Photocatalysts For H2 Generation Research Articles

Rational Design of Vinylene-Linked Covalent Organic Frameworks for Modulating Photocatalytic H2 Evolution.

Vinylene-linked covalent organic frameworks (COFs) have attracted enormous attention for photocatalytic H2 evolution from water because of their fully conjugated structures, high chemical stabilities, and enhanced charge-carrier mobilities. In this work, two novel vinylene-linked COFs with tuned cyano contents were successfully synthesized and then employed as photocatalysts for H2 generation. Notably, the photocatalytic H2 production rate of the COF with the higher cyano content reached 73 μmol h-1 under visible light irradiation, which is 2.4 times higher than that with the lower content (30 μmol h-1 ). Both the experimental and computational results demonstrated that the rational design incorporating cyano groups into COF skeletons could precisely tune the corresponding energy levels, expand the visible-light absorption, and improve the photoinduced charge separation. This work not only provides a simple method for modulating the photocatalytic activities of COFs at the molecular level, but also affords interesting insights into the relationship between their structures and photocatalytic performance.

Exploration of 1D-2D LaFeO3/RGO S-scheme heterojunction for photocatalytic water splitting

Sustainable energy innovation is spearheading the way to achieve decarbonisation through commercially viable and highly competitive renewable technologies for green hydrogen. Photocatalytic water splitting has received global attention, as it promotes the direct conversion of solar energy to chemical energy and hydrogen production. Lanthanum orthoferrite (LaFeO3) has been selected due to its narrow bandgap perovskite-oxides (ABO3) type nature, low cost and high chemical stability but it is limited with fast charge recombination. To circumvent its constraint of fast charge recombination, an efficient graphene-based nanocomposite has been prepared by employing reduced graphene oxide (RGO) nanosheets as charge separators for visible light driven photocatalytic water splitting. Here, we present a thorough physical and spectroscopic characterization of the Lanthanum orthoferrite/Reduced Graphene oxide (LaFeO3/RGO) nanocomposites, and investigate its photocatalytic and photoelectrochemical performance. The photocurrent density of the nanocomposites demonstrated ∼21 times higher in comparison to pure LaFeO3. The as-prepared nanocomposites have been successfully used as photocatalysts for H2 generation through water reduction under visible light. A significant enhancement in H2 generation has been recorded for nanocomposites (∼82 mmol g−1 h−1) as compared to that of bare LaFeO3 (∼9 mmol g−1 h−1) which is among the highest values obtained using noble-metal-free graphene-based photocatalytic nanocomposites. This work offers a facile approach for fabricating highly efficient 1D-2D heterostructure for photocatalysis application.

In situ synthesis of g-C3N4/Ti3C2Tx nano-heterostructures for enhanced photocatalytic H2 generation via water splitting.

Herein, we demonstrated the in situ synthesis of g-C3N4/Ti3C2Tx nano-heterostructures for hydrogen generation under UV visible light irradiation. The formation of the g-C3N4/Ti3C2Tx nano-heterostructures was confirmed via powder X-ray diffraction and supported by XPS. The FE-SEM images indicated the formation of layered structures of MXene and g-C3N4. HR-TEM images and SAED patterns confirmed the presence of g-C3N4 together with Ti3C2Tx nanosheets, i.e., the formation of nano-heterostructures of g-C3N4/Ti3C2Tx. The absorption spectra clearly showed the distinct band gaps of g-C3N4 and Ti3C2Tx in the nano-heterostructure. The increase in PL intensity and broadening of the peak with an increase in g-C3N4 indicated the suppression of electron-hole recombination. Furthermore, the nano-heterostructure was used as a photocatalyst for H2 generation from water and methylene blue dye degradation. The highest H2 evolution (1912.25 μmol/0.1 g) with good apparent quantum yield (3.1%) and an efficient degradation of MB were obtained for gCT-0.75, which was much higher compared to that of the pristine materials. The gCT-0.75 nano-heterostructure possessed a high surface area and abundant vacancy defects, facilitating the separation of charge carriers, which was ultimately responsible for this high photocatalytic activity. Additionally, TRPL clearly showed a higher decay time, which supports the enhancement in the photocatalytic activity of the gCT-0.75 nano-heterostructure. The nano-heterostructure with the optimum concentration of g-C3N4 formed a hetero-junction with the linked catalytic system, which facilitated efficient charge carrier separation also responsible for the enhanced photocatalytic activity.

Development of novel TiN–LiAlSiO4/SnS/AuPd multilayer films for H2 generation by photocatalysis

TiN–LiAlSiO4/SnS/AuPd multilayer films were deposited on glass substrates using screen-printing and SILAR (successive ionic layer adsorption reaction) methods. The films were used as photocatalysts for H2 generation using a solar simulator source. The incorporation of the SnS provokes the presence of particles of different morphology and size, allowing a better light absorption on the surface of the films. In addition, AuPd nanoparticles were deposited on the multilayer films to improve photocatalytic activity. XRD and TEM analysis revealed that whatever the proportion, the main phase is TiN. The SEM images showed that the electrostatic attraction between TiN and SnS is a key factor that influences the morphology of the films. The optical band-gap values for the films were similar (∼3.86 eV). According to profilometry measurements, the thickness of films varies from 218 to 887 nm, and PL results showed that when the thickness decreases the electron-hole pair recombination processes decrease too. The highest H2 generation was reached by T20:L80/SnS/AuPd, around 99 μmol/m2 after 3 h of irradiation, while T80:L20/SnS/AuPd produces only 33 μmol/m2, and coincidentally, showed lower and higher thickness, respectively. Moreover, the presence of AuPd nanoparticles did not provoke any structural and optical modification on the films. However, it caused the existence of active sites for H2 generation and diminish the electron-hole pair recombination, evidenced by PL analysis. Finally, these results corroborate that the film with a low thickness and higher LiAlSiO4 showed better photocatalytic activity because of the inhibition of the electro-hole pair recombination.

Metal-free SiOC/g-C3N4 heterojunction composites with efficient visible-light photocatalytic H2 production

Photocatalytic water splitting to product hydrogen (H2) is a desired route to utilize solar energy into chemical energy. At present, most photocatalysts for H2generation contains metallic elements with high-cost, confined reserves, and environmentallyunfriendly. The exploitation of metal-free and efficient photocatalysts becomesparticularlyimportant. Herein, a new metal-free g-C3N4-based heterojunction composites were facilely produced via the ultrasonic assisted deposition and subsequent calcination from g-C3N4 layers embedded with SiOC with prominent conductivity and large surface area. Systematic investigates suggested that the SiOC/g-C3N4 heterojunctions exhibited eminently meliorated photocatalytic H2 production efficiency. The optimum H2 production rate (1020 μmol h−1 g−1) of the SiOC/g-C3N4 photocatalyst is 3.2 and 25.5 folds that of g-C3N4 and SiOC, respectively. The boosted photocatalytic activity primarily originates from the increased absorptivity of light, the markedly expedited charge migration and the impeded recombination of charge carriers. This study offers a new perspective for the scientific contrive and fabrication of high-quality heterojunction photocatalysts for solar energy utilization.

Synthesis of uniform ordered mesoporous TiO2 microspheres with controllable phase junctions for efficient solar water splitting.

As a benchmark photocatalyst, commercial P25-TiO2 has been widely used for various photocatalytic applications. However, the low surface area and poorly porous structure greatly limit its performance. Herein, uniform ordered mesoporous TiO2 microspheres (denoted as Meso-TiO2-X; X represents the rutile percentage in the resultant microspheres) with controllable anatase/rutile phase junctions and radially oriented mesochannels are synthesized by a coordination-mediated self-assembly approach. The anatase/rutile ratio in the resultant microspheres can be facilely adjusted as desired (rutile percentage: 0-100) by changing the concentration of hydrochloric acid. As a typical one, the as-prepared Meso-TiO2-25 microspheres have a similar anatase/rutile ratio to commercial P25. But the surface area (78.6 m2 g-1) and pore volume (0.39 cm3 g-1) of the resultant microspheres are larger than those of commercial P25. When used as the photocatalyst for H2 generation, the Meso-TiO2-25 delivers high solar-driven H2 evolution rates under air mass 1.5 global (AM 1.5 G) and visible-light (λ > 400 nm), respectively, which are significantly larger than those of commercial P25. This coordination-mediated self-assembly method paves a new way toward the design and synthesis of high performance mesoporous photocatalysts.

Tailoring TiO2 Nanotube-Interlaced Graphite Carbon Nitride Nanosheets for Improving Visible-Light-Driven Photocatalytic Performance.

Rapid recombination of photoinduced electron–hole pairs is one of the major defects in graphitic carbon nitride (g‐C3N4)‐based photocatalysts. To address this issue, perforated ultralong TiO2 nanotube‐interlaced g‐C3N4 nanosheets (PGCN/TNTs) are prepared via a template‐based process by treating g‐C3N4 and TiO2 nanotubes polymerized hybrids in alkali solution. Shortened migration distance of charge transfer is achieved from perforated PGCN/TNTs on account of cutting redundant g‐C3N4 nanosheets, leading to subdued electron–hole recombination. When PGCN/TNTs are employed as photocatalysts for H2 generation, their in‐plane holes and high hydrophilicity accelerate cross‐plane diffusion to dramatically promote the photocatalytic reaction in kinetics and supply plentiful catalytic active centers. By having these unique features, PGCN/TNTs exhibit superb visible‐light H2‐generation activity of 1364 µmol h−1 g−1 (λ > 400 nm) and a notable quantum yield of 6.32% at 420 nm, which are much higher than that of bulk g‐C3N4 photocatalysts. This study demonstrates an ingenious design to weaken the electron recombination in g‐C3N4 for significantly enhancing its photocatalytic capability.

A nickel complex, an efficient cocatalyst for both electrochemical and photochemical driven hydrogen production from water

Inspired by that nickel complexes can act as molecular catalysts for hydrogen generation from organic acid or water, we choose a nickel(II) complex, [Bz-4-MePy]2[NiII(i-mnt)2] 1 (i-mnt2− = iso-maleonitriledithiolate, Bz-4-MePy+ = 1-benzyl-4′-methylpyridinium ion) as a potential catalyst. Electro- and photochemical investigations show that this nickel complex can act as both an electrocatalyst and a photocatalyst for H2 generation from water via an unstable nickel hydride intermediate. A homogeneous electrocatalytic system containing complex 1 can afford 577.4 mol of H2 per mole of catalyst per hour (mol H2/mol catalyst/h) from neutral water at an overpotential (OP) of 837.6 mV. Together with CdS nanorod (CdS NR) as a photosensitizer, and ascorbic acid (H2A) as a sacrificial electron donor in a pH 3.5 aqueous solution, under photoirradiation with blue light (λmax = 469 nm), complex 1 also can provide hydrogen with a turnover number (TON) of 55340 mol of H2 per mole of catalyst during first 60 h irradiation. The highest apparent quantum yield (AQY) is ∼26.8% at 420 nm.

Visible-light-driven photocatalytic system based on a nickel complex over CdS materials for hydrogen production from water

Based on that the evolution of renewable H2 by using a photochemical method requires photoinduced electron transfer (ET) from a light harvester to an electrocatalyst in water, we present a nickel complex, [BzPyN(CH3)2]2[Ni(i-mnt)2] 1 (i-mnt2−=2,2-dicyanoethylene-1,1-dithiolate) that acts as both an electrocatalyst and a photocatalyst for H2 generation from water via an unstable nickel hydride intermediate. As an electrocatalyst, this nickel complex can catalyze hydrogen generation from a neutral buffer with a turnover frequency (TOF) of 577.4mol of hydrogen per mole of catalyst per hour (mol H2/mol catalyst/h) at an overpotential (OP) of 837.6mV. As a photocatalyst, combining with CdS nanorods (CdS NRs) as a photosensitizer, and ascorbic acid (H2A) as a sacrificial electron donor, it can photocatalyze hydrogen evolution in heterogeneous environments and can work for 100h. Under an optimal condition, the highest hydrogen evolution turnover number (TON) reaches 85820mol of H2 per mol of catalyst during 70h irradiation, and the highest apparent quantum yield (AQY) is ∼23% at 420nm. And possible cycles for H2 production by the electrocatalytic and photocatalytic systems are afforded.

Double-Side Co-Catalytic Activation of Anodic TiO2 Nanotube Membranes with Sputter-Coated Pt for Photocatalytic H2 Generation from Water/Methanol Mixtures.

Self-standing TiO2 nanotube layers in the form of membranes are fabricated by self-organizing anodization of Ti metal and a potential shock technique. The membranes are then decorated by sputtering different Pt amounts i) only at the top, ii) only at the bottom or iii) at both top and bottom of the tube layers. The Pt-decorated membranes are transferred either in tube top-up or in tube top-down configuration onto FTO slides and are investigated, after crystallization, as photocatalysts for H2 generation using either front or back-side light irradiation. Double-side Pt-decoration of the tube membranes leads to higher H2 generation rates (independently of tube and light-irradiation configuration) compared to membranes decorated at only one side with similar overall Pt amounts. The results suggest that this effect cannot be only ascribed to the overall amount of Pt co-catalyst as such but also to its distribution at both tube extremities. This leads to optimized light absorption and electron diffusion/transfer dynamics: the central part of the membranes acts as light-harvesting zone and electrons therein generated can diffuse towards the Pt/TiO2 active zones (tube extremities) where they can react with the environment and generate H2 .

Ni(dmgH)2 complex coupled with metal-organic frameworks MIL-101(Cr) for photocatalytic H2 evolution under visible light irradiation

Metal-organic frameworks (MOFs) have been tremendously used as photocatalysts for H2 generation in recent years. Lacking native active sites (so-called co-catalyst) for H2 generation motivates the incorporation of noble metals and their molecular complexes, hydrogenase active site mimics into MOFs to promote H2 generation. We herein report an noble-metal-free photocatalytic H2 generation system consisting of Erythrosin B dye-sensitized MIL-101(Cr) as a light absorber and Ni(dmgH)2 as a co-catalyst. It is found that Ni(dmgH)2 can serve as an efficient co-catalyst to boost H2 generation in the presence of triethanolamine (TEOA) as an electron donor under visible light irradiation. The optimal MIL-101(Cr)/Ni(dmgH)2 hybrid (5 wt% Ni(dmgH)2) displays a hydrogen H2 rate of 45.5 μmol h−1, which is 10 times greater than the control sample without Ni(dmgH)2 loading. This paper provides a novel design route for active H2 generation systems by combining molecular complexes of earth-abundant metal and MOFs photocatalysts.

Pt-Au Triangular Nanoprisms with Strong Dipole Plasmon Resonance for Hydrogen Generation Studied by Single-Particle Spectroscopy.

Three anisotropic Pt-covered, Pt-edged, and Pt-tipped Au triangular nanoprisms (TNPs) were prepared by controlling the overgrowth of Pt as photocatalysts for H2 generation. With strong electric field and more interface for the hot electrons transfer, the H2 generation rate of Pt-edged Au TNPs was 3 and 5 times higher than those of Pt-tipped and Pt-covered Au TNPs. Single-particle photoluminescence (PL) spectra and finite-difference-time-domain (FDTD) simulations demonstrated that dipole surface plasmon resonance (DSPR) of Au TNPs enhanced the hot electrons transfer from Au to Pt leading to H2 generation. SPR bands of Au TNPs depending on the size play an important role on the photocatalytic activity of Pt-edged Au TNPs.

Eosin Y functionalized graphene for photocatalytic hydrogen production from water

Chemically reduced graphene oxide (RGO) can be functionalized by eosin Y (EY). The formation of the stable aqueous EY functionalized graphene (EY-RGO) suspension is due to the non-covalent interaction between EY and RGO surface via hydrogen bonding and π–π stacking. EY-RGO was characterized by transmission electron microscopy (TEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Spectral and photoelectrochemical studies indicate that photoinduced electron transfer occurs from EY to RGO. The EY-RGO is photocatalytic active for water reduction to produce hydrogen. The average production rate of H2 for the photocatalyst (wEY/wRGO = 1) in a 10 vol% triethanolamine aqueous solution can reach 3.35 mmol g−1 h−1 and 0.40 mmol g−1 h−1 under 30 h UV–vis and 10 h visible light irradiation, respectively. The photocatalytic activity of EY-RGO is superior to that of RGO, graphene oxide (GO), and EY-GO. Modification EY-RGO with Pt nanoparticles can further improve photocatalytic activity. All these features demonstrated that organic sensitizers functionalized graphene provided a nice candidate as a photocatalyst for H2 generation from water under solar light irradiation.