Light Absorption Processes Research Articles

Design and fabrication of an α-Fe2O3/TiO2/Si 3D hierarchical photoanode for improved photoelectrochemical water splitting

Ordered hierarchical structured films are considered promising photoanodes for high-performance water-splitting because of their large surface area for light absorption and excellent charge transfer process. In this paper, a novel hierarchical structured photoanode based on growing TiO2 on a Si nanowire substrate and coupling with Fe2O3 nanothorns was designed and fabricated via a simple method to obtain photoelectrochemical (PEC) properties. The Fe2O3/TiO2/Si nanowire arrays (NWs) were characterized in detail using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and UV–vis spectra. The 3D Fe2O3/TiO2/Si hierarchical photoanode exhibited a photocurrent density of 3.5 mA cm−2 at a bias potential of 1.23 V (vs. RHE), which is approximately 87.5 times higher than that of the Si nanowire array (NW) photoanode. The measurement results showed that the improved PEC performance could be attributed to the synergistic effect of the large surface area, low charge transfer resistance, and high charge separation in the hierarchical structure.

Correspondence between light-absorption spectrum and nonequilibrium work distribution as a mean to access free energy differences between electronic states.

The problem of recovering the free energy difference between two electronic states has been investigated by Frezzato [Chem. Phys. Lett. 533, 106 (2012)], exploring the equivalence between light-absorption spectra and work distribution, hence opening to the application of a spectroscopic version of the Jarzynski equality (JE) [Phys. Rev. Lett. 78, 2690 (1997)]. Here, assuming the validity of the time-dependent perturbation theory, we demonstrate that such equivalence does not lead to the known form of the JE. This is ascribed to the fact that light-absorption processes cannot be described as stochastic processes. To emphasize such an aspect, we devise a stochastic model for the UV-vis (ultraviolet and visible) absorption, suitable for determining the free energy difference between two generic quantum manifolds in a JE-like fashion. However, the model would require explicit knowledge of the transition dipole moments, which are in general not available. Nonetheless, we derive a spectroscopic version of the JE that allows us to recover the free energy difference between the ground and an excited electronic state when the latter state is the only one observed in the spectrum.

Photoelectrochemical Properties of a Cu2O Film/ZnO Nanorods Oxide p-n Heterojunction Photoelectrode for Solar-Driven Water Splitting

We report on the fabrication and photoelectrochemical(PEC) properties of a Cu2O thin film/ZnO nanorod array oxide p-n heterojunction structure with ZnO nanorods embedded in Cu2O thin film as an efficient photoelectrode for solardriven water splitting. A vertically oriented n-type ZnO nanorod array was first prepared on an indium-tin-oxide-coated glass substrate via a seed-mediated hydrothermal synthesis method and then a p-type Cu2O thin film was directly electrodeposited onto the vertically oriented ZnO nanorods array to form an oxide semiconductor heterostructure. The crystalline phases and morphologies of the heterojunction materials were characterized using X-ray diffraction and scanning electron microscopy as well as Raman scattering. The PEC properties of the fabricated Cu2O/ZnO p-n heterojunction photoelectrode were evaluated by photocurrent conversion efficiency measurements under white light illumination. From the observed PEC current density versus voltage (J-V) behavior, the Cu2O/ZnO photoelectrode was found to exhibit a negligible dark current and high photocurrent density, e.g., 0.77 mA/cm2 at 0.5 V vs Hg/HgCl2 in a 1 mM Na2SO4 electrolyte, revealing an effective operation of the oxide heterostructure. In particular, a significant PEC performance was observed even at an applied bias of 0 V vs Hg/HgCl2, which made the device self-powered. The observed PEC performance was attributed to some synergistic effect of the p-n bilayer heterostructure on the formation of a built-in potential, including the light absorption and separation processes of photoinduced charge carriers.

Proton-Coupled Electron Transfer in Artificial Photosynthetic Systems.

Artificial photosynthetic constructs can in principle operate more efficiently than natural photosynthesis because they can be rationally designed to optimize solar energy conversion for meeting human demands rather than the multiple needs of an organism competing for growth and reproduction in a complex ecosystem. The artificial photosynthetic constructs described in this Account consist primarily of covalently linked synthetic chromophores, electron donors and acceptors, and proton donors and acceptors that carry out the light absorption, electron transfer, and proton-coupled electron transfer (PCET) processes characteristic of photosynthetic cells. PCET is the movement of an electron from one site to another accompanied by proton transfer. PCET and the transport of protons over tens of angstroms are important in all living cells because they are a fundamental link between redox processes and the establishment of transmembrane gradients of proton electrochemical potential, known as proton-motive force (PMF), which is the unifying concept in bioenergetics. We have chosen a benzimidazole phenol (BIP) system as a platform for the study of PCET because with appropriate substitutions it is possible to design assemblies in which one or multiple proton transfers can accompany oxidation of the phenol. In BIP, oxidation of the phenol increases its acidity by more than ten pKa units; thus, electrochemical oxidation of the phenol is associated with a proton transfer to the imidazole. This is an example of a PCET process involving transfer of one electron and one proton, known as electron-proton transfer (EPT). When the benzimidazole moiety of BIP is substituted at the 4-position with good proton acceptor groups such as aliphatic amines, experimental and theoretical results indicate that two proton transfers occur upon one-electron oxidation of the phenol. This phenomenon is described as a one-electron-two-proton transfer (E2PT) process and results in translocation of protons over ∼7 Å via a Grotthuss-type mechanism, where the protons traverse a network of internally H-bonded sites. In the case of the E2TP process involving BIP analogues with amino group substituents, the thermodynamic price paid in redox potential to move a proton to the final proton acceptor is ∼300 mV. In this example, the decrease in redox potential limits the oxidizing power of the resulting phenoxyl radical. Thus, unlike the biological counterpart, the artificial construct is thermodynamically incapable of effectively advancing the redox state of a water oxidation catalyst. The design of systems where multiple proton transfer events are coupled to an oxidation reaction while a relatively high redox potential is maintained remains an outstanding challenge. The ability to control proton transfer and activity at defined distances and times is key to achieving proton management in the vicinity of catalysts operating at low overpotential in myriad biochemically important processes. Artificial photosynthetic constructs with well-defined structures, such as the ones described in this Account, can provide the means for discovering design principles upon which efficient redox catalysts for electrolysis and fuel cells can be based.

On the Assessment of Light Use Efficiency in Alfalfa (Medicago sativa L.) in the Eco-Climatic Conditions of Târgovişte Piedmont Plain

The overall objective of this research was to study the light use efficiency (LUE) of alfalfa (Medicago sativa L.) in the eco-climatic conditions of Târgovişte Piedmont Plain. A field experiment with two Romanian varieties (Roxana and Mihaela) developed by NARDI Fundulea was conducted in pure stand between 2012 and 2014, using a Latin rectangle design with four replicates and three cutting cycles per year. Dry matter accumulation, leaf parameters and light capture characteristics did not show significant differences between the two tested varieties (p>0.05) in non-irrigated conditions. The maximum LAI at the first cutting was 5.1 for Roxana, and 5.3 for Mihaela. The ELADP varied between 1.47 and 2.36, while light extinction coefficient (k) had a synthetic average of 0.8. The energy transmitted to the canopy ranged between 7.4 and 22.3 mol m-2 s -1 on average. The coefficient of absorption varied between 0.75 and 0.83 depending on variety and experimental year. LUE of alfalfa ranged between 0.23 and 0.27 g moles-1 m-2 day-1 . The resulted elements characterizing the light interception and absorption processes in alfalfa, which were determined dynamically during the growth seasons, are useful for the parameterisation and utilization in dedicated crop growth models.

Evaluating particle-suspension reactor designs for Z-scheme solar water splittingviatransport and kinetic modeling

Simulations of coupled light absorption, transport and kinetics processes in a tandem, vertically-stacked particle-suspension reactor design for solar-water splitting.

Elemental doping for optimizing photocatalysis in semiconductors.

The exploration of advanced photocatalysts has been attracting wide attention, especially in facing the recent growing challenges of energy and environment. Among the various strategies for achieving high solar energy utilization, elemental doping has been extensively explored owing to its advantages in tuning the electronic structures of semiconductor-based catalysts, where the key factors in photocatalysis can be generally optimized. This frontier article will focus on the impacts of elemental doping on light absorption, band positions and charge carrier processes of semiconductors.

Multipoint Fiber Bragg Grating Sensing Using Two-Photon Absorption Process in Silicon Avalanche Photodiode

We propose a multipoint fiber Bragg grating (FBG) sensing system that measures both spectral change of FBGs and precise distance to them by using intensity-modulated light and two-photon absorption process in a silicon avalanche photodiode. We experimentally confirm the principle using five cascaded FBGs with almost the same Bragg wavelength. The optical path length difference and the reflection spectrum for each FBG are successfully measured at the same time. The relative uncertainty of path length measurement is in the order of 10−4.

Synthesis and optical properties of cerium doped Li7La3Hf2O12 with tetragonal garnet structure

The new polycrystalline samples of Li7La3Hf2O12:xCe (x = 0.00–3.30%) with tetragonal garnet structure have been prepared using a conventional solid state reaction. The phase composition, morphology and optical properties of undoped and cerium doped compounds have been characterized by powder X-ray diffraction, scanning electron microscopy, electron paramagnetic resonance, diffuse reflectance and photoluminescence spectroscopy. The absorption bands at 2.5eV (496nm) and 3.3eV (375nm) are associated with processes of light absorption in F-centers in tetragonal Li7La3Hf2O12. The origin of luminescence in Li7La3Hf2O12 can be explained by the creation and relaxation of self-trapped exciton localized in [HfO6]8- system. The Ce ions in Li7La3Hf2O12:xCe (x = 0.00–3.30%) have predominantly the Ce4+ valence state, and no luminescence of Ce3+ is observed. The increase in the cerium concentration in Li7La3Hf2O12 leads to a reduction of the photoluminescence intensity associated with intrinsic luminescence centers. Using the temperature dependence of luminescence, the activation energy of temperature quenching of intrinsic luminance was calculated.

Modeling and simulation of experimentally fabricated QDSSC using ZnS as light absorbing and blocking layer

Abstract—Two main factors which limit the power conversion efficiency of solar cells are light absorption and recombination processes. In photovoltaic (PV) devices, low energy photons cannot be absorbed and excite electrons from valance band to conduction band, hence do not contribute to the current. On the other hand, high energy photons cannot be efficiently used due to a poor match to the energy gap. Existence of charge recombination in PV devices causes the low conversion performance, which is indicated by the low open-circuit voltage (V OC ). Using a blocking layer in system could effectively reduce the recombination of charge carriers. In this study, we simulated a solar cell with ITO/ZnO/P3HT&PCBM/Ag structure. To prevent the charge recombination, a ZnS QD layer was used which acts as a light absorbing and a recombination blocking layer in the ITO/ZnO film/ZnS QD/P3HT&PCBM/Ag structure. The simulated J–V characteristics of solar cells showed a close match with the experimental results. Simulate data showed an increase of conversion efficiency in ZnS QDSSC from 1.71 to 3.10%, which is relatively 81.28% increase.

Optical bandgap of semiconductor nanostructures: Methods for experimental data analysis

Determination of the optical bandgap (Eg) in semiconductor nanostructures is a key issue in understanding the extent of quantum confinement effects (QCE) on electronic properties and it usually involves some analytical approximation in experimental data reduction and modeling of the light absorption processes. Here, we compare some of the analytical procedures frequently used to evaluate the optical bandgap from reflectance (R) and transmittance (T) spectra. Ge quantum wells and quantum dots embedded in SiO2 were produced by plasma enhanced chemical vapor deposition, and light absorption was characterized by UV-Vis/NIR spectrophotometry. R&T elaboration to extract the absorption spectra was conducted by two approximated methods (single or double pass approximation, single pass analysis, and double pass analysis, respectively) followed by Eg evaluation through linear fit of Tauc or Cody plots. Direct fitting of R&T spectra through a Tauc-Lorentz oscillator model is used as comparison. Methods and data are discussed also in terms of the light absorption process in the presence of QCE. The reported data show that, despite the approximation, the DPA approach joined with Tauc plot gives reliable results, with clear advantages in terms of computational efforts and understanding of QCE.

Mechanochemical Tuning of Pyrene Absorption Spectrum Using Force Probes.

Control of absorption spectra in chromophores is a fundamental aspect of many photochemical and photophysical processes as it constitutes the first step of the global photoinduced process. Here we explore the use of mechanical forces to modulate the light absorption process. Specifically, we develop a computational formalism for determining the type of mechanical forces permitting a global tuning of the absorption spectrum. This control extends to the excitation wavelength, absorption bands overlap, and oscillator strength. The determination of these optimal forces permits us to rationally guide the design of new mechano-responsive chromophores. Pyrene has been chosen as the case study for applying these computational tools because significant absorption spectra information is available for the chromophore as well as for different strained derivatives. Additionally, pyrene presents a large flexibility, which makes it a good system to test the inclusion of force probes as the strategy to exert forces on the system.

Electronic and optical properties of metalloporphyrins of zinc on TiO2 cluster in dye-sensitized solar-cells (DSSC). A quantum chemistry study

Dye-sensitized solar-cell (DSSC) systems have been investigated by calculating light-absorption and electron-injection processes of zinc-porphyrin-dye based sensitizers adsorbed on a TiO2 cluster simulating the semiconductor.

Laser wavelength effect on nanosecond laser light reflection in ablation of metals

Reflection of nanosecond laser pulses with different wavelengths (1.06 and 0.69 µm) in ablation of titanium in air is studied experimentally. The laser wavelength effect on reflection is essential at low laser fluence values. However, it becomes negligible for laser fluence values by about an order of magnitude higher than the plasma ignition threshold. We speculate that the disappearance of the wavelength effect is explained by counter-acting processes of the laser light absorption in plasma, which increases with laser wavelength, and absorption in the surface layer, which decreases with increasing laser wavelength.

Fabrication and Photoelectrochemical Properties of a Cu2O/CuO Heterojunction Photoelectrode for Hydrogen Production from Solar Water Splitting

We report on the fabrication and characterization of a novel Cu2O/CuO heterojunction structure with CuO nanorods embedded in Cu2O thin film as an efficient photocathode for photoelectrochemical (PEC) solar water splitting. A CuO nanorod array was first prepared on an indium-tin-oxide-coated glass substrate via a seed-mediated hydrothermal synthesis method; then, a Cu2O thin film was electrodeposited onto the CuO nanorod array to form an oxide semiconductor heterostructure. The crystalline phases and morphologies of the heterojunction materials were examined using X-ray diffraction and scanning electron microscopy, as well as Raman scattering. The PEC properties of the fabricated Cu2O/CuO heterojunction photocathode were evaluated by photocurrent conversion efficiency measurements under white light illumination. From the observed PEC current density versus voltage (J-V) behavior, the Cu2O/CuO photocathode was found to exhibit negligible dark current and high photocurrent density, e.g. −1.05 mA/cm2 at −0.6 V vs. Hg/HgCl2 in 1 mM Na2SO4 electrolyte, revealing the effective operation of the oxide heterostructure. The photocurrent conversion efficiency of the Cu2O/CuO photocathode was estimated to be 1.27% at −0.6 V vs. Hg/HgCl2. Moreover, the PEC current density versus time (J-T) profile measured at −0.5 V vs. Hg/HgCl2 on the Cu2O/CuO photocathode indicated a 3-fold increase in the photocurrent density compared to that of a simple Cu2O thin film photocathode. The improved PEC performance was attributed to a certain synergistic effect of the bilayer heterostructure on the light absorption and electron-hole recombination processes.

Absorption of Light by Free Charge Carriers in the Crystalline CdS Under Intense Electron Irradiation

The process of light absorption by free electrons in the crystalline cadmium sulfide under irradiation by a nanosecond electron beam with the current density of 8–100 A/cm2 is studied. A superlinear increase in optical absorption is observed if the beam current density is increased from ~8 to 12 A/cm2. The nature of light absorption by thermalized electrons corresponds to the scattering on lattice defects. An increase in the exponent of the power dependence of light absorption on the wavelength with increasing beam current density is associated with the single and double ionization of donors and acceptors. It is concluded that accumulation of charge carriers occurs without capture by traps due to their impact ionization by secondary electrons, whose energy in the thermalization stage is comparable with the band gap of the crystal. According to the results of calculations, the capture cross section of electrons by holes at quadratic recombination is ~10–20 cm2, the Auger recombination coefficient is ~10-31 cm6∙s–1, and the charge carrier concentration is ~1.3∙1018–1.5∙1019 cm–3.

Solar Water Splitting Photoelectrodes Structured By Laser Interference Lithography

One of the most environmentally friendly ways to produce hydrogen is by electrochemical splitting of water, using only sunlight as an energy source. In a photoelectrochemical cell with a semiconductor photoanode, water oxidation takes place on the surface of the electrode, at the interface between semiconductor and electrolyte. While photoelectrodes with structured surfaces tend to outperform planar ones, an increased surface area can be accompanied by enhanced surface recombination, which is deleterious to photoelectrode performance. Furthermore, the geometry of the surface structuring can influence light absorption and transport processes, and consequently the water splitting efficiency. In order to investigate the effect of surface structuring on water splitting performance, model photoelectrodes are needed, where the geometry and aspect ratio of the surface features can be controlled. In this work, we report on the use of laser interference lithography to generate precisely defined surface patterns on n+Si substrates. The geometry and aspect ratio of the surface features is determined during a reactive ion etching step. After etching, the samples are conformally coated with TiO2 by atomic layer deposition. Photoelectrochemical characterization of the electrodes is complemented by extensive physicochemical studies in an effort to identify the structural parameters with highest impact on performance in the photodriven water oxidation reaction. Figure 1

Increase the absorption plasm and the flow of light energy in ultra-thin films gaas solar nano-structured metal

The silicon thin film solar cells in the visible region, The low absorption which reduces its efficiency. The use of metallic nanostructures help, to increase light absorption and reduce the size of the entire structure will be. The process of light absorption in solar cells is one of the factors in improving the performance of solar cells. Silicon thin film solar cells, in the visible region, the absorption is low. Using arrays of metallic nanostructures, Semiconductors such as silicon or germanium on, Radiation absorption, a significant amount used in the top layer. Metal nanostructures for their ability to excite PLASMONS, (Collective oscillations of free electrons) are being considered. PLASMONS, surface electromagnetic waves, They are strictly limited to the field level, And that's why the border field strength is high, As a result, the surface PLASMON excitation, Square in the vicinity of heavily reinforced silicon and thus the absorption of light, The silicon layer is increased significantly. Keyword: Plasmon light energy; NANO-structured metal

The Nature of the Donor Motif in Acceptor-Bridge-Donor Dyes as an Influence in the Electron Photo-Injection Mechanism in DSSCs.

The combination and balance of acceptor(A)-bridge-donor(D) architecture of molecules confer suitable attributes and/or properties to act as efficient light-harvesting and sensitizers in dye sensitized solar cells (DSSCs). An important process in a DSSC performance is the electron photoinjection (PI) mechanism which can take place either via type I (indirect), that consists in injecting from the excited state of the dye to the semiconductor, or type II (direct), where the PI is from the ground state of the dye to the semiconductor upon photoexcitation. Here, we present a computational study about the role of the donor motif in the PI mechanisms displayed from a family of 11 A-bridge-D structured dyes to a (TiO2)15 anatase cluster. To this end, different donor motifs (D1-D11) were evaluated while the A and bridge motifs remained the same. All the computations were carried out within the DFT framework, using the B3LYP, PW91, PBE, M06L and CAM-B3LYP functionals. The 6-31G(d) basis set was employed for nonmetallic atoms and the LANL2DZ pseudopotential for Ti atoms. The solvation effects were incorporated using the polarized continuum model (PCM) for acetonitrile. As benchmark systems, alizarin and naphthalenediol dyes were analyzed, as they are known to undergo Type I and Type II PI pathways in DSSCs, respectively. Donors in the studied family of dyes could influence to drive Type I or II PI since it was found that D2 could show some Type II PI route, showing a new absorption band, although with CAM-B3LYP this shows a very low oscillator strength, while the remaining dyes behave according to Type I photoinjectors. Finally, the photovoltaic parameters that govern the light absorption process were evaluated, as the use of these criteria could be applied to predict the efficiency of the studied dyes in DSSCs devices.

Semiconducting materials for photoelectrochemical energy conversion

To achieve a sustainable society with an energy mix primarily based on solar energy, we need methods of storing energy from sunlight as chemical fuels. Photoelectrochemical (PEC) devices offer the promise of solar fuel production through artificial photosynthesis. Although the idea of a carbon-neutral energy economy powered by such ‘artificial leaves’ is intriguing, viable PEC energy conversion on a global scale requires the development of devices that are highly efficient, stable and simple in design. In this Review, recently developed semiconductor materials for the direct conversion of light into fuels are scrutinized with respect to their atomic constitution, electronic structure and potential for practical performance as photoelectrodes in PEC cells. The processes of light absorption, charge separation and transport, and suitable energetics for energy conversion in PEC devices are emphasized. Both the advantageous and unfavourable aspects of multinary oxides, oxynitrides, chalcogenides, classic semiconductors and carbon-based semiconductors are critically considered on the basis of their experimentally demonstrated performance and predicted properties. Photoelectrochemical (PEC) devices offer the promise of efficient artificial photosynthesis. In this Review, recently developed light-harvesting materials for PEC application are scrutinized with respect to their atomic constitution, electronic structure and potential for practical performance in PEC cells.