Hydrogen induced interface engineering in Fe2O3-TiO2 heterostructures for efficient charge separation for solar-driven water oxidation in photoelectrochemical cells

Semiconductor heterostructure junctions are known to improve the water oxidation performance in photoelectrochemical (PEC) cells. Depending on the semiconductor materials involved, different kinds of junctions can appear, for instance, type II band alignment where the conduction and valence bands of the semiconductor materials are staggered with respect to each other. This band alignment allows for a charge separation of the photogenerated electron-hole pairs, where the holes will go from low-to-high valance band levels and vice versa for the electrons. For this reason, interface engineering has attracted intensive attention in recent years. In this work, a simplified model of the Fe2O3-TiO2 heterostructure was investigated via first-principles calculations. The results show that Fe2O3-TiO2 produces a type I band alignment in the heterojunction, which is detrimental to the water oxidation reaction. However, the results also show that interstitial hydrogens are energetically allowed in TiO2 and that they introduce states above the valance band, which can assist in the transfer of holes through the TiO2 layer. In response, well-defined planar Fe2O3-TiO2 heterostructures were manufactured, and measurements confirm the formation of a type I band alignment in the case of Fe2O3-TiO2, with very low photocurrent density as a result. However, once TiO2 was subjected to hydrogen treatment, there was a nine times higher photocurrent density at 1.50 V vs. the reversible hydrogen electrode under 1 sun illumination as compared to the original heterostructured photoanode. Via optical absorption, XPS analysis, and (photo)electrochemical measurements, it is clear that hydrogen treated TiO2 results in a type II band alignment in the Fe2O3-H:TiO2 heterostructure. This work is an example of how hydrogen doping in TiO2 can tailor the band alignment in TiO2-Fe2O3 heterostructures. As such, it provides valuable insights for the further development of similar material combinations. This journal i

Biomimetic zinc chlorin-poly(4-vinylpyridine) assemblies : doping level dependent emission-absorption regimes

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Broadband laser polarization control with aligned carbon nanotubes

We introduce a simple approach to fabricate aligned carbon nanotube (ACNT) device for broadband polarization control in fiber laser systems. The ACNT device was fabricated by pulling from as-fabricated vertically-aligned carbon nanotube arrays. Their anisotropic property is confirmed with optical and scanning electron microscopy, and with polarized Raman and absorption spectroscopy. The device was then integrated into fiber laser systems (at two technologically important wavelengths of 1 and 1.5 um) for polarization control. We obtained a linearly-polarized light output with the maximum extinction ratio of ~12 dB. The output polarization direction could be fully controlled by the ACNT alignment direction in both lasers. To the best of our knowledge, this is the first time that ACNT device is applied to polarization control in laser systems. Our results exhibit that the ACNT device is a simple, low-cost, and broadband polarizer to control laser polarization dynamics, for various photonic applications (such as material processing, polarization diversity detection in communications), where the linear polarization control is necessary.Comment: 5 pages, 6 figure

Focused ion beam high resolution grayscale lithography for silicon-based nanostructures

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Nonequilibrium Green's function simulation of Cu2 O photocathodes for photoelectrochemical hydrogen production

Funding Information: The authors acknowledge the financial support from the Academy of Finland Project No. 329406 and the Photonics Research and Innovation (PREIN) flagship program, Decision No. 320167. L.H. acknowledges funding from the Aalto ELEC doctoral school, and Walter Ahlström foundation. Finally, we acknowledge the computational resources provided by the Aalto Science-IT project. Publisher Copyright: © 2023 American Physical Society.In this work we present a simulation of the semiconductor electrodes of photoelectrochemical (PEC) water-splitting cells based on the nonequilibrium Green's function (NEGF) formalism. While the performance of simple PEC cells can be adequately explained with semiclassical drift-diffusion theory, the increasing interest towards thin-film cells and nanostructures, in general, requires theoretical treatment that can capture the quantum phenomena influencing the charge carrier dynamics in these devices. Specifically, we study a p-type Cu2O electrode and examine the influence of the bias voltage, reaction kinetics, and the thickness of the Cu2O layer on the generated photocurrent. The NEGF equations are solved in a self-consistent manner with the electrostatic potential from Poisson's equation, sunlight-induced photon scattering and the chemical overpotential required to drive the water-splitting reaction. We show that the NEGF simulation accurately reproduces experimental results from both voltammetry and impedance spectroscopy measurements, while providing an energy-resolved solution of the charge carrier densities and corresponding currents inside the semiconductor electrode at nanoscale.Peer reviewe

Computational Study Revealing the Influence of Surface Phenomena in p-GaAs Water-Splitting Cells

A computational model of a photoelectrochemical cell describing the influence of competing surface reactions to the operation of the cell is presented. The model combines an optical simulation for the incident light intensity with fully self-consistent solution of drift-diffusion equations to accurately calculate the electronic state of the semiconductor electrode in a photoelectrochemical cell under operation. The solution is calculated for the full thickness of a typical wafer, while simultaneously solving the thin surface charge region with sufficient precision. In addition to comparing the simulated current–voltage response with experimental data, the simulation is shown to replicate experimental results from electrochemical impedance spectroscopy (EIS) measurements. The results show that considering optical losses in the system is crucial for accurate simulation. The model is capable of selectively characterizing the impact of material parameters on both current–voltage response and interface capacitance, while revealing the internal dynamics of the quasi-Fermi levels that are inaccessible by experimental methods.Peer reviewe

Large-area thermal distribution sensor based on multilayer graphene ink

| openaire: EC/H2020/645241/EU//TransFlexTegEmergent applications in wearable electronics require inexpensive sensors suited to scalable manufacturing. This work demonstrates a large-area thermal sensor based on distributed thermocouple architecture and ink-based multilayer graphene film. The proposed device combines the exceptional mechanical properties of multilayer graphene nanocomposite with the reliability and passive sensing performance enabled by thermoelectrics. The Seebeck coefficient of the spray-deposited films revealed an inverse thickness dependence with the largest value of 44.7 µV K−1 at 78 nm, which makes thinner films preferable for sensor applications. Device performance was demonstrated by touch sensing and thermal distribution mapping-based shape detection. Sensor output voltage in the latter application was on the order of 300 µV with a signal-to-noise ratio (SNR) of 35, thus enabling accurate detection of objects of different shapes and sizes. The results imply that films based on multilayer graphene ink are highly suitable to thermoelectric sensing applications, while the ink phase enables facile integration into existing fabrication processes.Peer reviewe