nitrogen fixation coupled with the generation of value-added chemicals from N2 and cellulose through a photocatalysis strategy.

Accelerated Charge Transfer through Interface Chemical Bonds in MoS2/TiO2 for Photocatalytic Conversion of Lignocellulosic Biomass to H2.

Solar photocatalytic H2 production from lignocellulosic biomass has attracted great interest, but it suffers from low photocatalytic efficiency owing to the absence of highly efficient photocatalysts. Herein, we designed and constructed ultrathin MoS2-modified porous TiO2 microspheres (MT) with abundant interface Ti-S bonds as photocatalysts for photocatalytic H2 generation from lignocellulosic biomass. Owing to the accelerated charge transfer related to Ti-S bonds, as well as the abundant active sites for both H2 and ●OH generation, respectively, related to the high exposed edge of MoS2 and the large specific surface area of TiO2, MT photocatalysts demonstrate good performance in the photocatalytic conversion of α-cellulose and lignocellulosic biomass to H2. The highest H2 generation rate of 849 µmol·g-1·h-1 and apparent quantum yield of 4.45% at 380 nm was achieved in α-cellulose aqueous solution for the optimized MT photocatalyst. More importantly, lignocellulosic biomass of corncob, rice hull, bamboo, polar wood chip, and wheat straw were successfully converted to H2 over MT photocatalysts with H2 generation rate of 10, 19, 36, 29, and 8 µmol·g-1·h-1, respectively. This work provides a guiding design approach to develop highly active photocatalysts via interface engineering for solar H2 production from lignocellulosic biomass.

Visible-Light-Driven Photocatalytic Cellulose-to-H2 Conversion by MoS2 /ZnIn2 S4 Photocatalyst with Cellulase Assistance.

Visible-light-driven photocatalytic cellulose-to-H2 conversion system was successfully designed by using MoS2 /ZnIn2 S4 as the photocatalyst and cellulase as the enzyme catalyst. At first, the cellulose was converted to glucose by cellulase. The generated glucose acted as an efficient hole trapper and electron donor, which was further converted into H2 through photocatalytic reaction over MoS2 /ZnIn2 S4 under visible light irradiation. The optimum H2 generation rate achieved under visible light irradiation (λ>420 nm) was 12.2 µmol ⋅ h-1 ⋅ g-1 in the presence of 100 mg of 3 % MoS2 /ZnIn2 S4 , 100 mg cellulase and 2 g poplar wood chip. These results open up a new possibility for the development of efficient visible-light-responding photocatalytic cellulose to H2 conversion system that combine photocatalysis and enzyme technology.

A Water-Soluble Highly Oxidizing Cobalt Molecular Catalyst Designed for Bioinspired Water Oxidation.

Herein, we present a stable water-soluble cobalt complex supported by a dianionic 2,2'-([2,2'-bipyridine]-6,6'-diyl)bis(propan-2-ol) ligand scaffold, which is a rare example of a high-oxidation species, as demonstrated by structural, spectroscopic and theoretical data. Electron paramagnetic resonance (EPR) spectroscopy and magnetic susceptibility measurements revealed that the CoIV center of the mononuclear complex in the solid state resides in the high spin state (sextet, S=5/2). The complex can effectively catalyze water oxidation via a single-site water nucleophilic attack pathway with an overpotential of only 360 mV in a phosphate buffer with a pH of 6. The key intermediate toward water oxidation was speculated based on theoretical calculations and was identified by in situ spectroelectrochemical experiments. The results are important regarding the accessibility of high-oxidation state metal species in synthetic models for achieving robust and reactive oxidation catalysis.

Metal-complex chromophores for solar hydrogen generation.

Solar H2 generation from water has been intensively investigated as a clean method to convert solar energy into hydrogen fuel. During the past few decades, many studies have demonstrated that metal complexes can act as efficient photoactive materials for photocatalytic H2 production. Here, we review the recent progress in the application of metal-complex chromophores to solar-to-H2 conversion, including metal-complex photosensitizers and supramolecular photocatalysts. A brief overview of the fundamental principles of photocatalytic H2 production is given. Then, different metal-complex photosensitizers and supramolecular photocatalysts are introduced in detail, and the most important factors that strictly determine their photocatalytic performance are also discussed. Finally, we illustrate some challenges and opportunities for future research in this promising area.

Energy Transfer in a Hybrid Ir(III) Carbene-Pt(II) Acetylide Assembly for Efficient Hydrogen Production.

A new heterometallic supramolecular complex, consisting of an iridium carbene-based unit appended to a platinum terpyridine acetylide unit, representing a new Ir(III) -Pt(II) structural motif, was designed and developed to act as an active species for photocatalytic hydrogen production. The results also suggested that a light-harvesting process is essential to realize the solar-to-fuel conversion in an artificial system as illustrated in the natural photosynthetic system.

A Noble-Metal-Free Nickel(II) Polypyridyl Catalyst for Visible-Light-Driven Hydrogen Production from Water.

The complex [Ni(bpy)3](2+) (bpy=2,2'-bipyridine) is an active catalyst for visible-light-driven H2 production from water when employed with [Ir(dfppy)2 (Hdcbpy)] [dfppy=2-(3,4-difluorophenyl)pyridine, Hdcbpy=4-carboxy-2,2'-bipyridine-4'-carboxylate] as the photosensitizer and triethanolamine as the sacrificial electron donor. The highest turnover number of 520 with respect to the nickel(II) catalyst is obtained in a 8:2 acetonitrile/water solution at pH 9. The H2 -evolution system is more stable after the addition of an extra free bpy ligand, owing to faster catalyst regeneration. The photocatalytic results demonstrate that the nickel(II) polypyridyl catalyst can act as a more effective catalyst than the commonly utilized [Co(bpy)3 ](2+). This study may offer a new paradigm for constructing simple and noble-metal-free catalysts for photocatalytic hydrogen production.

Charge-neutral amidinate-containing iridium complexes capable of efficient photocatalytic water reduction.

Two new charge-neutral iridium complexes, [Ir(tfm-ppy)(2)(N,N'-diisopropyl-benzamidinate)] (1) and [Ir(tfm-ppy)(2)(N,N'-diisopropyl-4-diethylamino-3,5-dimethyl-benzamidinate)] (2) (tfm-ppy=4-trifluoromethyl-2-phenylpyridine) containing an amidinate ligand and two phenylpyridine ligands were designed and characterised. The photophysical properties, electrochemical behaviours and emission quenching properties of these species were investigated. In concert with the cobalt catalyst [Co(bpy)(3)](2+), members of this new class of iridium complexes enable the photocatalytic generation of hydrogen from mixed aqueous solutions via an oxidative quenching pathway and display long-term photostability under constant illumination over 72 h; one of these species achieved a relatively high turnover number of 1880 during this time period. In the case of complex 1, the three-component homogeneous photocatalytic system proved to be more efficient than a related system containing a charged complex, [Ir(tfm-ppy)(2)(dtb-bpy)](+) (3, dtb-bpy=4,4'-di-tert-butyl-2,2'-dipyridyl). In combination with a rhodium complex as a water reduction catalyst, the performances of the systems using both complexes were also evaluated, and these systems exhibited a more efficient catalytic propensity for water splitting than did the cobalt-based systems that have been studied previously.

Tricyclometalated iridium complexes as highly stable photosensitizers for light-induced hydrogen evolution.

The development of an efficient and stable artificial photosensitizer for visible-light-driven hydrogen production is highly desirable. Herein, a new series of charge-neutral, heteroleptic tricyclometalated iridium(III) complexes, [Ir(thpy)2(bt)] (1-4; thpy = 2,2'-thienylpyridine, bt = 2-phenylbenzothiazole and its derivatives), were systematically synthesized and their structural, photophysical, and electrochemical properties were established. Three solid-state structures were studied by X-ray crystallographic analysis. This design offers the unique opportunity to drive the metal-to-ligand charge-transfer (MLCT) band to longer wavelengths for these iridium complexes. We describe new molecular platforms that are based on these neutral iridium complexes for the production of hydrogen through visible-light-induced photocatalysis over an extended period of time in the presence of [Co(bpy)3](2+) and triethanolamine (TEOA). The maximum amount of hydrogen was obtained under constant irradiation over 72 h and the system could regenerate its activity upon the addition of cobalt-based catalysts when hydrogen evolution ceased. Our results demonstrated that the dissociation of the [Co(bpy)3](2+) catalyst contributed to the loss of catalytic activity and limited the long-term catalytic performance of the systems. The properties of the neutral complexes are compared in detail to those of two known non-neutral bpy-type complexes, [Ir(thpy)2(dtb-bpy)](+) (5) and [Ir(ppy)2(dtb-bpy)](+) (6; ppy = 2-phenylpyridine, dtb-bpy = 4,4'-di-tert-butyl-2,2'-dipyridyl). This work is expected to contribute toward the development of long-lasting solar hydrogen-production systems.

MoS2@N-doped graphitic carbon/TiO2 photocatalysts for photocatalytic H2 production from lignocellulosic biomass.

TiO2 nanoparticles grown on MoS2/N-doped graphitic carbon were demonstrated to be efficient noble-metal-free photocatalysts for H2 production from lignocellulosic biomass, and the H2 generation rate from wheat straw, corncob, polar wood chip, bamboo, rice hull, corn straw and rice straw aqueous solution respectively reaches 4.9, 6.7, 11.7, 14.5, 8.4, 7.3 and 6.2 µmol g-1 h-1.

Solar-Driven Photothermal Catalytic Lignocellulosic Biomass-to-H2 Conversion.

The conversion of lignocellulosic biomass to chemical fuel can achieve the sustainable use of lignocellulosic biomass, but it was limited by the lack of an effective conversion strategy. Here, we reported a unique approach of photothermal catalysis by using MoS2-reduced graphene oxide (MoS2/RGO) as the catalyst to convert lignocellulosic biomass into H2 fuel in alkaline solution. The RGO acting as a support for the growth of MoS2 results in the high exposed Mo edges, which act as efficient Lewis acidic sites for the oxygenolysis of lignocellulosic biomass dissolved in alkaline solution. The broad light absorption capacity and abundant Lewis acidic sites make MoS2/RGO to be efficient catalysts for photothermal catalytic H2 production from lignocellulosic biomass, and the H2 generation rate with respect to catalyst under 300 W Xe lamp irradiation in cellulose, rice straw, wheat straw, polar wood chip, bamboo, rice hull, and corncob aqueous solution achieve 223, 168, 230, 564, 390, 234, and 55 µmol·h-1·g-1, respectively. It is believed that this photothermal catalysis is a simple and "green" approach for the lignocellulosic biomass-to-H2 conversion, which would have great potential as a promising approach for solar energy-driven H2 production from lignocellulosic biomass.

Impact of ligand modification on hydrogen photogeneration and light-harvesting applications using cyclometalated iridium complexes.

To explore structure-activity relationships with respect to light-harvesting behavior, a family of bis-cyclometalated iridium complexes [Ir(C^N)(2)(Hbpdc)] 2-5 (where C^N = 2-phenylbenzothiazole and its functionalized derivatives, and H(2)bpdc =2,2'-bipyridine-4,4'-dicarboxylate) was synthesized using a facile method. The photophysical and electrochemical properties of these complexes were investigated and compared to those of analogue 1 (C^N = (4-trifluoromethyl)-2-phenylbenzothiazole); they were also investigated theoretically using density functional theory. The molecular structures of complexes 2-4 were determined by X-ray crystallography, which revealed typical octahedral coordination geometry. The structural modifications involved in the complexes were accomplished through the attributes of electron-withdrawing CF(3) and electron-donating NMe(2) substituents. The UV-vis spectra of these species, except for that of 5, displayed a broad absorption in the low-energy region, which originated from metal-to-ligand charge-transfer transitions. These complexes were found to exhibit visible-light-induced hydrogen production and light-to-electricity conversion in photoelectrochemical cells. The yield of hydrogen production from water using these complexes was compared, which revealed substantial dependences on their structures, particularly on the substituent of the cyclometalated ligand. Among the systems, the highest turnover number of 1501 was achieved with complex 2, in which the electron-withdrawing CF(3) substituent was connected to a phenyl ring of the cyclometalated ligand. The carboxylate anchoring groups made the complexes highly suitable for grafting onto TiO(2) (P25) surfaces for efficient electron transfer and thus resulted in an enhancement of hydrogen evolution compared to the unattached homogeneous systems. In addition, the combined incorporation of the electron-donating NMe(2) group and the electron-withdrawing CF(3) substituent on the cyclometalated ligand caused complex 5 to not work well for hydrogen production. Their incorporation, however, enhanced the performance of 5 in the light-harvesting application in nanocrystalline TiO(2) dye-sensitized solar cells, which was attributed to the intense absorption in the visible region.

SiP Nanosheets: A Metal-Free Two-Dimensional Photocatalyst for Visible-Light Photocatalytic H2 Production and Nitrogen Fixation.

Two-dimensional (2D) semiconductors for photocatalysis are more advantageous than the other photocatalytic materials since the 2D semiconductors generally have large specific surface area and abundant active sites. Phosphorus silicon (SiP), with an indirect bandgap in bulk and a direct bandgap in the monolayer, has recently emerged as an attractive 2D material because of its anisotropic layered structure, tunable bandgap, and high charge carrier mobility. However, the utilization of SiP as a photocatalyst for photocatalysis has been scarcely studied experimentally. Herein, we reported the synthesis of SiP nanosheets (SiP NSs) prepared from bulk SiP by an ultrasound-assisted liquid-phase exfoliation approach which can act as a metal-free, efficient, and visible-light-responsive photocatalyst for photocatalytic H2 production and nitrogen fixation. In a half-reaction system, the maximal H2 and NH3 generation rate under visible light irradiation achieves 528 and 35 µmol·h-1·g-1, respectively. Additionally, the apparent quantum yield for H2 production at 420 nm reaches 3.56%. Furthermore, a Z-scheme photocatalytic overall water-splitting system was successfully constructed by using Pt-loaded SiP NSs as the H2-evolving photocatalyst, Co3O4/BiVO4 as the O2-evolving photocatalyst, and Co(bpy)33+/2+ as an electron mediator. Notably, the highest H2 and O2 generation rate with respect to Pt/SiP NSs achieves 71 and 31 µmol·h-1·g-1, respectively. This study explores the potential application of 2D SiP as a metal-free visible-light-responsive photocatalyst for photocatalysis.

Understanding the hierarchical behavior of Bi2WO6 with enhanced photocatalytic nitrogen fixation activity.

Hierarchical Bi2WO6 nanostructures self-assembled with planar arranged nanosheets and dispersed Bi2WO6 nanosheets were synthesized with different dosages of EG via a simple hydrothermal route. The Bi2WO6 photocatalysts were analyzed by X-ray diffraction (XRD), transmission electron microscopy (TEM), UV-vis diffuse reflectance spectroscopy (DRS) and X-ray photoelectron spectroscopy (XPS). A control experiment was conducted to test the effect of EG dosage on the growth mechanism and behavior of the highly (010) exposed hierarchical lamellar nanostructures and dispersed nanosheets. The photocatalytic nitrogen fixation rate of the hierarchical Bi2WO6 nanostructures was estimated to be 948 µmol g-1 h-1 across the full spectrum, which was 23% higher than that of the dispersed nanosheets (770 µmol g-1 h-1) due to chemisorption on the hierarchical structures and enhanced surface oxygen vacancies (OVs).

Visible-light-responsive Z-scheme system for photocatalytic lignocellulose-to-H2 conversion.

A Z-scheme system was successfully constructed for visible-light-driven photocatalytic H2 production from lignocelluloses, the highest H2 evolution rate of this Z-scheme system is 5.3 and 1.6 µmol h-1 in α-cellulose and poplar wood chip aqueous solutions, respectively, under visible light irradiation.

Construction of hierarchical CoP@Ni2P core-shell nanoarrays for efficient electrocatalytic hydrogen evolution in alkaline solution.

Design and synthesis of non-noble electrocatalyst with controlled structure and composition for hydrogen evolution reaction (HER) are significant for large-scale water electrolysis. Here, an elegant multi-step templating strategy is developed for the fabrication of vertically aligned CoP@Ni2P nanowire-nanosheet architecture on Ni foam. Cobalt-carbonate hydroxides nanowires grown on Ni foam are first synthesized as the self-template. Afterward, a layer of amorphous Ni(OH)2 nanosheets is grown on the Co-based precursors through a chemical bath process, which is then transformed into the hierarchical CoP@Ni2P nanoarrays by a co-phosphatization treatment. Owing to the synergistic effect of the compositions and the advantages of the hierarchical heterostructures, the resulting hybrid electrocatalyst with dense heterointerfaces is revealed as an excellent HER catalyst, with a low overpotential of 101 mV at the current density of 10 mA cm-2, a relatively small Tafel slope of 79 mV dec-1, and favorable long-term stability of at least 20 h in 1 M KOH.

Solar-Driven Lignocellulose-to-H2 Conversion in Water using 2D-2D MoS2 /TiO2 Photocatalysts.

As an alternative strategy for H2 production under ambient conditions, solar-driven lignocellulose-to-H2 conversion provides a very attractive approach to store and utilize solar energy sustainably. Exploiting efficient photocatalyst for photocatalytic lignocellulose-to-H2 conversion is of huge significance and remains the key challenge for development of solar H2 generation from lignocellulose. Herein, 2D-2D MoS2 /TiO2 photocatalysts with large 2D nanojunction were constructed for photocatalytic lignocellulose-to-H2 conversion. In this smart structure, the 2D nanojunctions acted as efficient channel for charge transfer from TiO2 to MoS2 to improve charge separation efficiency and thus enhance photocatalytic lignocellulose-to-H2 conversion activity. The 2 % MoS2 /TiO2 photocatalyst showed the highest photocatalytic lignocellulose-to-H2 conversion performance with the maximal H2 generation rate of 201 and 21.4 µmol h-1 g-1 in α-cellulose and poplar wood chip aqueous solution, respectively. The apparent quantum yield at 380 nm reached 1.45 % for 2 % 2D-2D TiO2 /MoS2 photocatalyst in α-cellulose aqueous solution. This work highlights the importance of optimizing the interface structures of photocatalyst for solar-driven lignocellulose-to-H2 conversion.

Highly dispersed BiOCl decahedra with a highly exposed (001) facet and exceptional photocatalytic performance.

BiOCl has been identified to be a promising photocatalyst for the rapid photodegradation of organic pollutants, but its practical application was restricted by its limited photocatalytic activity. In this work, a highly reactive BiOCl decahedron photocatalyst with an exposed (001) facet was successfully hydrothermally synthesized via a simple hydrothermal method using bismuth nitrate (Bi(NO3)3·5H2O) and ammonium chloride (NH4Cl) as raw materials. By adjusting the dosage of NH4Cl, the BiOCl nanoplates transformed from hexahedra with quadrilateral {110} oblique facets to decahedra with octagonal {110} and {100} oblique facets. As compared to the original BiOCl nanoplates, decahedral BiOCl possesses much more oxygen-enriched surfaces and a narrowed bandgap, resulting in enhanced photocatalytic performance. The decahedral BiOCl photocatalyst achieves a high degradability of 98% for the photodegradation of RhB after 6 min of irradiation, which is much faster than that of a hexahedral BiOCl sample.

Few-Layer Black Phosphorus Nanosheets: A Metal-Free Cocatalyst for Photocatalytic Nitrogen Fixation.

Exploiting an appropriate strategy to prepare fine crystal quality black phosphorus nanosheet (BPNS) catalyst is a major challenge for its practical application in catalysis. Herein, we address this challenge by developing a rapid electrochemical expansion strategy for scale preparation of fine crystal quality BPNSs from bulk black phosphorus, which was demonstrated to be an active cocatalyst for photocatalytic nitrogen fixation in the presence of CdS as a photocatalyst. The transient photocurrent and charge density studies show that the BPNSs can efficiently accelerate charge separation of CdS, leading to the enhanced photocatalytic activities of BPNS/CdS nanocomposites for nitrogen fixation. The 1.5% BPNS/CdS photocatalyst exhibits the highest photocatalytic activity for nitrogen fixation with an NH3 evolution rate of 57.64 µmol·L-1·h-1. This study not only affords a rapid and simple strategy for scale synthesis of fine crystal quality BPNSs but also provides new insights into the design and development of black phosphorus-based materials as low-cost metal-free cocatalysts for photocatalytic nitrogen fixation.