Photochemical Water Splitting Research Articles

Cobalt Phosphide Nanowires: Efficient Nanostructures for Fluorescence Sensing of Biomolecules and Photocatalytic Evolution of Dihydrogen from Water under Visible Light

AbstractThe detection of specific DNA sequences plays an important role in the identification of disease‐causing pathogens and genetic diseases, and photochemical water splitting offers a promising avenue to sustainable, environmentally friendly hydrogen production. Cobalt–phosphorus nanowires (CoP NWs) show a high fluorescence quenching ability and different affinity toward single‐ versus double‐stranded DNA. Based on this result, the utilization of CoP NWs as fluorescent DNA nanosensors with a detection limit of 100 pM and a selectivity down to single‐base mismatch was demonstrated. The use of a thrombin‐specific DNA aptamer also enabled the selective detection of thrombin. The photoinduced electron transfer from the excited dye that labels the oligonucleotide probe to the CoP semiconductor led to efficient fluorescence quenching, and largely enhanced the photocatalytic evolution of hydrogen from water under visible light.

Cobalt phosphide nanowires: efficient nanostructures for fluorescence sensing of biomolecules and photocatalytic evolution of dihydrogen from water under visible light.

The detection of specific DNA sequences plays an important role in the identification of disease-causing pathogens and genetic diseases, and photochemical water splitting offers a promising avenue to sustainable, environmentally friendly hydrogen production. Cobalt-phosphorus nanowires (CoP NWs) show a high fluorescence quenching ability and different affinity toward single- versus double-stranded DNA. Based on this result, the utilization of CoP NWs as fluorescent DNA nanosensors with a detection limit of 100 pM and a selectivity down to single-base mismatch was demonstrated. The use of a thrombin-specific DNA aptamer also enabled the selective detection of thrombin. The photoinduced electron transfer from the excited dye that labels the oligonucleotide probe to the CoP semiconductor led to efficient fluorescence quenching, and largely enhanced the photocatalytic evolution of hydrogen from water under visible light.

Unusual stability of dyads during photochemical hydrogen production.

Dyads for photochemical water splitting often suffer from instability during irradiation with visible light. However, the use of bis(bidentate) phosphines forming a five-membered ring enhances their stability. The coordination of these phosphor based chelates to soft metals like Pd(ii) prolongs the photocatalytic activity to 1000 hours. To avoid contribution to hydrogen production by colloidal metal, a small amount of Hg is added to the reaction mixture. In the course of our investigations, it turned out that colloidal palladium was not able to produce hydrogen under our irradiation conditions. As soon as metallic palladium emerged in our reaction vessels, no further hydrogen production was detected. This is confirmed by the observation that the hydrogen production depends on the kind of ancillary ligands present in the dyads. The first dyads of the type [MI(bpy)2(dppcb)MII(bpy)](4+) are presented (MI = Os, MII = Pd (1); MI = Ru, MII = Pd (2); MI = Os, MII = Pt (3); MI = Ru, MII = Pt (4)). In [Os(bpy)2(dppcb)Pd(dppm)](PF6)4 (5) the ancillary ligand is varied. Furthermore, it is also possible to produce hydrogen in an intermolecular way. Using different bidentate diphosphines instead of a bis(bidentate) tetraphosphine leads to this intermolecular approach, where the chromophore and the water reduction catalyst (WRC) belong now to two molecules. In this case the TON is sensitive to the type of diphosphine, which is only possible if intact molecules act as catalysts and no free palladium(0) is formed.

Carbon- and Nitrogen-Based Porous Solids: A Recently Emerging Class of Materials

Abstract In this account, the emergence of a new family of porous solid-state materials based on carbon and nitrogen is discussed. This started with the observation that carbon nitride, a polymeric material with a composition close to C3N4, is not only unexpectedly thermally and chemically stable, but a semiconductor at the same time and able to catalyze a variety of chemical reactions, such as Diels–Alder cyclizations, oxidations, and photochemical water splitting. Carbon nitride is however already sufficiently reviewed but related materials with similar potential have essentially been not sufficiently covered. The remaining structures are manifold and cover the range from on the one hand covalent triazine frameworks (CTFs) as new semiconducting porous solid-state materials to the other porous nitrogen-doped carbons (NdCs) as catalysts which are known to be applicable in a variety of electrocatalytic reactions. CTFs are porous polymeric frameworks constituted of triazine rings and other aromatic rings. Chemical binding motifs and crystal structures can be used to control the band structure of such an (organic) solid-state material. Porous NdCs are usually made by adding nitrogen sources to classical carbonization recipes, and they are attracting a lot of interest because of their catalytic activity for electrochemical processes, such as oxygen reduction reaction (ORR). This account will summarize state-of-the-art work on those systems being next-generation catalysts for different kinds of reactions, which are affordable and high-performance catalysts with unique principle for emergence of catalytic activities, combining theoretical and experimental studies together with basic and applied science in a range of scientific fields from chemistry to physics. Indeed, the first exploration of the above materials as candidates for components of fuel cells and rechargeable metal–air batteries are discussed.

Electron-transfer sensitization of H 2 oxidation and CO 2 reduction catalysts using a single chromophore

Energy-storing artificial-photosynthetic systems for CO2 reduction must derive the reducing equivalents from a renewable source rather than from sacrificial donors. To this end, a homogeneous, integrated chromophore/two-catalyst system is described that is thermodynamically capable of photochemically driving the energy-storing reverse water-gas shift reaction (CO2 + H2 → CO + H2O), where the reducing equivalents are provided by renewable H2. The system consists of the chromophore zinc tetraphenylporphyrin (ZnTPP), H2 oxidation catalysts of the form [Cp(R)Cr(CO)3](-), and CO2 reduction catalysts of the type Re(bpy-4,4'-R2)(CO)3Cl. Using time-resolved spectroscopic methods, a comprehensive mechanistic and kinetic picture of the photoinitiated reactions of mixtures of these compounds has been developed. It has been found that absorption of a single photon by broadly absorbing ZnTPP sensitizes intercatalyst electron transfer to produce the substrate-active forms of each. The initial photochemical step is the heretofore unobserved reductive quenching of the low-energy T1 state of ZnTPP. Under the experimental conditions, the catalytically competent state decays with a second-order half-life of ∼15 μs, which is of the right magnitude for substrate trapping of sensitized catalyst intermediates.

Photochemical water splitting of WO3 nanoparticles layers on indium tin oxide glass substance after absorbing chlorophyll

N-type semiconductor tungsten oxide is an interesting photocatalytic material because it has a good band gap (2·6 eV). This experiment using the surfactants citric acid and tannic acid acts on an aqueous solution of tungsten oxide, helps in the formation of different crystal growth surfaces and thus achieves a mixed solution of tungsten oxide to obtain a different optical, surface morphology and its two kinds of citric acid and tannic acid mixed ratios of the tungsten oxide specimen test the efficiency of solar hydrogen production. The efficiency test results in maximum efficiency, reaching 0·85% WO3 (citric acid:tannic acid = 1:3) of the solar hydrogen production efficiency. The efficiency will possibly be up to 3% after optimization.

The Influence of Functionals on Density Functional Theory Calculations of the Properties of Reducible Transition Metal Oxide Catalysts

Reducible transition metal oxides (RTMOs) comprise an important class of catalytic materials that are used for the selective oxidation and electro- and photochemical splitting of water, and as supports for metal nanoparticles. It is, therefore, highly desirable to model the properties of these materials accurately using density functional theory (DFT) in order to understand how oxide structure and performance are related and to guide the search for materials exhibiting superior performance. Unfortunately, accurate description of the structural and electronic properties of RTMOs using DFT has proven particularly challenging. The M06-L density functional, which has been shown to be broadly accurate for calculations of gas phase clusters, has recently become available to researchers carrying out calculations in the solid state, but its performance in determining the properties RTMOs has been little investigated. The aim of this work was to assess the performance of the M06-L functional for describing the structural and electronic properties of a family of RTMOs: MoO2, MoO3, and Bi2Mo3O12. Lattice constants, band gaps, and densities of states calculated using the M06-L functional are compared to those obtained from DFT+U. We have also used the M06-L functional to determine the reaction barrier for propene activation over Bi2Mo3O12, the rate-limiting step in the oxidation of propene to acrolein. We find that while DFT calculations carried out with the M06-L functional are roughly five times more expensive computationally than those performed with DFT+U, the results obtained using the M06-L functional provide sensible results for all properties investigated, while avoiding the necessary trade-off between accurate electronic structure and accurate thermochemistry that occurs in DFT+U.

PEC electrode of ZnO nanorods sensitized by CdS with different size and its photoelectric properties

The gradient PEC electrodes of ZnO nanorods sensitized by CdS with different size were fabricated via successive ionic layer adsorption and reaction (SILAR) method and applied in photochemical water splitting. The concentrations of reaction solution and SILAR cycles were investigated in the synthetic process and the working mechanism of the gradient PEC electrode was suggested. The results showed that the hydrogen generation efficiency of 4.88% was achieved for the ZnO/CdS gradient PEC electrode constructed by decreasing of the CdS quantum dots size on ZnO nanorods due to the improved absorption and appropriate energy gap structure, which was confirmed by enhanced absorption spectrum. The expected products have potential application in photoelectrochemical water splitting.

Nonadiabatic Dynamics of Positive Charge during Photocatalytic Water Splitting on GaN(10-10) Surface: Charge Localization Governs Splitting Efficiency

Photochemical water splitting is a promising avenue to sustainable, clean energy and fuel production. Gallium nitride (GaN) and its solid solutions are excellent photocatalytic materials; however, the efficiency of the process is low on pure GaN, and cocatalysts are required to increase the yields. We present the first time-domain theoretical study of the initial steps of photocatalytic water splitting on a GaN surface. Our state-of-the-art simulation technique, combining nonadiabatic molecular dynamics and time-dependent density functional theory, allows us to characterize the mechanisms and time scales of the evolution of the photogenerated positive charge (hole) and the subsequent proton transfer at the GaN/water interface. The calculations show that the hole loses its excess energy within 100 fs and localizes primarily on the nitrogen atoms of the GaN surface, initiating a sequence of proton-transfer events from the surface N-H group to the nearby OH groups and bulk water molecules. Water splitting requires hole localization on oxygen rather than nitrogen, necessitating nonadiabatic transitions uphill in energy on pure GaN. Such transitions happen rarely, resulting in low yields of the photocatalytic water splitting observed experimentally. We conclude that efficient cocatalysts should favor localization of the photogenerated hole on oxygen-containing species at the semiconductor/water interface.

High-Efficiency Photochemical Water Splitting of CdZnS/CdZnSe Nanostructures

We have prepared and employed TiO2/CdZnS/CdZnSe electrodes for photochemical water splitting. The TiO2/CdZnS/CdZnSe electrodes consisting of sheet-like CdZnS/CdZnSe nanostructures (8–10 μm in length and 5–8 nm in width) were prepared through chemical bath deposition on TiO2 substrates. The TiO2/CdZnS/CdZnSe electrodes have light absorption over the wavelength 400–700 nm and a band gap of 1.87 eV. Upon one sun illumination of 100 mW cm−2, the TiO2/CdZnS/CdZnSe electrodes provide a significant photocurrent density of 9.7 mA cm−2 at −0.9 V versus a saturated calomel electrode (SCE). Incident photon-to-current conversion efficiency (IPCE) spectrum of the electrodes displays a maximum IPCE value of 80% at 500 nm. Moreover, the TiO2/CdZnS/CdZnSe electrodes prepared from three different batches provide a remarkable photon-to-hydrogen efficiency of 7.3 ± 0.1% (the rate of the photocatalytically produced H2 by water splitting is about 172.8 mmol·h−1·g−1), which is the most efficient quantum-dots-based photocatalysts used in solar water splitting.

Strong excitonic effects in the optical properties of graphitic carbon nitrideg-C3N4from first principles

Graphitic carbon nitride ($g$-C${}_{3}$N${}_{4}$) has recently triggered extensive investigations due to its potential applications, such as in direct photochemical water splitting, CO${}_{2}$ activation, and transition-metal-free spintronics. However, electronic, and particularly the optical properties of $g$-C${}_{3}$N${}_{4}$ still have not been well established. Based on one of the state-of-the-art approaches---many-body Green's function theory (i.e., $GW$ $+$ BSE)---absorption of ultraviolet light by $g$-C${}_{3}$N${}_{4}$ is found to be determined by strong excitonic effects with a significantly large binding energy assigned to the bound excitons. Dark states have also been found in $g$-C${}_{3}$N${}_{4}$, which can affect the photoluminescence yield of $g$-C${}_{3}$N${}_{4}$. We find that the band gap of $g$-C${}_{3}$N${}_{4}$ probably can be tuned by adjusting the condensation (dimensionality) to initiate excitonic absorption in the visible light region, which might help improve the solar energy conversion efficiency.

Water splitting by infrared femtosecond laser excitation of surface plasmon

Gold nanoparticles supported on soda-lime glass exhibit a photochemical water splitting activity under infrared femtosecond laser excitation. Both H2 and hydroxyl radicals productions were characterized. The hydroxyl radicals production mechanism was identified by comparison with three prototypal mechanisms, photoionization of organic compound in the UV, VUV dissociation of water and water gamma radiolysis. The mechanisms involved in the case of laser femtosecond seem to be water ionization events occurring at distance from the gold particles.

Polymeric Graphitic Carbon Nitride for Heterogeneous Photocatalysis

Polymeric graphitic carbon nitride (for simplicity, g-C3N4) is a layered material similar to graphene, being composed of only C, N, and some impurity H. Contrary to graphenes, g-C3N4 is a medium band gap semiconductor and an effective photocatalyst for a broad variety of reactions, and it possesses a high thermal and chemical stability In this Perspective, we describe the polycondensation of this structure, how to modify band positions and band gap by doping and copolymerization, and how to texture the organic solid to make it an effective photocatalyst. We then describe the photochemical splitting of water and some mild and selective photooxidation reactions catalyzed by g-C3N4.

Oxidative quenching within photosensitizer–acceptor dyads based on bis(bidentate) phosphine-connected osmium(II) bipyridyl light absorbers and reactive metal sites

For the first time oxidative quenching of OsP2N4 chromophores by reactive PtII or PdII sites containing cis, trans, cis-1,2,3,4-tetrakis(diphenylphosphino)cyclobutane (dppcb) is directly observed despite the presence of a saturated cyclobutane backbone “bridge”. This dramatic effect is measured as a sudden temperature-dependent onset of a reduction in phosphorescence lifetime in [Os(bpy)2(dppcb)MCl2](SbF6)2 (M = Pt, 1; Pd, 2). The appearance of this additional energy release is not detectable in [Os(bpy)2(dppcbO2)](PF6)2 (3), where dppcbO2 is cis, trans, cis-1,2-bis(diphenylphosphinoyl)-3,4-bis(diphenylphosphino)cyclobutane. Obviously, the square-planar metal centers in 1 and 2 are responsible for this effect. In line with these observations, the emission quantum yields at room temperature for 1 and 2 are drastically reduced compared with 3. Since this luminescence quenching implies strong intramolecular interaction between the OsII excited states and the acceptor sites and depends on the metal⋯metal distances, also the single crystal X-ray structures of 1–3 are given.

ChemInform Abstract: Polymeric Graphitic Carbon Nitride as a Heterogeneous Organocatalyst: From Photochemistry to Multipurpose Catalysis to Sustainable Chemistry.

AbstractReview: 207 refs.

Polymeric Graphitic Carbon Nitride as a Heterogeneous Organocatalyst: From Photochemistry to Multipurpose Catalysis to Sustainable Chemistry

Polymeric graphitic carbon nitride materials (for simplicity: g-C(3)N(4)) have attracted much attention in recent years because of their similarity to graphene. They are composed of C, N, and some minor H content only. In contrast to graphenes, g-C(3)N(4) is a medium-bandgap semiconductor and in that role an effective photocatalyst and chemical catalyst for a broad variety of reactions. In this Review, we describe the "polymer chemistry" of this structure, how band positions and bandgap can be varied by doping and copolymerization, and how the organic solid can be textured to make it an effective heterogenous catalyst. g-C(3)N(4) and its modifications have a high thermal and chemical stability and can catalyze a number of "dream reactions", such as photochemical splitting of water, mild and selective oxidation reactions, and--as a coactive catalytic support--superactive hydrogenation reactions. As carbon nitride is metal-free as such, it also tolerates functional groups and is therefore suited for multipurpose applications in biomass conversion and sustainable chemistry.

Towards an artificial leaf?

Photosynthesis in plants converts energy from the Sun into chemical fuel in the form of glucose. Now, a strategy to combine carbon dioxide reduction and photochemical water splitting, using a recyclable hydrogen donor, offers the opportunity to develop non-biomimetic photosynthesis.

Analysis of a photochemical water splitting reactor with supramolecular catalysts and a proton exchange membrane

In this paper, a new type of photochemical water splitting system is proposed and analyzed. The system comprises two reactors in which photocatalytic half-reactions of water reduction and water oxidation are conducted. The two reactors are divided by a proton conducting membrane. Complex molecular devices based on ruthenium-(bipyridine) 3 2+ photosensitizers are dissolved in both reactors, which generate electrons or holes when exposed to high energy photonic radiation, and act as catalysts for water splitting. The selected molecular devices for water reduction have a unique property to enhance the existence time of photoelectrons, such that the likelihood of generated electron pairs to produce a molecule of hydrogen is increased. In this study, the physical–chemical processes are analyzed and the main design parameters of the reactor are determined. The engineering design consists of a photochemical reactor, a mass flow controller to supply the fresh water, and two fans that extract the products of water splitting.

A renewable amine for photochemical reduction of CO2

Photochemical reduction of CO₂ (to produce formic acid) can be seen both as a method to produce a transportable hydrogen-based fuel and also to reduce levels of CO₂ in the atmosphere. However, an often overlooked necessity for photochemical CO₂ reduction is the need for a sacrificial electron donor, usually a tertiary amine. Here, we describe a new strategy for coupling the photochemical reduction of CO₂ to photochemical water splitting, and illustrate this with a prototype example. Instead of seeking to eliminate the use of an external reducing agent altogether, our alternative strategy makes the reducing agent recyclable. This has two potential advantages over the direct coupling of CO₂ reduction and water oxidation. First, it allows the two redox reactions to be carried out with existing chemistry, and second, it permits these reactions to be conducted under mutually incompatible conditions.

Editorial: A Current Perspective on Photocatalysis

The efficient conversion of solar photons into solar electricity and solar fuels is one of the most important scientific challenges of this century owing to dwindling fossil fuel reserves and the need for clean energy. While research in the direct conversion of solar energy to electricity in the areas of low-cost photovoltaic (PV) systems based on all-inorganic semiconductors, dye-sensitized solar cells, organic, and molecular PV is more technically advanced than its direct conversion to fuels, electricity may not be the ultimate primary solar energy conversion choice owing to the intermittence of solar radiation, the considerable energy loss during transmission, the availability of cost-effective storage media for electricity, and the continuing need for liquid transportation fuels. On the other hand, the direct conversion of solar photons to fuels such as H{sub 2}, CO, alcohols, and hydrocarbons using H{sub 2}O and CO{sub 2} as feedstocks offers a solution for the storage and distribution of solar energy in the form of stable chemical bonds that can be activated to provide energy at arbitrary times and locations. The latter approach to photocatalysis is generally called artificial photosynthesis, and has received renewed interest over the past five or so years. While 'photocatalysis' has not traditionallymore » been restricted to the generation of 'solar fuels,' and has included the production of other useful chemicals, polymerization, and environmental remediation applications, the recent upsurge of interest has been driven mostly by renewable energy issues. It was the pioneering work on photo-electrochemical splitting of water to H{sub 2} and O{sub 2} by n-type TiO{sub 2} using ultraviolet light, by Fujishima and Honda in 1972, that ushered in the area of research that has come to be known as 'solar fuels,' and that has led to the terms 'photocatalysis' and 'solar fuels' becoming almost synonymous. This special issue of ChemSusChem is devoted to providing a current perspective on the field of photocatalysis. It contains invited papers from leading researchers in a wide range of important aspects of the field that address materials, photophysical, photochemical, and electrocatalysis issues. The area remains primarily the domain of basic research studies because progress toward the promise offered by the early work has (at least until recently) been slow, despite its significance having become increasingly recognized. The present collection of papers deals with new semiconductor photocatalysts, molecular catalysts for hydrogen production and water oxidation, dye-sensitized photoelectrochemical cells, and electrochemical CO{sub 2} reduction. Overall photochemical water splitting without any applied bias potential is achieved in several systems, especially under UV irradiation. Further advances are also achieved in a few semiconductor systems, such as GaZn oxynitrides or two-step (so-called 'Z-scheme') systems to produce H{sub 2} and O{sub 2} without any sacrificial reagent under visible irradiation. When band gaps of semiconductors are narrowed to absorb more visible light for greater efficiency, or when band positions are not suitable for carrying out one-electron redox processes, multielectron catalysts are required to promote proton-coupled electron transfer reactions in producing solar fuels. In homogeneous photocatalysis systems, sacrificial reagents are typically used to investigate the catalytic activity, detailed kinetics, and mechanisms of a half reaction. Photoelectrolysis systems with immobilized catalysts (metals, metal oxides, or molecular catalysts) on electrodes can separate oxidized products, such as O{sub 2}, and reduced products, such as H{sub 2}, CO, CH{sub 3}OH, and others, by means of proton- or hydroxide-conducting membranes. The following paragraphs briefly summarize these contributions. In the area of UV-driven water splitting, Townsend et al. prepared Pt-and/or IrO{sub x}-coated niobate (Nb{sub 6}O{sub 17}{sup 4-}) nanoscrolls and tested photochemical water reduction with methanol as a sacrificial reagent, and water oxidation with AgNO{sub 3} as a sacrificial reagent. In this work, factors for improving the limited photocatalytic activity of the system are explored. Nishiyama et al. investigated factors controlling the photocatalytic water splitting activity, such as preparation temperature and crystallinity of the semiconductor materials, the amount of co-catalyst loading, and the degree of dispersion of the co-catalyst with RuO{sub 2}-loaded niobium and tantalum bronzes, M{sub 8}P{sub 4}O{sub 32}{sup 4-} (M=Nb, Ta). Their DFT calculations demonstrate that severe distortion of NbO{sub 6} octahedra plays an important role in photocatalytic water splitting.« less