Semiconductor-liquid Interface Research Articles

The Influence of Electrolytes on the Performance of Self-Powered Photoelectrochemical Photodetector Based on α-Ga2O3 Nanorods.

Photodetectors have a wide range of applications across various fields. Self-powered photodetectors that do not require external energy have garnered significant attention. The photoelectrochemical type of photodetector is a self-powered device that is both simple to fabricate and offers high performance. However, developing photoelectrochemical photodetectors with superior quality and performance remains a significant challenge. The electrolyte, which is a key component in these detectors, must maintain extensive contact with the semiconductor without degrading its material quality and efficiently catalyze the redox reactions of photogenerated electrons and holes, while also facilitating rapid charge carrier transport. In this study, α-Ga2O3 nanorod arrays were synthesized via a cost-effective hydrothermal method to achieve a self-powered solar-blind photodetector. The impacts of different electrolytes-Na2SO4, NaOH, and Na2CO3-on the photodetector was investigated. Ultimately, a self-powered photodetector with Na2SO4 as the electrolyte demonstrated a stable photoresponse, with the maximum responsivity of 0.2 mA/W at 262 nm with the light intensity of 3.0 mW/cm2, and it exhibited rise and decay times of 0.16 s and 0.10 s, respectively. The α-Ga2O3 nanorod arrays and Na2SO4 electrolyte provided a rapid pathway for the transport of photogenerated carriers and the built-in electric field at the semiconductor-liquid heterojunction interface, which was largely responsible for the effective separation of photogenerated electron-hole pairs that provided the outstanding performance of our photodetector.

Investigation of the photovoltaic effect and tribovoltaic effect at the GaAs semiconductor-liquid interface

Contact electrification (CE) is a physical phenomenon that occurs between two materials. When two materials with different electron affinity are contacted, electrons move along the contacting surfaces due to triboelectrification. The result of this effect is direct current. In this study, the triboelectric current is generated by the bindington energy generated by sliding a water drop on n-type, p-type and GaAs p-n junction. In addition, since the water drop is transparent, it is illuminated with 100 mW/cm2 while sliding and tribovoltaic and photovoltaic effects are evaluated together. The fact that the current value obtained was in the order of microampere (μA) showed that GaAs can be used in triboelectric nanogenerators (TENGs).

(Invited) Solar Flow Batteries: Integrated Photoelectrochemical Solar Energy Conversion and Redox Flow Battery Systems

Due to the intermittent nature of sunlight, practical solar energy utilization systems demand both efficient solar energy conversion and inexpensive large scale energy storage. We have developed hybrid solar-charged storage devices called solar flow batteries (SFBs) that integrate photoelectrochemical solar cells with redox flow batteries (RFBs). In these devices, photoexcited carriers collected at the semiconductor-liquid electrolyte interface convert the redox couples to charge up the RFB without external electric bias; which can be discharged to generate the electricity when needed. By matching various silicon solar cells and tandem III-V solar cells (Chem 2018, 4, 2644) with various organic redox couples and optimizing the SFB device design, we improved their round trip solar-to-output electricity efficiency (SOEE) and eventually achieved a record SOEE of 20% with a 500-hour lifetime using high performance tandem perovskite/silicon solar cells (Nature Mater. 2020, 19, 1326). The design principles and the quantitative analysis model for voltage matching solar cells with RFBs (Acc. Chem. Res. 2020, 53, 2611) light up the pathways for achieving even better performance and stability yet maintaining a low cost, which would make these SFBs practical for standalone solar energy conversion and storage systems in remote off-grid locations.

(Invited) In Situ spectroscopy of Electrochemical and Photoelectrochemical Interfaces

We explore various aspects of electrochemistry and photoelectrochemistry using in situ spectroscopy of electrode (metal) and photoelectrode (semiconductor) interfaces under electrochemical working conditions. Using sum frequency generation (SFG) spectroscopy, we measure the voltage dependence of the orientation of D2O molecules at a graphene electrode surface, which enables us to extract the free energy orienting potential of interfacial water.1 In particular, we measure the “free OD” feature in the spectra, which corresponds to the topmost water molecule that is rotated up out of the bulk water solution and is, therefore, not hydrogen bonded. Using transient absorption spectroscopy (TAS), we measure the lifetime of hot electrons photoexcited in plasmon resonant nanostructures.5 In a similar study, we use transient reflectance spectroscopy (TRS) to measure the photoexcited carrier dynamics in a GaP/TiO2 photoelectrode, as well as the electrostatic field dynamics at this semiconductor-liquid interface in situ under various electrochemical potentials.2 Here, the electrostatic fields at the surface of the semiconductor are measured via Franz−Keldysh oscillations (FKO). These spectra reveal that the nanoscale TiO2 protection layer enhances the built-in field and charge separation performance of GaP photoelectrodes. Using surface enhanced Raman scattering (SERS) spectroscopy, we monitor local electric fields via Stark-shifts of nitrile-functionalized silicon photoelectrodes.6 By monitoring Stark shifts in graphene-enhanced Raman spectroscopy (GERS), we measure local electric fields and local charge densities at monolayer graphene electrode surfaces.3 Lastly, we measure the stacking dependence of monolayer WSe2/MoSe2 heterostructures and observe resonant excitation of interlayer excitons for photocatalytic energy conversion.4 Montenegro, A., C. Dutta, M. Mammetkuliev, H.T. Shi, B.Y. Hou, D. Bhattacharyya, B.F. Zhao, S.B. Cronin and A.V. Benderskii, Asymmetric response of interfacial water to applied electric fields. Nature, 594, 62 (2021).Xu, Z.H., B.Y. Hou, F.Y. Zhao, Z. Cai, H.T. Shi, Y.W. Liu, C.L. Hill, D.G. Musaev, M. Mecklenburg, S.B. Cronin and T.Q. Lian, Nanoscale TiO2 Protection Layer Enhances the Built-In Field and Charge Separation Performance of GaP Photoelectrodes. Nano Letters, 21, 8017-8024 (2021).Shi, H.T., B.F. Zhao, J. Ma, M.J. Bronson, Z. Cai, J.H. Chen, Y. Wang, M. Cronin, L. Jensen and S.B. Cronin, Measuring Local Electric Fields and Local Charge Densities at Electrode Surfaces Using Graphene-Enhanced Raman Spectroscopy (GERS)-Based Stark-Shifts. ACS Applied Materials & Interfaces, 11, 36252-36258 (2019).Chen, J., C.S. Bailey, D. Cui, Y. Wang, B. Wang, H. Shi, Z. Cai, E. Pop, C. Zhou and S.B. Cronin, Stacking Independence and Resonant Interlayer Excitation of Monolayer WSe2/MoSe2 Heterostructures for Photocatalytic Energy Conversion. ACS Applied Nano Materials, DOI:10.1021/acsanm.9b01898 (2020).Yu Wang, Yi Wang, Indu Aravind, Zhi Cai, Lang Shen, Boxin Zhang, Bo Wang, Jihan Chen, Bofan Zhao, Haotian Shi, Jahan M. Dawlaty, and Stephen B. Cronin. In Situ Investigation of Ultrafast Dynamics of Hot Electron-Driven Photocatalysis in Plasmon-Resonant Grating Structures. Journal of the American Chemical Society. DOI: 10.1021/jacs.1c12069 (2022).Haotian Shi, Ryan T. Pekarek, Ran Chen, Boxin Zhang, Yu Wang, Indu Aravind, Zhi Cai, Lasse Jensen, Nathan R. Neale, and Stephen B. Cronin. Monitoring Local Electric Fields using Stark Shifts on Napthyl Nitrile-Functionalized Silicon Photoelectrodes. The Journal of Physical Chemistry C, 124, 17000-17005 (2020).

Electrochemical Study of Semiconductor Properties for Bismuth Silicate-Based Photocatalysts Obtained via Hydro-/Solvothermal Approach.

Three bismuth silicate-based photocatalysts (composites of Bi2SiO5 and Bi12SiO20) prepared via the hydro-/solvothermal approach were studied using electrochemical methods. The characteristic parameters of semiconductors, such as flat band potential, donor density, and mobility of their charge carriers, were obtained and compared with the materials’ photocatalytic activity. An attempt was made to study the effect of solution components on the semiconductor/liquid interface (SLI). In particular, the Mott–Schottky characterization was made in a common model electrolyte (Na2SO4) and with the addition of glycerol as a model organic compound for photocatalysis. Thus, a medium close to those in photocatalytic experiments was simulated, at least within the limits allowed by electrochemical measurements. Zeta-potential measurements and electrochemical impedance spectroscopy were used to reveal the processes taking place at the SLI. It was found that the medium in which measurements were carried out dramatically impacted the results. The flat band potential values (Efb) obtained via the Mott–Schottky technique were shown to differ significantly depending on the solution used in the experiment, which is explained by different processes taking place at the SLI. A strong influence of specific adsorption of commonly used sulfate ions and neutral molecules on the measured values of Efb was shown.

Interpretation and Use of Mott-Schottky Plots at the Semiconductor-liquid Interfaces

Interpretation and Use of Mott-Schottky Plots at the Semiconductor-liquid Interfaces

Impact of Bulk Trapping Phenomena on the Maximum Attainable Photovoltage of Semiconductor–Liquid Interfaces

In this work, we present a generic semiclassical approach for theoretically evaluating the impact of bulk trap states on the maximum attainable photovoltage in solar-assisted water-splitting reactions. Our method explicitly considers the charge contribution due to the capture and emission of conduction band electrons and valence band holes by trap states situated in the band gap, integrated within a self-consistent solution procedure for calculating electrostatic and charge transport properties. Utilizing the hematite photoanode as our model system, our approach reveals that bulk trap states may significantly degrade both the maximum attainable photovoltage and the concentration of mobile electrons (majority carriers) in the conduction band. These results suggest that bulk trap states can have two primary consequences in photoanodes. First, they may limit the degree to which a favorable cathodic shift in the photocurrent can be achieved (by lowering the maximum attainable photovoltage). Second, they may i...

Spectroscopic Study of Charge Transport at Organic Solid–Water Interface

Charge transport in an organic solid and its coupling with the neighboring aqueous biological environment dictates the performance of many organic bioelectronic devices. Understanding how the transport property at the solid–water interface is influenced by the surface structure characteristics of the organic solid is essential for rational design of organic bioelectronics and chemical sensors. However, in situ probing such structure–property relationships has been difficult due to lack of experimental techniques with sufficient sensitivity to the water-buried interface. Here, we demonstrate a charge accumulation spectroscopy (CAS)-based protocol, exploiting water-gated organic field-effect transistor as the testing platform, to investigate the structure-dependent localization of polaronic charge carriers at the organic semiconductor–liquid interface. Our results reveal that the degree of charge delocalization is reduced drastically when the charge carriers are moved from the bulk semiconductor to the semiconductor–water interface, suggesting the existence of a highly disordered surface layer in contact with water. It is also found that the charge delocalization could be further reduced by intercalation of chloride ions (from salt water) in the semiconductor surface layer. This study suggests that the spectroscopic signatures of polaronic charge carriers could be a sensitive probe to detect the structure-dependent charge localization at organic solid–liquid interfaces.

Single measurement detection of individual cell ionic oscillations using an n-type semiconductor \u2013 electrolyte interface

Pollen tubes are used as models in studies on the type of tip-growth in plants. They are an example of polarised and rapid growth because pollen tubes are able to quickly invade the flower pistil in order to accomplish fertilisation. How different ionic fluxes are perceived, processed or generated in the pollen tube is still not satisfactorily understood. In order to measure the H+, K+, Ca2+ and Cl− fluxes of a single pollen tube, we developed an Electrical Lab on a Photovoltaic-Chip (ELoPvC) on which the evolving cell was immersed in an electrolyte of a germination medium. Pollen from Hyacinthus orientalis L. was investigated ex vivo. We observed that the growing cell changed the (redox) potential in the medium in a periodic manner. This subtle measurement was feasible due to the effects that were taking place at the semiconductor-liquid interface. The experiment confirmed the existence of the ionic oscillations that accompany the periodic extension of pollen tubes, thereby providing – in a single run – the complete discrete frequency spectrum and phase relationships of the ion gradients and fluxes, while all of the metabolic and enzymatic functions of the cell life cycle were preserved. Furthermore, the global 1/fα characteristic of the power spectral density, which corresponds to the membrane channel noise, was found.

(Invited) Designing Efficient Photoelectrochemical Solar Energy Conversion Devices and Their Integration with Redox Flow Battery Devices

Due to the intermittent nature of sunlight, practical solar energy utilization systems demand both efficient solar energy conversion and inexpensive large scale energy storage. We will first discuss the rational design and demonstration of efficient photoelectrochemical hydrogen generation systems using efficient semiconductors and earth-abundant catalyst materials. We have further developed novel hybrid solar-charged storage devices that integrate regenerative photoelectrochemical solar cells and redox flow batteries (RFBs) that share the same pair of redox couples. In these integrated solar flow batteries (SFBs), solar energy is absorbed by semiconductor electrodes and photoexcited caries are collected at the semiconductor-liquid electrolyte interface and used to convert the redox couples in the RFB to fully charge up the battery. When electricity is needed, the charged up redox couples will be discharged on carbon electrodes to generate the electricity as in a RFB. We have demonstrated that such SFB devices can be charged under solar light without external electric bias and deliver a high discharge capacity comparable with state-of-the-art RFBs over many cycles. After developing silicon solar cells and high performance solar cells, carefully matching them with various organic or inorganic redox couples, and optimizing several generations of SFB device designs, we have recently achieved integrated SFB device with an overall direct solar-to-output electricity efficiency (SOEE) of 14%. These high performance SFBs can serve as distributed and standalone solar energy conversion and storage systems in remote locations and enable practical off-gird electrification.

Simultaneously Solving the Photovoltage and Photocurrent at Semiconductor–Liquid Interfaces

In this work, we present a general theoretical and numerical approach for simultaneously solving the photovoltage and photocurrent at semiconductor–liquid interfaces. Our methodology extends drift-diffusion methods developed for metal–semiconductor Schottky contacts in the device physics community into the domain of semiconductor–liquid “pseudo-Schottky” contacts. This model is applied to the study of photoelectrochemical anodes, utilized in the oxidative splitting of water. To capture both the photovoltage and photocurrent at semiconductor–liquid interfaces, we show that it is necessary to solve both the electron and hole current continuity equations simultaneously. The electron continuity equation is needed to primarily capture the photovoltage formation at photoanodes, whereas the hole continuity equation must be solved to obtain the photocurrent. Both continuity equations are solved through coupled recombination and generation terms. Moreover, to capture charge transfer at the semiconductor–liquid int...

3D Si/ITO/WO3 photoelectrode with a micropost array structure for photocatalysis enhancement

In this paper, a three-dimensional (3D) tandem Si/ITO/WO3 photoelectrode system with a high-aspect-ratio (HAR) micropost array structure based on a ‘Z-scheme’ is designed to enhance photocatalytic performance. To produce such a photoelectrode system, microposts were first fabricated on a Si substrate by deep reactive ion etching (DRIE), followed by deposition of an indium tin oxide (ITO) layer and tungsten trioxide (WO3) layer on microposts. The photocatalytic performance was verified by the degradation of methylene blue (MB, C16H18ClN3S) under visible light conditions. In the photocatalytic test, photoelectrode samples of a 0.5 cm2 footprint were used to degrade 15 ml MB (10 mg l−1). Experimental results indicate that the 3D micropost WO3/ITO/Si structure has a higher efficiency compared with the 2D planar WO3/ITO/Si structure. A decrease of 83.6% in MB concentration within 30 min is observed in the preliminary degradation test. The HAR photoelectrode configuration can dramatically increase the surface-to-volume ratio, not only increasing the total incident light absorption efficiency, but also facilitating the oxidation–reduction reaction in the semiconductor-liquid interface.

Integrated Photoelectrochemical Solar Energy Conversion and Redox Flow Battery Devices

The practical utilization of solar energy demands not only efficient energy conversion but also inexpensive large scale energy storage. Building on mature regenerative photoelectrochemical solar cells and emerging electrochemical redox flow batteries (RFBs), more efficient, scalable, compact and cost-effective hybrid energy conversion and storage devices could be realized. Here we present an integrated PEC solar energy conversion and electrochemical storage device by integrating regenerative photoelectrochemical solar cells in aqueous electrolytes with RFBs using the same pair of organic redox couples. In such an integrated PEC-RFB device, solar energy is absorbed by semiconductor electrodes and photoexcited caries are collected at the semiconductor-liquid electrolyte interface and used to convert the redox couples in the RFB to fully charge up the battery (i.e. store the solar energy into the redox couples). When electricity is needed, the charged up redox couples will be discharged on the surface of carbon felt electrodes as one would do in the discharge of a RFB to generate the electricity. We demonstrated that such an integrated PEC-RFB device can be charged under solar illumination without external electric bias and deliver a high discharge capacity comparable with state of the art RFBs over many cycles. This integrated device can utilize solar energy efficiently -- an overall direct solar-to-output electricity efficiency (SOEE) of 1.7% have been achieved without significant performance optimization. Figure 1

Correlation of Ti3+ states with photocatalytic enhancement in TiO2-passivated p-GaAs

Recently, we reported enhanced H2 evolution on TiO2-passivated GaAs. Based on density functional theory (DFT) calculations, this enhancement was attributed to Ti3+ states, which bind reactant species and increase charge transfer across the semiconductor–liquid interface. Here, we provide a quantitative correlation between Ti3+ density, as measured by X-ray photoemission spectroscopy (XPS) and photoluminescence (PL) spectroscopy, and photocatalytic performance, which substantiates the hypothesis put forth previously. In the photo-I–V characteristics reported here, passivating GaAs with TiO2 produces a shift in the onset potential of +0.35V at 1mA/cm2 and enhances the photocurrent by 32-fold over bare GaAs (at 0V vs. RHE), resulting in a peak photoconversion efficiency of 1.5% under AM1.5 G illumination. We find that just 1nm of TiO2 produces the best conditions for photocatalysis. XPS spectra show that thinner TiO2 films (1nm) have a higher density of Ti3+ states than thicker films (5nm), which have lower photocatalytic performance. PL spectroscopy provides further evidence for these Ti3+ surface states, which cause increased surface recombination. While it is apparent that the TiO2 films cause strong electron–hole recombination, the benefit that they provide by providing catalytically active sites outweighs their detriment associated with charge recombination. No enhancement is observed for TiO2 thicknesses above 10nm, which are crystalline and, therefore, considerably more insulating than thinner amorphous TiO2 films.

Effect of Oxygen Content on the Photoelectrochemical Activity of Crystallographically Preferred Oriented Porous Ta3N5 Nanotubes

Crystallographically preferred oriented porous Ta3N5 nanotubes (NTs) were synthesized by thermal nitridation of vertically oriented, thick-walled Ta2O5 NTs, strongly adhered to the substrate. The adherence on the substrate and the wall thickness of the Ta2O5 NTs were fine-tuned by anodization, thereby helping to preserve their tubular morphology for nitridation at higher temperatures. Samples were studied by scanning electron microscopy, high-resolution electron microscopy, X-ray diffraction, Rietveld refinements, ultraviolet–visible spectrophotometry, X-ray photoelectron spectroscopy, photoluminescence spectra, and electrochemical techniques. Oxygen content in the structure of porous Ta3N5 NTs strongly influenced their photoelectrochemical activity. Structural analyses revealed that the nitridation temperature has crystallographically controlled the preferential orientation along the (110) direction, reduced the oxygen content in the crystalline structure and the tubular matrix, and increased the grain size. The preferred oriented porous Ta3N5 NTs optimized by the nitridation temperature presented an enhanced photocurrent of 7.4 mA cm–2 at 1.23 V vs RHE under AM 1.5 (1 Sun) illumination. Hydrogen production was evaluated by gas chromatography, resulting in 32.8 μmol of H2 in 1 h from the pristine porous Ta3N5 NTs. Electrochemical impedance spectroscopy has shown an effect of nitridation temperature on the interfacial charge transport resistance at the semiconductor–liquid interface; however, the flat band of Ta3N5 NTs remained unchanged.

On the photocatalytic reduction of MTT tetrazolium salt on the surface of TiO2 nanoparticles: Formazan production kinetics and mechanism

HypothesisThe MTT [3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium-bromide] cell cytotoxicity indicator is photocatalytically reduced on the surface of TiO2 nanoparticles in phosphate-buffered-saline (PBS) environment. We hypothesize that specific phosphate adsorption may be used to modulate the efficiency of the TiO2–MTT reaction through colloidal and semiconductor-liquid interface processes. ExperimentsThe TiO2–MTT reaction kinetics was studied in PBS, with respect to photocatalyst and MTT concentrations and irradiation wavelength. The effects of PBS and electron scavengers (Fe3+ ions) on reaction efficiency and the role of colloidal surface charge in the photocatalytic process were investigated. The structural and spectroscopic characteristics of relevant TiO2-formazan systems were studied by X-ray diffraction, transmission electron microscopy and IR-spectroscopy. FindingsThe reaction was pseudo-first order with respect to photocatalyst and showed a negative and fractional partial order with respect to MTT. Formazan production rates were directly proportional to radiation wavelength and TiO2 concentration and inversely proportional to the MTT initial concentration. The addition of Fe3+ ions, as well as the absence of PBS, induced strong reaction inhibition. Reaction efficiency and catalyst Zeta potential were enhanced by Na2HPO4 (PBS component) and showed a maximum around the phosphate concentration 0.005M.Structural/spectroscopic characterization confirmed the formation of amorphous MTT-formazan on the surface of TiO2 and the TiO2–phosphate binding.

Electron Transfer at Oxide/Water Interfaces Induced by Ionizing Radiation

The electron transfer from oxide into water is studied in nanoparticle suspensions of various oxides (SiO2, ZnO, Al2O3, Nd2O3, Sm2O3, and Er2O3) by means of pulse and γ radiolysis. The time-resolved and steady-state investigations of the present study demonstrate independently that whatever the band gap and the electron affinity of the oxide, the electron transfer always takes place in these nanometric systems: Irradiation generates hot electrons which have enough energy to cross the semiconductor–liquid interface. Moreover, picosecond measurements evidence that the spectrum of the solvated electron is the same as in water. Lastly, the decay of the solvated electron is similar on the picosecond to nanosecond time scale in water and in these suspensions, but it is clearly different on the nanosecond to microsecond time scale.

Metal Oxide Photoelectrodes for Solar Fuel Production, Surface Traps, and Catalysis.

The photoelectrochemical reduction of water or CO2 is a promising route to sustainable solar fuels but hinges on the identification of a stable photoanode for water oxidation. Semiconductor oxides like Fe2O3 and BiVO4 have been gaining significant attention as promising materials. However, they exhibit a major drawback of a large required overpotential for solar water oxidation. In this Perspective, recent efforts to characterize and reduce the overpotential are critically examined. The accumulation of photogenerated holes at the semiconductor-liquid interface, recently observed with multiple techniques, is rationalized with surface state models. Transient absorption spectroscopy and electrochemical impedance spectroscopy suggest that surface treatments designed to either passivate surface traps or increase reaction rates (as catalysts) actually perform identically. This calls into question the definition of a catalyst when coupled to a semiconductor photoelectrode. In contrast, results from transient photocurrent spectroscopy suggest that two separate loss mechanisms are indeed occurring and can be addressed separately.

A Wave Packet Model for Electron Transfer and Its Implications for the Semiconductor−Liquid Interface

This paper establishes the computational feasibility and examines the implications of a particular technique for simulations of time dependent electron transfer (ET) at semiconductor−liquid interfaces (SLIs). The methodology uses a one electron formalism employing wave packets, pseudopotentials, and molecular dynamics, which we dub WPMD. We describe a detailed mechanism for SLI ET by using the methodology. The model is versatile enough to address conventional SLI ET, surface state and adsorption mediated ET, photoexcited ET, and ET between quantum dots and other microstructures. We contrast the perspectives of our WPMD model of SLI ET with those in traditional literature and find substantial differences. The use of standard Landau−Zener theory for SLI ET is found particularly problematic.

Theoretical Studies of Electron Transfer and Electron Localization at the Semiconductor−Liquid Interface

We present new models and simulations of electron transfer (ET) at the semiconductor−liquid interface (SLI). The simulations are of a “first principles” molecular dynamics type and therefore incorporate electronic structure calculations. An In(H2O)62+/3+ redox species, water, and InP semiconductor system are focused on. We discuss the problem of electron localization at this interface, especially as it relates to ET. The study allows the mechanism of the ET process to be analyzed. Rate constant calculations are performed with the dynamics of the entire system incorporated. We apparently present the first calculation of electron transfer coupling matrix elements for the SLI.