RESUMEN
The ground- and excited-state electronic structures of four polymeric carbon nitride (PCN) materials have been investigated using a combination of photoemission and optical absorption spectroscopy. To establish the driving forces for photocatalytic water-splitting reactions, the ground-state data was used to produce a band diagram of the PCN materials and the triethanolamine electron scavenger, commonly implemented in water-splitting devices. The ultrafast charge-carrier dynamics of the same PCN materials were also investigated using two femtosecond-time-resolved pump-probe techniques: extreme-ultraviolet (EUV) photoemission and ultraviolet-visible (UV-Vis) transient absorption spectroscopy. The complementary combination of these surface- and bulk-sensitive methods facilitated photoinduced kinetic measurements spanning the sub-picosecond to few nanosecond time range. The results show that 400 nm (3.1 eV) excitation sequentially populates a pair of short-lived transient species, which subsequently produce two different long-lived excited states on a sub-picosecond time scale. Based on the spectro-temporal characteristics of the long-lived signals, they are assigned to singlet-exciton and charge-transfer states. The associated charge-separation efficiency was inferred to be between 65% and 78% for the different studied materials. A comparison of results from differently synthesized PCNs revealed that the early-time processes do not differ qualitatively between sample batches, but that materials of more voluminous character tend to have higher charge separation efficiencies, compared to exfoliated colloidal materials. This finding was corroborated via a series of experiments that revealed an absence of any pump-fluence dependence of the initial excited-state decay kinetics and characteristic carrier-concentration effects that emerge beyond few-picosecond timescales. The initial dynamics of the photoinduced charge carriers in the PCNs are correspondingly determined to be spatially localised in the immediate vicinity of the lattice-constituting motif, while the long-time behaviour is dominated by charge-transport and recombination processes. Suppressing the latter by confining excited species within nanoscale volumes should therefore affect the usability of PCN materials in photocatalytic devices.
RESUMEN
Triazine-based graphitic carbon nitride (TGCN) is the most recent addition to the family of graphene-type, two-dimensional, and metal-free materials. Although hailed as a promising low-band-gap semiconductor for electronic applications, so far, only its structure and optical properties have been known. Here, we combine direction-dependent electrical measurements and time-resolved optical spectroscopy to determine the macroscopic conductivity and microscopic charge-carrier mobilities in this layered material "beyond graphene". Electrical conductivity along the basal plane of TGCN is 65 times lower than through the stacked layers, as opposed to graphite. Furthermore, we develop a model for this charge-transport behavior based on observed carrier dynamics and random-walk simulations. Our combined methods provide a path towards intrinsic charge transport in a direction-dependent layered semiconductor for applications in field-effect transistors (FETs) and sensors.
RESUMEN
Solar light harvesting by photocatalytic H2 evolution from water could solve the problem of greenhouse gas emission from fossil fuels with alternative clean energy. However, the development of more efficient and robust catalytic systems remains a great challenge for the technological use on a large scale. Here we report the synthesis of a sol-gel prepared mesoporous graphitic carbon nitride (sg-CN) combined with nickel phosphide (Ni2 P) which acts as a superior co-catalyst for efficient photocatalytic H2 evolution by visible light. This integrated system shows a much higher catalytic activity than the physical mixture of Ni2 P and sg-CN or metallic nickel on sg-CN under similar conditions. Time-resolved photoluminescence and electron paramagnetic resonance (EPR) spectroscopic studies revealed that the enhanced carrier transfer at the Ni2 P-sg-CN heterojunction is the prime source for improved activity.
RESUMEN
Solvated electrons are among the most reductive species in an aqueous environment. Diamond materials have been proposed as a promising source of solvated electrons, but the underlying emission process in water remains elusive so far. Here, we show spectroscopic evidence for the emission of solvated electrons from detonation nanodiamonds upon excitation with both deep ultraviolet (225 nm) and visible (400 nm) light using ultrafast transient absorption. The crucial role of surface termination in the emission process is evidenced by comparing hydrogenated, hydroxylated and carboxylated nanodiamonds. In particular, a transient response that we attribute to solvated electrons is observed on hydrogenated nanodiamonds upon visible light excitation, while it shows a sub-ps recombination due to trap states when excited with deep ultraviolet light. The essential role of surface reconstructions on the nanodiamonds in these processes is proposed based on density functional theory calculations. These results open new perspectives for solar-driven emission of solvated electrons in an aqueous phase using nanodiamonds.
RESUMEN
The synthesis and characterization of the first heterobimetallic methylzinc-magnesium alkoxide clusters [Me(6)MgZn(6)(OR)(8)] [R=Et (1 a), n-Pr (1 b), nBu (1 c)] with a bis-cubane-shaped MgZn(6)O(8) core is described. The thermal degradation of 1 a-c and mixtures of 1 a and the homometallic MeZn-alkoxide cubane [{MeZnOtBu}(4)] (2) in dry synthetic air led to wide-band-gap semiconducting Mg(x)Zn(1-x)O nanoparticles at temperatures below 500 °C. The final materials were characterized by different analytical techniques such as PXRD, REM, TEM, EDX, and IR spectroscopy. The morphology of the as-prepared magnesium-containing ZnO samples is influenced by the different organo groups (alkoxo) in the precursors. EDX mapping showed that magnesium is uniformly distributed in the ZnO matrix. The incorporation of magnesium led to a distortion of the ZnO lattice with increased a axis and decreased c axis parameters. Room-temperature photoluminescence (PL) spectra reveal that the near-band-edge (NBE) emission of Mg(2+)-doped ZnO is shifted to higher energies relative to that of pure ZnO.
RESUMEN
The kinetics of ultrafast photoinduced structural changes in linkage isomers is investigated using Na2[Fe(CN)5NO] as a model complex. The buildup of the metastable side-on configuration of the NO ligand, as well as the electronic energy levels of ground, excited, and metastable states, has been revealed by means of time-resolved extreme UV (XUV) photoelectron spectroscopy in aqueous solution, aided by theoretical calculations. Evidence of a short-lived intermediate state in the isomerization process and its nature are discussed, finding that the complete isomerization process occurs in less than 240 fs after photoexcitation.
RESUMEN
Charge transport in polymeric graphitic carbon nitrides is shown to proceed via diffusive hopping of electron and hole polarons with reasonably high mobilities >10(-5) cm(2) V(-1) s(-1). The power-law behavior of the ultrafast luminescence decay exhibits that the predominant transport direction is perpendicular to the graphitic polymer sheets, thus complementing 2D materials like graphene.
RESUMEN
Recently, it has been shown that an abundant material, polymeric carbon nitride, can produce hydrogen from water under visible-light irradiation in the presence of a sacrificial donor. We present herein the preparation and characterization of graphitic carbon nitride (g-C(3)N(4)) films on p-type semiconducting CuGaSe(2) chalcopyrite thin-film substrates by thermal condensation of a dicyandiamide precursor under inert-gas conditions. Structural and surface morphological studies of the carbon nitride films suggest a high porosity of g-C(3)N(4) thin films consisting of a network of nanocrystallites. Photoelectrochemical investigations show light-induced hydrogen evolution upon cathodic polarization for a wide range of proton concentrations in the aqueous electrolyte. Additionally, synchrotron radiation-based photoelectron spectroscopy has been applied to study the surface/near-surface chemical composition of the utilized g-C(3)N(4) film photocathodes. For the first time, it has been shown that g-C(3)N(4) films coated on p-type CuGaSe(2) thin films can be successfully applied as new photoelectrochemical composite photocathodes for light-induced hydrogen evolution.