Directional Charge Transport Research Articles

Photo-Electrochemical Solar-to-Fuel Energy Conversion by Hematite-Based Photo-Anodes – The Role of 1D Nanostructuring

Abstract Photo-electrochemical (PEC) water splitting (WS) using metal oxide semiconductors is regarded as a promising approach for the renewable production of fuels and energy vectors such as hydrogen (H2). Among metal oxide semiconductors, iron oxide in the form of hematite (α-Fe2O3) is one of the most researched photo-anode materials, mainly due to its ability to absorb photons up to 600 nm combined to a set of desirable properties such as high photocorrosion resistance, environmental friendliness, large abundance and relatively low production costs. However, hematite main disadvantages are a low electrical conductivity and a high rate of charge recombination; both these shortcomings drastically limit functionality and efficiency of hematite-based photo-anodes in PEC devices. One-dimensional (1D) nanostructuring is a powerful tool to tackle such disadvantages as it provides the photoelectrode material with increased surface area along with directional charge transport properties and short charge diffusion distances to the electrolyte – these features can improve the lifetime of photo-generated charges and/or enhance the charge transfer efficiency, and can consequently lead to a superior photo-electrochemical performance. At the same time, chemical/physical modification can also compensate natural weaknesses of hematite in water photoelectolysis. The present mini-review outlines a series of most effective strategies for the fabrication of 1D hematite nanostructures as well as for their physicochemical modification, mainly by doping or co-catalyst decoration, to achieve superior PEC activity.

Optical Shading Induces an In‐Plane Potential Gradient in a Semiartificial Photosynthetic System Bringing Photoelectric Synergy

AbstractSemiartificial photosynthetic systems have opened up new avenues for harvesting solar energy using natural photosynthetic materials in combination with synthetic components. This work reports a new, semiartificial system for solar energy conversion that synergistically combines photoreactions in a purple bacterial photosynthetic membrane with those in three types of transition metal–semiconductor Schottky junctions. A transparent film of a common transition metal interfaced with an n‐doped silicon semiconductor exhibits an in‐plane potential gradient when a light‐penetration variance is established on its surface by optical shading of photoabsorbing photosynthetic membranes. The in‐plane potential gradients (0.08–0.3 V) enable a directional charge transport between the synthetic and natural photoelectric systems, which is further enhanced in a device setting by a biocompatible thixotropic gel electrolyte that permeates the membrane multilayer, facilitating a strong and steady photoelectric current as high as 1.3 mA cm−2, the highest achieved so far with any anoxygenic photosynthetic system.

Directional Charge Transport in Layered Two-Dimensional Triazine-Based Graphitic Carbon Nitride.

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.

Directional Charge Transport in Layered Two‐Dimensional Triazine‐Based Graphitic Carbon Nitride

AbstractTriazine‐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.

TiO2 Based Nanostructures for Photocatalytic CO2 Conversion to Valuable Chemicals.

Photocatalytic conversion of CO2 to useful products is an alluring approach for acquiring the two-fold benefits of normalizing excess atmospheric CO2 levels and the production of solar chemicals/fuels. Therefore, photocatalytic materials are continuously being developed with enhanced performance in accordance with their respective domains. In recent years, nanostructured photocatalysts such as one dimensional (1-D), two dimensional (2-D) and three dimensional (3-D)/hierarchical have been a subject of great importance because of their explicit advantages over 0-D photocatalysts, including high surface areas, effective charge separation, directional charge transport, and light trapping/scattering effects. Furthermore, the strategy of doping (metals and non-metals), as well as coupling with a secondary material (noble metals, another semiconductor material, graphene, etc.), of nanostructured photocatalysts has resulted in an amplified photocatalytic performance. In the present review article, various titanium dioxide (TiO2)-based nanostructured photocatalysts are briefly overviewed with respect to their application in photocatalytic CO2 conversion to value-added chemicals. This review primarily focuses on the latest developments in TiO2-based nanostructures, specifically 1-D (TiO2 nanotubes, nanorods, nanowires, nanobelts etc.) and 2-D (TiO2 nanosheets, nanolayers), and the reaction conditions and analysis of key parameters and their role in the up-grading and augmentation of photocatalytic performance. Moreover, TiO2-based 3-D and/or hierarchical nanostructures for CO2 conversions are also briefly scrutinized, as they exhibit excellent performance based on the special nanostructure framework, and can be an exemplary photocatalyst architecture demonstrating an admirable performance in the near future.

Ultra-high-aspect-ratio vertically aligned 2D MoS2-1D TiO2 nanobelt heterostructured forests for enhanced photoelectrochemical performance

Vertically aligned one-dimensional (1D) materials offer numerous advantages for photoelectrochemical (PEC) energy conversion due to highly accessible surface area, direct charge transport pathway along the aligned direction, and suppressed electron-hole recombination. Hierarchical 1D-2D materials generate heterogeneous interfaces that can modify intrinsic chemical and physical material properties. Here, we report an ultra-high-aspect-ratio perfectly aligned 2D MoS2-1D TiO2 nanobelts (NBs) heterostructured nanoforest electrode and demonstrate enhanced PEC performance. The vertically aligned nanoforest electrode with an ultra-high aspect ratio of approximately 3000 was achieved using high-voltage electrophoretic deposition (EPD). The thickness of the vertical nanoforest correlated well with the length of the NBs and there was no evidence of vertical stacking. In addition, both semi-conductive 2H phase and metallic 1T phase MoS2 were discovered to form on TiO2 (011) and (001) surfaces, respectively. The nanoforest drastically enhanced light penetration into the interstice of the electrode and increased the probability of photon–semiconductor interaction due to reduced band gap as well as local surface plasmonic resonance (LSPR) evidenced by numerical studies. A remarkable 98% enhancement of the PEC photocurrent was achieved for the MoS2–TiO2 NB forest electrode compared with its un-aligned counterpart.

Quantum plasmonic hot-electron injection in lateral WSe2/MoSe2 heterostructures

Lateral two-dimensional (2D) transitional metal dichalcogenide (TMD) heterostructures have recently attracted a wide attention as promising materials for optoelectronic nanodevices. Due to the nanoscale width of lateral heterojunctions, the study of their optical properties is challenging and requires using subwavelength optical characterization techniques. We investigated the photoresponse of a lateral 2D WSe2/MoSe2 heterostructure using tip-enhanced photoluminescence (TEPL) with nanoscale spatial resolution and with picoscale tip-sample distance dependence. We demonstrate the observation of quantum plasmonic effects in 2D heterostructures on a non-metallic substrate, and we report the nano-optical measurements of the lateral 2D TMD heterojunction width of ~ 150 nm and the charge tunneling distance of ~ 20 pm. Controlling the plasmonic tip location allows for both nano-optical imaging and plasmon-induced hot electron injection into the heterostructure. By adjusting the tip-sample distance, we demonstrated the controllability of the hot-electron injection via the competition of two quantum plasmonic photoluminescence (PL) enhancement and quenching mechanisms. The directional charge transport in the depletion region leads to the increased hot electron injection, enhancing the MoSe2 PL signal. The properties of the directional hot-electron injection in the quantum plasmonic regime make the lateral 2D MoSe2/WSe2 heterostructures promising for quantum nanodevices with tunable photoresponse.

P-Type CuFeO2 nanoflakes prepared by sodium alginate-assisted hydrothermal method for photoelectrochemical water reduction

p-Type delafossite CuFeO2 is a promising material for photoelectrochemical water reduction due to its significant light absorption, chemical stability, and earth abundance. Herein, the CuFeO2 nanoflakes are synthesized by using a sodium alginate-assisted hydrothermal method in a strong alkali environment with NaOH as the mineralizer. The carboxyl and hydroxyl group of sodium alginate adsorbs on the exposed facets of CuFeO2 and promotes the generation of nanoflakes with small thickness. Moreover, the thin CuFeO2 nanoflakes provide a direct and short charge transport channel for photoinduced charge carriers with an improved photoelectrochemical water reduction performance.

Heterostructured TiO2/NiTiO3 Nanorod Arrays for Inorganic Sensitized Solar Cells with Significantly Enhanced Photovoltaic Performance and Stability.

Organic dyes used in the conventional dye-sensitized solar cells (DSSCs) suffer from poor light stability and high cost. In this work, we demonstrate a new inorganic sensitized solar cell based on ordered one-dimensional semiconductor nanorod arrays of TiO2/NiTiO3 (NTO) heterostructures prepared via a facile two-step hydrothermal approach. The semiconductor heterostructure arrays are highly desirable and promising for DSSCs because of their direct charge transport capability and slow charge recombination rate. The low-cost NTO inorganic semiconductor possesses an appropriate band gap that matches well with TiO2, which behaves like a "dye" to enable efficient light harvesting and fast electron-hole separation. The solar cells constructed by the ordered TiO2/NTO heterostructure photoanodes show a significantly improved power conversion efficiency, high fill factor, and more promising, outstanding life stability. The present work will open up an avenue to design heterostructured inorganics for high-performance solar cells.

Probing the density of trap states in the middle of the bandgap using ambipolar organic field-effect transistors

The number of trap states in the band gap of organic semiconductors directly influences the charge transport as well as the threshold and turn-on voltage. Direct charge transport measurements have been used until now to probe the trap states rather close to the transport level, whereas their number in the middle of the band gap has been elusive. In this study, we use PDIF-CN2, a well known n-type semiconductor, together with vanadium pentoxide electrodes to build ambipolar field-effect transistors. Employing three different methods, we study the density of trap states in the band gap of the semiconductor. These methods give consistent results, and no pool of defect states was found. Additionally, we show first evidence that the number of trap states close to the transport level is correlated with the number of traps in the middle of the band-gap, meaning that a high number of trap states close to the transport level also implies a high number of trap states in the middle of the band gap. This points to a common origin of the trap states over a wide energy range.

Molecular Rectification Enhancement Based On Conformational and Chemical Modifications

Design principles for molecules with intrinsic directional charge transport will likely prove crucial for breakthroughs in nanotechnology and other emerging fields like biosensors and advanced phot...

Hierarchical cobalt oxide-functionalized silicon carbide nanowire array for efficient and robust oxygen evolution electro-catalysis

A hierarchical cobalt oxide (Co3O4)-functionalized silicon carbide nanowire (SiC NW) array electrode is synthesized on 4H–SiC(0001) substrates and investigated for use in the oxygen evolution reaction (OER) of water splitting. Ni-catalyzed chemical vapor deposition is employed to obtain vertically-aligned SiC NWs, and electrodeposition is used to functionalize the nanowires with hierarchical Co3O4 nano-clusters and a thin Co3O4 layer on the top side and the side-walls of the NWs, respectively. By using the Co3O4-functionalized SiC NW electrode for OER, the overpotential required to drive an anodic current density (Janodic) of 10 mA cm−2 is 0.22 V, a value much lower than those of Co3O4 nano-clusters (0.42 V), of bare SiC NWs (1.27 V) electrodes and of Pt foil (0.56 V). The overpotential value of 0.22 V is among the most active for the noble metal-free electro-catalytic electrode to produce Janodic of 10 mA cm−2. The deposition of Co3O4 on the SiC NW substrate produces an extremely hydrophilic surface, which improves the charge transfer characteristics at the interface between the electrode and the aqueous electrolyte. Moreover, the vertically-aligned SiC NWs provide a directional charge transport route along the nanowire length, as seen from a lower charge transfer resistance (Rct) in the Co3O4-functionalized SiC NW electrode compared to the Co3O4-functionalized SiC electrode under the same OER operating conditions. Finally, the Co3O4-functionalized SiC NW electrode maintains stable performance in an alkaline electrolyte (i.e., 1 M KOH aqueous solution) during continuous electrolysis testing over 16 h. The outstanding electro-catalytic performance and stability make the Co3O4-functionalized SiC NW electrode a promising noble metal-free electro-catalytic electrode for the OER and other renewable energy applications.

Ordered Single-Crystalline Anatase TiO2 Nanorod Clusters Planted on Graphene for Fast Charge Transfer in Photoelectrochemical Solar Cells.

Achieving efficient charge transport is a great challenge in nanostructured TiO2 -electrode-based photoelectrochemical cells. Inspired by excellent directional charge transport and the well-known electroconductibility of 1D anatase TiO2 nanostructured materials and graphene, respectively, planting ordered, single-crystalline anatase TiO2 nanorod clusters on graphene sheets (rGO/ATRCs) via a facial one-pot solvothermal method is reported. The hierarchical rGO/ATRCs nanostructure can serve as an efficient light-harvesting electrode for dye-sensitized solar cells. In addition, the obtained high-crystallinity anatase TiO2 nanorods in rGO/ATRCs possess a lower density of trap states, thus facilitating diffusion-driven charge transport and suppressing electron recombination. Moreover, the novel architecture significantly enhances the trap-free charge diffusion coefficient, which contributes to superior electron mobility properties. By virtue of more efficient charge transport and higher energy conversion efficiency, the rGO/ATRCs developed in this work show significant advantages over conventional rGO-TiO2 nanoparticle counterparts in photoelectrochemical cells.

Constructing aligned single-crystalline TiO2 nanorod array photoelectrode for PbS quantum dot-sensitized solar cell with high fill factor

Low bandgap PbS quantum dots (QDs) have attracted special interests serving as promising sensitizers for developing high performance near-infrared responsive photovoltaics. However, the commonly studied successive ionic layer adsorption and reaction (SILAR) processed PbS QD-sensitized solar cells (QDSCs) constructed with mesoporous TiO2 nanoparticle-photoelectrodes generally experience poor fill factor (FF) values lower than 0.50, arising from severe interfacial charge recombination because of the structural disorder of nanoparticles. This work demonstrates the construction of aligned TiO2 nanorod arrays (NRAs), vertically grown on substrates, for SILAR processed PbS QDSCs with high FF. The microscopic, structural, and optical characterizations reveal a rutile phase of single-crystalline structure in nature for TiO2 NRAs. The as-designed solar cell affords the advantages of panchromatic absorption for adequate light harvesting, reasonable band structure for efficient electron injection, and one dimensional (1D) oriented architecture for radial directional charge transport. A solar cell based on a 1.9 μm TiO2 NRA photoelectrode yields a considerable power conversion efficiency of 1.15% under 1 sun, coupled with a significantly enhanced FF of 0.51 in contrast to a conventional TiO2 nanoparticle-based device (FF = 0.38). The high FF is attributed to the outstanding electron transport behavior from QD sensitizers to conducting substrates via a shortest pathway (i.e., through radial direction of NRAs), thereby suppressing charge recombinations in solar devices. Therefore, the architecture of aligned 1D TiO2 NRAs is of great prospect to construct high performance PbS QDSCs. The performance improvement is promisingly expected through fully understanding of the radial directional charge transport mechanism and further optimization of NRA photoelectrodes and QD assemblies.

One-Dimensional Electron Transport Layers for Perovskite Solar Cells.

The electron diffusion length (Ln) is smaller than the hole diffusion length (Lp) in many halide perovskite semiconductors meaning that the use of ordered one-dimensional (1D) structures such as nanowires (NWs) and nanotubes (NTs) as electron transport layers (ETLs) is a promising method of achieving high performance halide perovskite solar cells (HPSCs). ETLs consisting of oriented and aligned NWs and NTs offer the potential not merely for improved directional charge transport but also for the enhanced absorption of incoming light and thermodynamically efficient management of photogenerated carrier populations. The ordered architecture of NW/NT arrays affords superior infiltration of a deposited material making them ideal for use in HPSCs. Photoconversion efficiencies (PCEs) as high as 18% have been demonstrated for HPSCs using 1D ETLs. Despite the advantages of 1D ETLs, there are still challenges that need to be overcome to achieve even higher PCEs, such as better methods to eliminate or passivate surface traps, improved understanding of the hetero-interface and optimization of the morphology (i.e., length, diameter, and spacing of NWs/NTs). This review introduces the general considerations of ETLs for HPSCs, deposition techniques used, and the current research and challenges in the field of 1D ETLs for perovskite solar cells.

Photoelectrochemical Probing of Cellular Interfaces and Evaluation of Cellular H2S Production Based on In Situ‐Generated CdS‐Enhanced TiO2 Nanotube Heterostructures

AbstractJudiciously integrated nanostructured materials/cellular interfaces could be applied for various applications including physiological monitoring. Physiological H2S is the third most important endogenously generated gasotransmitter and is believed to be related with many diseases. This work reports the novel photoelectrochemical (PEC) probing of cellular interfaces and the evaluation of cellular H2S production based on in situ‐generated CdS‐enhanced TiO2 nanotube (NT) heterostructures. We chose the TiO2 NT array in particular, owing to its structure of regular oriented vertical NTs (which permits fast directional charge transport) and its substantial inner cavity (which guarantees the highly efficient guest loading and molecule capture). HepG2 cells directly grown on and interfacing with the TiO2 NTs are then used for the activation of the TiO2 NTs through the CdS generated in situ on the TiO2 NTs. Owing to the aforementioned unique properties of TiO2 NTs, even small amounts of in situ‐generated CdS could be detected and result in apparent electrical signals. To the best of our knowledge, such TiO2 NT‐based in situ PEC probing of cellular interfaces and evaluation of H2S cellular production have not previously been reported.

A novel material NiOOH directly grown on in-situ etched Cu(OH)2 nanowire with high performance of electrochemical energy storage

The 1D nanowire is prepared by in-situ etching current collector displaying faster electron transportation and better connection, plusing the directional charge transport properties, which could enable it become one of the most appropriate core of 3D core@shell structure. We apply novel nanowire-like Cu(OH)2, which is synthesized by etching copper foil, as core structure to be coated by novel NiOOH with high theoretical capacity. The fast electronic conductive core structure and promising faradic electrode material candidate material enable the Cu(OH)2@NiOOH/copper-foil electrode deliver high specific capacity of 1300Cg−1 at current density of 10mAcm−2 and outstanding cycling stability of capacity retention of 88% after 10000 charge-discharge cycles, indicating the as-prepared Cu(OH)2@NiOOH/copper-foil electrode has potential application in electrochemical capacitors.

Synthetic approaches towards structurally-defined electrochemically and (photo)redox-active polymer architectures.

Electrochemically active polymers are widely used in (opto)electronic applications, e.g. organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), solar cells or other light harvesting devices. In this regard, the progress in controlled polymerization techniques opens new possibilities to construct covalently linked polymer architectures, e.g. block copolymers or graft copolymers, from defined building blocks. These architectures represent promising candidates for application in (opto)electronic devices as they allow control of the morphology through self-assembly processes and, furthermore, are capable of mimicking fundamental processes for directional charge transport or light harvesting. This review details synthetic approaches to prepare functional redox-active and conjugated homopolymers as well as the construction of covalently linked well-defined architectures thereof, e.g. block copolymers, graft copolymers or chain end functionalized assemblies.

Bismuth Iodide Perovskite Materials for Solar Cell Applications: Electronic Structure, Optical Transitions, and Directional Charge Transport

Cesium and methylammonium bismuth iodides (Cs3Bi2I9 and MA3Bi2I9) are new low-toxic and air stable compounds in the perovskite solar cell family with promising characteristics. Here, the electronic structure and the nature of their optical transitions, dielectric constant, and charge carrier properties are assessed for photovoltaic applications with density functional theory (DFT) calculations and experiments. The calculated direct and indirect band gap values for Cs3Bi2I9 (2.17 and 2.0 eV) and MA3Bi2I9 (2.17 and 1.97 eV) are found to be in good agreement with the experimental optical band gaps (2.2, 2.0 eV and 2.4, 2.1 eV for Cs3Bi2I9 and MA3Bi2I9, respectively) estimated for solution-processed films. There is an error cancelation in the DFT calculated band gap similar to that for lead perovskites. However, fully relativistic DFT calculations indicate that the size of the spin orbit coupling (SOC) error cancelation for bismuth perovskite (0.5 eV) is less than for lead perovskite (1 eV), and other factors...

Templating C60 on MoS2 Nanosheets for 2D Hybrid van der Waals p–n Nanoheterojunctions

C60 and single-layer MoS2 nanocomposites were facilely prepared via a combined solvent transfer and surface deposition (STSD) method by templating C60 aggregates on 2D MoS2 nanosheets to construct hybrid van der Waals heterojunctions. The electronic property of the hybrid nanomaterials was investigated in a direct charge transport diode device configuration of ITO/C60–MoS2 nanocomposites/Al; rewritable nonvolatile resistive switching with low SET/RESET voltage (∼3 V), high ON/OFF resistance ratio (∼4 × 103), and superior electrical bistability (>104 s) of a flash memory behavior was observed. This particular electrical property of C60–MoS2 nanocomposites, not possessed by either C60 or MoS2 nanosheets, was supposed to be due to the efficiently established C60/MoS2 p–n nanojunction, which controls the electron tunneling via junction barriers modulated by electric-field-induced polarization. Thus, our 2D templating method through STSD is promising to massively allocate van der Waals p–n heterojunctions in 2...