Photocurrent Measurements Research Articles

NH4Cl and KCl effect on the structural, morphological, optical and electrochemical properties of Cu2O nanoparticles and Cu2O nanostructures, galvanostatically electrodeposited

In this work, Cu2O polyhedrons with two different shapes were synthesized, using the galvanostatic electrochemical deposited method. In copper nitrate (Cu(NO3)2) and Potassium chloride (KCl) bath, the Cu2O show highly symmetric 26-facet polyhedral microcrystals with well-defined {100}, {110}, {111} crystallographic faces. While, in the presence of additive (NH4Cl), Cu2O shows 18-polyhedral with well-developed {100} and {110} faces. The two morphologies revealed p-type semiconducting behavior. Cu2O 26-facet polyhedral show a high photoelectrochemical (PEC) stability of 73.5%. Such a feature is of great importance for future water splitting applications. Furthermore, the influence of the potassium chloride (KCl) upon their structural, morphological, conduction type, and optical properties, were investigated, studying the Cu2O nanostructures electrodeposited. Cu2O thin films were prepared from copper sulfate as a precursor (CuSO4), using different concentrations of potassium chloride (KCl). Structural, optical, and morphological properties of films, which were obtained by galvanostatic mode, were observed using X-ray diffraction, UV–visible spectrophotometer, and scanning electron microscopy, in addition to Mott-shotcky measurement, for an electrochemical characterization, and photocurrent measurement. . It was shown that the conduction type of Cu2O nanostructures does not change regardless of the KCl concentration in the electrolyte. However, chlorine ions can ameliorate the density of the donor carriers (30 times largest) and the morphologie can be totally changed when the concentration of Cl− ions attends 0.1 ​M from nanocubes to dendrites of polyhedral shape.

Enhanced Visible Light-Driven Photoelectrocatalytic Degradation of Paracetamol at a Ternary z-Scheme Heterojunction of Bi2WO6 with Carbon Nanoparticles and TiO2 Nanotube Arrays Electrode.

In this study, a ternary z-scheme heterojunction of Bi2WO6 with carbon nanoparticles and TiO2 nanotube arrays was used to remove paracetamol from water by photoelectrocatalysis. The materials and z-scheme electrode were characterised using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDS), EDS mapping, ultraviolet diffuse reflection spectroscopy (UV-DRS), photocurrent measurement, electrochemical impedance spectroscopy (EIS), uv-vis spectroscopy and total organic carbon measurement (TOC). The effect of parameters such as current density and pH were studied. At optimal conditions, the electrode was applied for photoelectrocatalytic degradation of paracetamol, which gave a degradation efficiency of 84% within 180 min. The total organic carbon removal percentage obtained when using this electrode was 72%. Scavenger studies revealed that the holes played a crucial role during the photoelectrocatalytic degradation of paracetamol. The electrode showed high stability and reusability therefore suggesting that the z-scheme Bi2WO6-CNP-TiO2 nanotube arrays electrode is an efficient photoanode for the degradation of pharmaceuticals in wastewater.

Multifunctional Catecholate-Modified 3D TiO2 Nanostructures for Highly Sensitive Photoelectrochemical IL-6 Immunosensors

Protein immunosensors are valuable analytical tools that allow for the detection of proteins with high sensitivity and specificity based on affinity-based interaction between antigen/antibody pairs.1 These devices are increasingly important in clinical settings due to their roles in detecting inflammatory processes, autoimmune disorders, cardiovascular disorders and cancers.1,2 Interleukin-6 (IL-6) is a protein biomarker that plays key roles in immunomodulation, hematopoiesis and inflammatory processes in the human body.3 Elevated levels of IL-6 can be used as evidence for diagnosis of various cancers and autoimmune disorders and even show the progression of COVID-19.3,4 In this work, we developed multifunctional three dimensional (3D) TiO2 nanostructured photoelectrodes as a sensing platform which combine photoelectrochemical (PEC) transduction with a signal-off direct immunoassay to detect IL-6. Surface modification with catecholate ligands was used to modify the properties of TiO2 nanostructures in a three-pronged approach of nanomorphology tuning, photocurrent signal enhancement, and facilitating bioconjugation.3D TiO2 nanostructures were synthesized via acid-hydrothermal process using titanium (IV) butoxide as a precursor. During the synthesis process 3,4-dihydroxybenzaldehyde (DHBA) was added in-situ into the reaction vessel. DHBA is part of the catecholate family of organic ligands which adsorb readily onto TiO2 surfaces. During the synthesis process, DHBA acted as a surfactant with preferential binding affinity for specific crystal facets of TiO2. The addition of DHBA resulted in anisotropic TiO2 crystal growth during synthesis, leading to globular nanorod cluster morphologies with high surface area-to-volume ratios. The 3D TiO2 nanostructures were further modified with extra DHBA after their synthesis. The 3D morphology combined with the extra DHBA resulted in higher UV/visible light absorption and photocurrent density measurements. The synthesized 3D TiO2 nanostructures were used to fabricate photoelectrodes for PEC signal-off immunoassay.The signal-off PEC IL-6 direct immunoassay was constructed by immobilizing a probe layer of anti-IL-6 on the surface 3D TiO2 photoelectrodes. When solutions containing target IL-6 encounter the photoelectrode, IL-6 is immobilized onto the photoelectrode surface through antibody/antigen complexation. Il-6 immobilization on the photoelectrode results in photocurrent signal decrease due to steric hinderance and this signal decrease can be directly correlated to the target concentration. The 3D nanostructure morphology contributed to increased sensitivity for the immunosensor as the higher electroactive surface area can accommodate a higher probe immobilization efficiency. Higher photocurrent generation also resulted in improved limit of detection (LOD) and dynamic range for the sensor. Functionalization by DHBA not only improves photocurrent generation, but also creates an avenue for biofunctionalization through Schiff-base linking between -COH groups of DHBA and -NH2 groups of anti-IL-6. The resultant immunosensor demonstrated excellent performance with an LOD of 3.6 pg mL-1 in plasma and 1.6 pg mL-1 in buffer solution, while having a log-linear dynamic range of 2-2000 pg mL-1. The 3D TiO2 photoelectrodes also demonstrate excellent short-term and long-term stability with little degradation after repeated use.It is evident from our investigations that combining PEC transduction mechanisms with direct signal-off immunoassays can result in high performance biosensors. In particular, the surface modification of 3D TiO2 nanostructure scaffolds with DHBA facilitates the fabrication of materials that are highly nanoporous and generate high photocurrent, resulting in biosensors with large dynamic ranges and low LOD.(1) Zhao, W. W.; Xu, J. J.; Chen, H. Y. Photoelectrochemical Immunoassays. Anal. Chem. 2018, 90 (1), 615–627. https://doi.org/10.1021/acs.analchem.7b04672.(2) Chikkaveeraiah, B. V.; Bhirde, A. A.; Morgan, N. Y.; Eden, H. S.; Chen, X. Electrochemical Immunosensors for Detection of Cancer Protein Biomarkers. ACS Nano 2012, 6 (8), 6546–6561. https://doi.org/10.1021/nn3023969.(3) Khan, M. A.; Mujahid, M. Recent Advances in Electrochemical and Optical Biosensors Designed for Detection of Interleukin 6. Sensors (Switzerland). MDPI AG February 1, 2020. https://doi.org/10.3390/s20030646.(4) Rubin, E. J.; Longo, D. L.; Baden, L. R. Interleukin-6 Receptor Inhibition in Covid-19 — Cooling the Inflammatory Soup. N. Engl. J. Med. 2021, 384 (16), 1564–1565. https://doi.org/10.1056/nejme2103108.

Photosynthetic Reaction Centres Assembled on a Gold Electrode and the Photocurrent - Potential Response

The photosynthetic reaction centre (RC) from Rhodobacter sphaeroides has been studied for use in biohybrid solar cells. Much of the previous work has focussed on improving photocurrent generation by loading the electrode surface with many copies of the protein resulting in multilayers. The primary disadvantage with this approach is the random orientation of proteins, with some supposedly oriented properly. We used RCs with Cys for covalent attachment to a gold electrode and for proper orientation. Areas of bare electrode surface and RCs bound non-specifically (i.e. not bound via the Cys) were competitively displaced by an insulating, non-redox layer of mercaptohexanol (MCH). The adsorbed monolayer of RCs was imaged using atomic force microscopy to detail the distribution of RCs on the gold surface for surfaces prepared with different RC deposition concentrations. Photocurrents were measured for all RC modified surfaces using a LED modulation method which enabled measurement of photocurrent in the presence of large faradaic currents from the sacrificial reactant (hydroquinone) at a variety of applied potentials.[1] The photocurrents generated from a monolayer composed of RCs and MCH resulted in consistent photocurrent currents. which enabled modeling of the photocurrent generation using the Marcus-Hush-Chidsey theory to extract a reorganization energy for this process. Multilayers of adsorbed RCs were distinctly different and revealed that the local environment in which the RCs are embedded significantly influenced photocurrent generation.[1] Jun, D.; Beatty, J. T.; Bizzotto, D. Highly Sensitive Method to Isolate Photocurrent Signals From Large Background Redox Currents on Protein‐Modified Electrodes. ChemElectroChem 2019, 6 (11), 2870–2875.

Photoelectrochemical CO2 Reduction to Formate over Hybrid System of CdS Photoanode and Formate Dehydrogenase Under Visible Light Irradiation

1. IntroductionPhotocatalytic and photoelectrochemical (PEC) CO2 reductions using semiconductor materials under solar light have attracted significant attention. The development of CO2-reduction systems operating under visible-light irradiation is essential for the practical use of solar energy because visible light takes up almost half of the solar radiation spectrum. Most metal sulfides, such as CdS, have suitable conduction band levels for CO2 reduction and narrow bandgaps, enabling visible-light absorption. However, most metal sulfides are unstable in aqueous solution under photoirradiation due to the occurrence of self-oxidative deactivation (photocorrosion) by photogenerated holes (e.g., CdS + 2h+ → Cd2+ + S). CO2 reduction in an aqueous solution competes with H2 evolution because the CO2 reduction potential is more negative compared to the water reduction potential. Therefore, the catalysts that can selectively reduce CO2 have been utilized. Enzymes are biocatalysts that only act on specific substrates. For example, formate dehydrogenase (FDH) can reduce CO2 to formate with 100% selectivity by accepting electrons from coenzyme NADH. We have recently reported PEC reduction of CO2 to formate using a sacrificial reagent-free system consisting of a TiO2 photoanode and FDH.1 However, this system cannot utilize visible light owing to the wide bandgap of TiO2. In this study, we attempted to fabricate stable CdS photoanode and construct a hybrid system consisting of the CdS photoanode, NADH, and FDH for the reduction of CO2 to formate under visible-light irradiation.2. ExperimentalThe CdS photoanode was fabricated by the chemical bath deposition (CBD) method according to a previous report.2 The CdS electrode was calcined in N2 flow at 200, 300, and 400 °C for 30 min. The electrochemical cell used for photocurrent measurements comprised a prepared electrode, a counter electrode (Pt wire), an Ag/AgCl reference electrode, and a borate buffer solution (pH 8) containing K4[Fe(CN)6]•3H2O. The electrodes were irradiated using a 300 W Xe lamp fitted with an L-42 cut-off filter. CO2 reduction was carried out using two-electrode system (counter electrode: carbon paper) in phosphate buffer solution (pH 7) under CO2 bubbling. Rh complex [Cp*Rh(bpy)(H2O)]2+,3 NAD+, and FDH was added in the counter side.3. Results and discussionThe stabilities of the photocurrents of the CdS electrodes were evaluated at a fixed potential (–0.5 V vs. Ag/AgCl) under continuous visible light irradiation. As shown in Fig. 1a, for CdS electrodes with and without calcination at 200 and 300 °C, the photocurrent densities gradually decreased with the start of light irradiation. After the reaction, large cubic particles with a particle size of approximately 100–300 nm, reflecting the crystal structure of K2Cd[Fe(CN)6], were observed on the electrodes. XRD analysis also confirmed the formation of K2Cd[Fe(CN)6]. As previously reported, K2Cd[Fe(CN)6] forms spontaneously on the CdS surface during photoirradiation by reaction of dissolved Cd2+ cations via photocorrosion and [Fe(CN)6]4− anions in the solution.4 Therefore, this gradual decrease in photocurrent density was mainly due to the photocorrosion of CdS. In contrast, calcination at 400 °C positively influenced the CdS electrode, gradually increasing the photocurrent density. The number of electrons passing through the outer circuit for over 1 h (19.7 C, corresponding to 205 μmol) exceeded the number of moles of CdS (approximately 22 μmol) in the electrode. Although large K2Cd[Fe(CN)6] particles were observed on the CdS electrode after the reaction, the amount was significantly lower than that observed on the other electrodes. Instead, fine particles of K2Cd[Fe(CN)6] (particle size ~10 nm) were densely formed on the CdS surface. Therefore, fine K2Cd[Fe(CN)6] particles generated on the surface effectively scavenged photogenerated holes in CdS and enabled the oxidation of [Fe(CN)6]4− to [Fe(CN)6]3−.4 Reduction of NAD+ to NADH using [Cp*Rh(bpy)(H2O)]2+ as an electron mediator was examined in two-electrode system consisting of the CdS photoanode (calcined at 400 °C) and a carbon paper counter electrode with no applied bias (0 V). Relatively stable photocurrent (0.2 mA cm–2) was observed for 60 min. UV-vis spectroscopy and liquid chromatography analyses revealed that the amount of 1,4-NADH, which functions as a coenzyme, was increased with progression of reaction by reduction of NAD+ over the carbon electrode accepting electrons from CdS photoanode. The PEC reduction of CO2 to formate was attempted by introducing FDH in the system. As shown in Fig. 1b, the amount of produced formate was increased with the increasing irradiation time and reached 1285 nmol, which exceeded that of FDH (64 nmol). Ishibashi, M. Higashi, S. Ikeda, Y. Amao, ChemCatChem, 2019, 11, 6227.Ikeda, T. et al, ChemSusChem, 2011, 4, 262.Ruppert, et al, Tetrahedron Lett., 1987, 28, 6583.Shirakawa, M. Higashi, O. Tomita, R. Abe, Sustain. Energy Fuels, 2017, 1, 1065. Figure 1

Graphene/SOI-based self-powered Schottky barrier photodiode array

We have fabricated a four-element graphene/silicon on insulator (SOI) based Schottky barrier photodiode array (PDA) and investigated its optoelectronic device performance. In our device design, monolayer graphene is utilized as a common electrode on a lithographically defined linear array of n-type Si channels on a SOI substrate. As revealed by wavelength resolved photocurrent spectroscopy measurements, each element in the PDA structure exhibited a maximum spectral responsivity of around 0.1 A/W under a self-powered operational mode. Time-dependent photocurrent spectroscopy measurements showed excellent photocurrent reversibility of the device with ∼1.36 and ∼1.27 μs rise time and fall time, respectively. Each element in the array displayed an average specific detectivity of around 1.3 × 1012 Jones and a substantially small noise equivalent power of ∼0.14 pW/Hz−1/2. The study presented here is expected to offer exciting opportunities in terms of high value-added graphene/Si based PDA device applications such as multi-wavelength light measurement, level metering, high-speed photometry, and position/motion detection.

Probing the defects states in MAPbI3 perovskite thin films through photoluminescence and photoluminescence excitation spectroscopy studies

The luminescence properties of methylammonium lead iodide (MAPbI3) perovskite thin films deposited on corning 1737 glass substrates are studied using photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy. From the PL spectra analysis, it is realized that there are two emission peaks, a high-intensity peak at 1.58 eV (~783 nm) and a smaller shoulder peak at 1.64 eV (~754 nm) in the bandgap energy region. The peak at ~1.64 eV corresponds to the bandgap of MAPbI3, whereas the high-intensity peak at photon energy ~1.58 eV is due to the presence of shallow trap states. The calculated optical bandgap energy 1.58 ± 0.01 eV is close to the reported bandgap energy 1.60–1.61 eV and consistent with the PL peak position. The slight difference in bandgap value is due to shallow defect states in MAPbI3 films. The slow decay of photocurrent observed in the transient photocurrent measurements further confirms the presence of shallow trap states in MAPbI3 perovskite.

Fabrication of novel neodymium oxide coupled mesoporous titania for effective visible light-induced photocatalyst for decomposition of Ciprofloxacin

A novel procedure for constructing mesoporous Nd2O3/TiO2 nanocomposites with different Nd2O3 (0.5–2%) was performed utilizing a soft template and impregnation method. Aqueous solutions of ciprofloxacin (CIP) as a pollutant antibiotics model were utilized to estimate the photocatalytic ability of the mesoporous Nd2O3/TiO2 nanocomposites under visible exposure. The Nd2O3/TiO2 nanocomposites displayed promoted photocatalytic ability for CIP degradation compared with TiO2 NPs, with a maximum degradation of 100% over 1.5%Nd2O3/TiO2 nanocomposite during 150 min illumination time, which is almost eight folds greater than TiO2 NPs. The apparent rate constant of 1.5% Nd2O3/TiO2 nanocomposite exhibited 11.8 times larger than TiO2 NPs. The promotion of photocatalytic ability over Nd2O3/TiO2 nanocomposites was ascribed to the combination between Nd2O3 and TiO2, leading to enhanced formation of the •OH and •O2− accountable for CIP decomposition. The introduction of Nd2O3 into TiO2 was promoted transportation and separation of charge carriers. The Nd2O3/TiO2 photocatalysts exhibited good corrosive resistance and mechanical stability after five consecutive runs of CIP degradation. The photocatalytic performances and mechanism over Nd2O3/TiO2 nanocomposites were verified utilizing photoluminescence and photocurrent density measurements. The prepared mesoporous Nd2O3/TiO2 nanocomposites are promising for environmental remediation.

Visualizing the Surface Photocurrent Distribution in Perovskite Photovoltaics

Defect states play an important role in the photovoltaic performance of metal halide perovskites. Particularly, the passivation of surface defects has made great contributions to high-performance perovskite photovoltaics. This highlights the importance of understanding the surface defects from a fundamental level by developing more accurate and operando characterization techniques. Herein, a strategy to enable the surface carriers and photocurrent distributions on perovskite films to be visualized in the horizontal direction is put forward. The visual image of photocurrent distribution is realized by combining the static local distribution of carriers provided by scanning near-field optical microscopy with the dynamic transporting of carriers achieved via a scanning photocurrent measurement system. Taking a surface passivated molecule as an example, a comprehensive defect scene including static and dynamic as well as local and entire conditions is obtained using this strategy. The comprehensive analysis of the trap states in perovskite films is pioneered vertically and horizontally, which will powerfully promote the deep understanding of defect mechanisms and carrier behavior for the goal of fabricating high-performance perovskite optoelectronic devices.

The effect of thermal annealing on defect states, light intensity and photocurrent of organic solar cells

Due to the low conversion efficiency and instability, organic solar cells (OSCs) still cannot be compared with conventional silicon-based solar cells that have been produced commercially. The low performance is attributed to the existence of defect states in solar cells. The defect states will prevent the carrier from transporting and then reduce the power conversion efficiency (PCE). In this work, the external quantum efficiency (EQE) measurement and current-voltage (I-V) measurement are used to characterize the performance of OSCs. The experimental results show that the power conversion efficiency of the cells increases from 0.56% to 3.52% after thermal annealing. The relationship between light intensity and photocurrent and the transient photocurrent (TPC) measurement is used to study the effect of annealing on the defect states of OSCs. The relationship between light intensity and photocurrent under different light intensities for cells before and after thermal annealing is compared. It is found that there is a linear relationship between light intensity and photocurrent of the cells after thermal annealing. Analysis of experimental results indicates that thermal annealing could reduce the density of defect states, effectively inhibits carrier recombination, promotes carrier transport, and ultimately improve the performance of cells.

Improvement of physical and electrochemical properties of Cu2O thin films with Fe ions doping towards optoelectronic applications

Pure and doped Cu2O thin films were deposited on FTO substrate with various Fe dopant concentrations employing low-cost electrodeposition technique. All thin films were successfully characterized employing XRD, SEM, PL spectroscopy, UV–vis spectrophotometer, and photocurrent measurement. XRD results indicate the formation of cubic structure of thin films where all grown grains were well oriented along (111) diffraction plane at different dopant concentrations. The crystal size of doped Cu2O thin films was calculated from the Scherrer equation were the values range from 24.95 to 31.75 nm. The creation of grains similar pyramid-shaped for grown thin films is proven by SEM results. PL results reveal the characteristic emission peaks of Cu2O thin films at 521 and 704 nm which are attributed to the inter-band transitions and Cu vacancies or O interstitial, respectively. Moreover, PL intensity changed with doping process due to the integration of Fe3+ ions in interstitial positions or/and Cu sites. The optical measurements indicate that thin films have optical absorption edge which located between 490 and 550 nm that attributed to Cu2O band gap. The estimated Eg for pure and doped Cu2O thin films was founded to be in the range from 2.1 to 2.59 eV. The photocurrent measurements signal that electrodeposited Cu2O films are p-type semiconductors. Furthermore, the doping process enhancement the photocurrent where more visible light could be more captured and therefore improvement of electron-hole production. The results show that Fe doped Cu2O thin films are talented candidate for the optoelectronic applications such as sensors and solar cells.

Microscopic Structures, Dynamics, and Spin Configuration of the Charge Carriers in Organic Photovoltaic Solar Cells Studied by Advanced Time-Resolved Spectroscopic Methods.

Organic photovoltaics (OPVs) are promising solutions for renewable energy and sustainable technologies and have attracted much attention in recent years. Two types of organic semiconductors are used as donor materials to fabricate OPV cells. One type is a photoconductive polymer, and the other type is a small-molecule-based compound. The discovery of a bulk-heterojunction (BHJ) structure using a mixture of p- and n-type organic semiconductors has dramatically increased the power conversion efficiency (PCE) of OPV cells. In this feature article, we review our recent studies on organic BHJ thin films and OPVs by using advanced time-resolved spectroscopic techniques. Two topics regarding the microscopic behaviors of the charge carriers are discussed. The first topic is focused on how to quantify the local mobility of the charge carriers. Here, we discuss charge carrier dynamics in diketopyrrolopyrrole-linked tetrabenzoporphyrin (DPP-BP) BHJ thin films studied by time-resolved terahertz spectroscopy on a subpicosecond to several tens of picoseconds time scale and by transient photocurrent measurements on a microsecond time scale. The second topic concerns the spin configuration and interaction of the electron and hole of the polaron pairs in polymer-based BHJ thin films and OPV cells studied by the time-resolved electron paramagnetic resonance method, time-resolved simultaneous optical and electrical detection, and measurement of the magnetoconductance effect.

On the Response Speed of Narrowband Organic Optical Upconversion Devices

AbstractOrganic upconversion devices (OUCs) consist of an organic infrared photodetector and an organic visible light‐emitting diode (OLED), connected in series. OUCs convert photons from the infrared to the visible and are of use in applications such as process control or imaging. Many applications require a fast OUC response speed, namely the ability to accurately detect in the visible a rapidly changing infrared signal. Here, high image‐contrast, narrowband OUCs are reported that convert near‐infrared (NIR) light at 980 and 976 nm with a full‐width at half maximum of 130 nm into visible light. Transient photocurrent measurements show that the response speed decreases when lowering the NIR light intensity. This is contrary to conventional organic photodetectors that show the opposite speed‐versus‐light trend. It is further found that the response speed increases (when using a phosphorescent OLED) or decreases (for a fluorescent OLED) when increasing the driving voltage. To understand these surprising results, an analysis by numerical simulation is conducted. Results show that the response speed behavior is primarily determined by the electron mobility in the OLED. It is proposed that the low electron drift velocity in the emitter layer sets a fundamental limit to the response speed of OUCs.

Conversion of CO2 to formate using activated carbon fiber-supported g-C3N4-NiCoWO4 photoanode in a microbial electrosynthesis system

Microbial electrosynthesis (MES) integrated with photocatalytic materials is a novel carbon dioxide (CO2) utilization technology with considerable potential to mitigate CO2 emission. The present study demonstrates for the first time an efficient formation of formate from CO2 in a photo-assisted MES system utilizing the activated carbon fiber (ACF)-supported g-C3N4-NiCoWO4 photoanode and g-C3N4 biocathode. The prepared NiCoWO4-g-C3N4/ACF formed a Z-scheme heterojunction resulting in the enhanced suppression of electron-hole pairs. Several characterization techniques confirm a successful incorporation of NiCoWO4 over g-C3N4/ACF. Atomic absorption spectroscopy revealed high chemical stability of photoanode without any metal leaching in the reaction medium. Cyclic voltammetry test showed an improved oxygen evolution reaction (OER) at the photoanode with a significantly low overpotential (∼0.12 V). Transient photocurrent measurements showed ∼1.05 times higher photocurrent for NiCoWO4-g-C3N4/ACF relative to g-C3N4/ACF upon illumination of light, demonstrating the formation of the Z-scheme heterojunction between NiCoWO4 and g-C3N4/ACF. The band alignment, scavenger analysis, and electron spin resonance spectroscopy results further confirmed the Z-scheme charge transfer mechanism. Approximately 12.8 mM of formate was synthesized per day in the MES system under visible light irradiation, which is approximately twice that in dark. The solar to formate conversion efficiency was determined to be 1.48%. The proposed mechanism indicates the photoinduced holes-initiated OER at the photoanode, and the release of formate dehydrogenase by Escherichia coli, isolated from a wastewater, at the biocathode. This study provides an efficient supported photoanode for harvesting solar light, combined with a biocathode in the MES cell for achieving a sustainable high rate of biochemicals formation.

Manipulating Picosecond Photoresponse in van der Waals Heterostructure Photodetectors

AbstractSelf‐powered ultrafast 2D photodetectors have demonstrated great potential in imaging, sensing, and communication. Understanding the intrinsic ultrafast charge carrier generation and separation processes is essential for achieving high‐performance devices. However, probing and manipulating the ultrafast photoresponse is limited either by the temporal resolution of the conventional methods or the required sophisticated device configurations. Here, van der Waals heterostructure photodetectors are constructed based on MoS2/WSe2 p–n and n–n junctions and manipulate the picosecond photoresponse by combining photovoltaic (PV) and photothermoelectric (PTE) effects. Taking time‐resolved photocurrent (TRPC) measurements, a TRPC peak at zero time delay is observed with decay time down to 4 ps in the n–n junction device, in contrast to the TRPC dip in the p–n junction and pure WSe2 devices, indicating an opposite current polarity between PV and PTE. More importantly, with an ultrafast photocurrent modulation, a transition from a TRPC peak to a TRPC dip is realized, and detailed carrier transport dynamics are analyzed. This study provides a deeper understanding of the ultrafast photocurrent generation mechanism in van der Waals heterostructures and offers a new perspective in instruction for designing more efficient self‐powered photodetectors.

Fabrication of a novel Z-scheme Bi2MoO6/GQDs/MoS2 hierarchical nanocomposite for the photo-oxidation of ofloxacin and photoreduction of Cr(VI) as aqueous pollutants

In this study, a novel MoS2/Bi2MoO6 composite photocatalyst, which was further sensitized using graphene quantum dots (GQDs), was developed to realize multiple photocatalytic applications in antibiotic remediation and Cr(VI) reduction. A simple chemical mixing procedure was used to prepare the GQD-MoS2/BMO composite photocatalyst, which was comprehensively characterized to confirm the structural purity, morphology, and distribution of GQDs on the MoS2/BMO catalyst. The enhanced charge separation and channeling of fast electron migration were confirmed by photocurrent and impedance measurements obtained for the 5 GQD-30 MoS2/BMO ternary catalyst system. The photoefficiency observed for the degradation of ofloxacin and reduction of Cr(VI) was 99.5% and 97.2% after 100 and 30 min, respectively, of photochemical reaction under visible-light irradiation. The reaction rate with 5 GQD-30 MoS2/BMO was 14.3- and 1.8-fold higher than that using pristine Bi2MoO6 and 30 MoS2/BMO photocatalysts, respectively. Persulfate (3 mM) improved the photo-oxidation of ofloxacin, with > 80% degradation occurring in the first 20 min. The excellent photocatalytic performance is attributed to the improved light absorption owing to the GQDs and the promotion of charge transfer between Bi2MoO6 and MoS2 via GQDs through a Z-scheme mechanism and the electron-attracting properties of the GQDs.

Insights into the charge carrier dynamics in perovskite/Si tandem solar cells using transient photocurrent spectroscopy

Direct bandgap perovskite and indirect bandgap Si, which form the two active layers in a tandem solar cell configuration, have different optoelectronic properties and thicknesses. The charge-carrier dynamics of the two-terminal perovskite-on-Si tandem solar cell in response to a supercontinuum light pulse is studied using transient photocurrent (TPC) measurements. Spectral dependence of TPC lifetime is observed and can be classified into two distinct timescales based on their respective carrier generation regions. The faster timescale (∼500 ns) corresponding to the spectral window (300–750 nm) represents the top-perovskite sub-cell, while the slower timescale regime of ∼25 μs corresponds to the bottom-Si sub-cell (>700 nm). Additionally, under light-bias conditions, the transient carrier dynamics of the perovskite sub-cell is observed to be coupled with that of the Si sub-cell. A sharp crossover from the fast-response to a slow-response of the device as a function of the light-bias intensity is observed. These results along with a model based on transfer matrix formulation highlight the role of charge-carrier dynamics in accessing higher efficiencies in tandem solar cells. The carrier transit times and lifetimes in addition to their optical properties need to be taken into account for optimizing the performance.

Enhanced Optical Response of Zinc-Doped Tin Disulfide Layered Crystals Grown with the Chemical Vapor Transport Method.

Tin disulfide (SnS2) is a promising semiconductor for use in nanoelectronics and optoelectronics. Doping plays an essential role in SnS2 applications, because it can increase the functionality of SnS2 by tuning its original properties. In this study, the effect of zinc (Zn) doping on the photoelectric characteristics of SnS2 crystals was explored. The chemical vapor transport method was adopted to grow pristine and Zn-doped SnS2 crystals. Scanning electron microscopy images indicated that the grown SnS2 crystals were layered materials. The ratio of the normalized photocurrent of the Zn-doped specimen to that of the pristine specimen increased with an increasing illumination frequency, reaching approximately five at 104 Hz. Time-resolved photocurrent measurements revealed that the Zn-doped specimen had shorter rise and fall times and a higher current amplitude than the pristine specimen. The photoresponsivity of the specimens increased with an increasing bias voltage or decreasing laser power. The Zn-doped SnS2 crystals had 7.18 and 3.44 times higher photoresponsivity, respectively, than the pristine crystals at a bias voltage of 20 V and a laser power of 4 × 10−8 W. The experimental results of this study indicate that Zn doping markedly enhances the optical response of SnS2 layered crystals.

Shining light on the underlying mechanisms in photomultiplication-type organic photodetectors

Measurements of the transient photocurrent, photovoltage, capacitance show photomultiplication is a slow process, aim to optimize device structure and performance

Highly Mobile Excitons in Single Crystal Methylammonium Lead Tribromide Perovskite Microribbons.

Excitons are often given negative connotation in solar energy harvesting in part due to their presumed short diffusion lengths. We investigate exciton transport in single-crystal methylammonium lead tribromide (MAPbBr3) microribbons via spectrally, spatially, and temporally resolved photocurrent and photoluminescence measurements. Distinct peaks in the photocurrent spectra unambiguously confirm exciton formation and allow for accurate extraction of the low temperature exciton binding energy (39 meV). Photocurrent decays within a few μm at room temperature, while a gate-tunable long-range photocurrent component appears at lower temperatures (about 100 μm below 140 K). Carrier lifetimes of 1.2 μs or shorter exclude the possibility of the long decay length arising from slow trapped-carrier hopping. Free carrier diffusion is also an unlikely source of the highly nonlocal photocurrent, due to their small fraction at low temperatures. We attribute the long-distance transport to high-mobility excitons, which may open up new opportunities for novel exciton-based photovoltaic applications.