No abstract

A photocatalytic noble metal-free system for the generation of hydrogen has been constructed using Eosin Y (1) as a photosensitizer, the complex [Co(dmgH)(2)pyCl](2+) (5, dmgH = dimethylglyoximate, py = pyridine) as a molecular catalyst, and triethanolamine (TEOA) as a sacrificial reducing agent. The system produces H(2) with an initial rate of approximately 100 turnovers per hour upon irradiation with visible light (lambda > 450 nm). Addition of free dmgH(2) greatly increases the durability of the system addition of 12 equiv of dmgH(2) (vs cobalt) to the system produces approximately 900 turnovers of H(2) after 14 h of irradiation. The rate of H(2) evolution is maximum at pH = 7 and decreases sharply at more acidic or basic pH. Spectroscopic study of photolysis solutions suggests that hydrogen production occurs through protonation of a Co(I) species to give a Co(III) hydride, which then reacts further by reduction and protolysis to give Co(II) and molecular hydrogen.

A series of cobaloxime complexes([Co(dmgH)(2)pyCl] (1), [Co(dmgH)(2)(4-COOMe-py)Cl] (2), [Co(dmgH)(2)(4-Me(2)N-py)Cl] (3), [Co(dmgH)(dmgH(2))Cl(2)] (4), [Co(dmgH)(2)(py)(2)](PF(6)) (5), [Co(dmgH)(2)(P(n-Bu)(3))Cl] (6), and [Co(dmgBF(2))(2)(OH(2))(2)] (7), where dmgH = dimethylglyoximate monoanion, dmgH(2) = dimethylglyoxime, dmgBF(2) = (difluoroboryl)dimethylglyoximate anion, and py = pyridinewere synthesized and studied as molecular catalysts for the photogeneration of hydrogen from systems containing a Pt terpyridyl acetylide chromophore and triethanolamine (TEOA) as a sacrificial donor in aqueous acetonitrile. All cobaloxime complexes 1-7 are able to quench the luminescence of the Pt(II) chromophore [Pt(ttpy)(CCPh)]ClO(4) (C1) (ttpy = 4'-p-tolyterpyridine). The most effective electron acceptor for hydrogen evolution is found to be complex 2, which provides the fastest luminescence quenching rate constant for C1 of 1.7 x 10(9) M(-1) s(-1). The rate of hydrogen evolution depends on many factors, including the stability of the catalysts, the driving force for proton reduction, the relative and absolute concentrations of system components (TEOA, Co molecular catalyst, and sensitizer), and the ratio of MeCN/water in the reaction medium. For example, when the concentration of TEOA increases, the rate of H(2) photogeneration is faster and the induction period is shorter. Colloidal cobalt experiments and mercury tests were run to verify that the system is homogeneous and that catalysis does not occur from in situ generated colloidal particles during photolysis. The most effective system examined to date consists of the chromophore C1 (1.1 x 10(-5) M), TEOA (0.27 M), and catalyst complex 1 (2.0 x 10(-4) M) in a MeCN/water mixture (24:1 v/v, total 25 mL); this system has produced approximately 2150 turnovers of H(2) after only 10 h of photolysis with lambda > 410 nm.

Metal-free elemental photocatalysts for hydrogen (H ) evolution are more advantageous than the traditional metal-based inorganic photocatalysts since the nonmetal elements are generally cheaper, more earth-abundant, and environmentally friendly. Black phosphorus (BP) has been attracting increasing attention in recent years based on its anisotropic 2D layered structure with tunable bandgap in the range of 0.3-2.0 eV; however, the application of BP for photocatalytic H evolution has been scarcely reported experimentally although being theoretically predicted. Herein, for the first time, the visible light photocatalytic H evolution of BP nanosheets prepared via a facile solid-state mechanochemical method by ball-milling bulk BP is reported. Without using any noble metal cocatalyst, the visible light photocatalytic hydrogen evolution rate of BP nanosheets reaches 512 µmol h g , which is ≈18 times higher than that of the bulk BP, and is comparable or even higher than that of graphitic carbon nitrides (g-C N ).

The complex [Co(dmgH)2pyCl]2+ (1, dmgH = dimethylglyoximate, py = pridine) has been used as a molecular catalyst for visible light driven hydrogen production in the presence of [Pt(tolylterpyridine)(phenylacetylide)]+ (3) as a photosensitizer and triethanolamine (TEOA) as a sacrificial reducing agent. Complex 3 is quenched oxidatively by [Co(dmgH)pyCl]2+ (1) with a rate constant kq of 1.27 x 10(9) M(-1) s(-1). Photogeneration of H2 is only seen when 1 + 3 + TEOA are all present. H2 production is maximized for this system at pH 8.5 and declines to very low levels at pH < 7 and pH > 12. Irradiation of the reaction solution initially containing 1.61 x 10(-2) M TEOA, 1.11 x 10(-5) M of 3, and 1.99 x 10(-4) M of Co catalyst 1 in MeCN/water (3:2 v/v) at pH = 8.5 for 10 h with lambda > 410 nm yields 400 turnovers of H2. When TEOA is 0.27 M, approximately 1000 turnovers are obtained after 10 h of irradiation. Spectroscopic study of the photolyses solutions suggests that H2 formation proceeds via Co(I) and protonation to form Co(III) hydride species.

Photocatalytic hydrogen evolution via water splitting is an attractive scientific and technological goal to address the increasing global demand for clean energy and to reduce the climate change impact of CO2 emission. Although tremendous efforts have been made, hydrogen production by a robust and highly efficient system driven by visible light still remains a significant challenge. Herein we report that nickel phosphide, as a cocatalyst to form a well-designed integrated photocatalyst with one-dimensional semiconductor nanorods, highly improves the efficiency and durability for photogeneration of hydrogen in water. The highest rate for hydrogen production reached ~1,200 μmol·h -1 •mg -1 based on the photocatalyst.The turnover number (TON) reached ~3,270,000 in 90 hours with a turnover frequency (TOF) of 36,400 for Ni2P, and the apparent quantum yield was ~41% at 450 nm. The photoinduced charge transfer process was further confirmed by steady-state photoluminescence spectra and time-resolved photoluminescence spectra. Such extraordinary performance of a noble-metal-free artificial photosynthetic hydrogen production system has, to our knowledge, not been reported to date.

Exfoliated black phosphorus (BP), as a monolayer or few-layer material, has attracted tremendous attention owing to its unique physical properties for applications ranging from optoelectronics to photocatalytic hydrogen production. Approaching intrinsic properties has been, however, challenged by chemical reactions and structure degradation of BP under ambient conditions. Surface passivation by capping agents has been proposed to extend the processing time window, yet contamination or structure damage rise challenges for BP applications. Here, we report experiments combined with first-principle calculations that address the degradation chemistry of BP. Our results show that BP reacts with oxygen in water even without light illumination. The reaction follows a pseudo-first-order parallel reaction kinetics, produces PO, PO, and PO with reaction rate constants of 0.019, 0.034, and 0.023 per day, respectively, and occurs preferentially from the P atoms locating at BP edges, which yields structural decay from the nanoflake edges in water. In addition, a negligible decay ratio (0.9 ± 0.3 mol %) and preserved photocatalytic activity of BP are observed after storage in deoxygenated water for 15 days without surface passivation under ambient light. Our results reveal the chemistry of BP degradation and provide a practical approach for exfoliation, delivery, and application of BP.

Photodriven charge-transfer dynamics and catalytic properties have been investigated for a hybrid system containing CdSe/ZnS core/shell quantum dots (QDs) and surface-bound molecular cobaloxime catalysts. The electron transfer from light-excited QDs to cobaloxime, revealed by optical transient absorption spectroscopy, takes place with an average time constant of 105 ps, followed a much slower charge recombination process with a time constant of ≫3 ns. More interestingly, we also observed photocatalytic hydrogen generation by this QD/cobaloxime hybrid system, with >10,000 turnovers of H(2) per QD in 10 h, using triethanolamine as a sacrificial electron donor. These results suggest that QD/cobaloxime hybrids succeed in coupling single-photon events with multielectron redox catalytic reactions, and such systems could have potential applications in long-lived artificial photosynthetic devices for fuel generation from sunlight.