High Density Of Surface States Research Articles

Effective surface passivation of GaAs nanowire photodetectors by a thin ZnO capping.

The III-V nanowire (NW) structure is a good candidate for developing photodetectors. However, high-density surface states caused by the large surface-to-volume ratio severely limit their performance, which is difficult to solve in conventional ways. Here, a robust surface passivation method, using a thin layer of ZnO capping, is developed for promoting NW photodetector performance. 11 cycles of ZnO, grown on pure zinc blende high-quality GaAs NWs by atomic layer deposition, significantly alleviates the undesirable effect of the surface states, without noticeable degradation in NW morphology. An average 20-fold increase in micro-photoluminescence intensity is observed for passivated NWs, which leads to the development of detectors with high responsivity, specific detectivity, and optical gain of 9.46 × 105 A W-1, 3.93 × 1014 Jones, and 2.2 × 108 %, respectively, under low-intensity 532 nm illumination. Passivated NW detectors outperform their counterparts treated by conventional methods, so far as we know, which shows the potential and effectiveness of thin ZnO surface passivation on NW devices.

Enhanced ultrafast dynamics and saturable absorption response of Nb2SiTe4/graphene heterostructure for femtosecond mode-locked bulk lasers

Two-dimensional (2D) heterostructure provides a promising opportunity to create new hybrid materials with unique tailored properties for novel electronic and photonic devices. Here, 2D Nb2SiTe4/Graphene (NST/G) heterostructure samples with different thicknesses of Nb2SiTe4 are fabricated by mechanical exfoliation and dry transfer methods. Their photocarrier dynamics and nonlinear optical responses are systematically investigated by pump–probe and Z-scan measurements. The results illustrate that Graphene acts as an electron transfer channel effectively shortening the photocarrier lifetime in NST. Moreover, the thinner of NST, the larger of the photocarrier recombination rate in NST/G heterostructure because of the high surface state density and strong electron–phonon coupling effect. Significantly, the 2D NST/G heterostructure with thinner NST also exhibits a larger nonlinear absorption coefficient and moderate saturable absorption parameters, which endow the high-performance passively mode-locked femtosecond laser. With 2 nm NST/G heterostructure as saturable absorber (SA), passively mode-locked Yb:KYW bulk laser is realized with the pulse width of 415 fs and output power of 498 mW. Our results pave a versatile way for designing desirable photonic devices and applications.

Sliding Schottky diode triboelectric nanogenerators with current output of 109 A/m2 by molecular engineering of Si(211) surfaces

Triboelectric nanogenerators (TENGs) are an autonomous and sustainable power-generation technology, seeking to harvest small vibrations into electricity. Here, by achieving molecular control of oxide-free Si crystals and using conductive atomic force microscopy, we address key open questions and use this knowledge to demonstrate zero-applied-bias current densities as high as 109 A/m2. Key to achieve this output, is to use a proton-exchangeable organic monolayer that simultaneously introduces a sufficiently high density of surface states (assessed as changes to carrier recombination velocities) coupled to a strong surface dipole in the form of a surface alkoxide anion (Si–monolayer–O−). We also demonstrate that the DC output of a Schottky diode TENG does not track the energy released as friction. This removes the complexity of controlling an unavoidable stick–slip motion, bypassing the requirement of aligning sliding motion and substrate topographical features. We reveal that there is no apparent correlation between the current of a static (biased) junction and the tribocurrent of the same junction when under motion and unbiased.

Effective Passivation of InGaAs Nanowires for Telecommunication Wavelength Optoelectronics

AbstractCatalyst‐free InGaAs nanowires are promising building blocks for optoelectronic devices operating at telecommunication wavelengths. Despite progress, the applications of InGaAs nanowires remain limited due to their high density of surface states that degrade their optical properties. Here, InGaAs nanowires with superior optical properties are achieved by effectively suppressing their surface states with an InP passivation shell. Optimal InP shell growth conditions and thickness to maximize the minority carrier lifetime are identified. The photoluminescence intensity of these passivated InGaAs nanowires is up to three orders of magnitude higher than that of their bare counterparts. Moreover, a long minority carrier lifetime of up to ≈13 ns is measured with these passivated nanowires at room temperature. Optimal passivation of InGaAs nanowires with an emission wavelength of 1530 nm results in an ultra‐low surface recombination velocity of ≈280 cm s−1. In addition to the shell, the crystal structure of these nanowires plays an important role in the luminescence intensity. Combined cathodoluminescence mapping and high‐resolution transmission electron microscopy along the nanowires reveal significantly lower emission intensities in wurtzite predominant sections of the nanowires than zinc blende predominant ones.These insights on the optimal passivation of InGaAs provide directions for engineering high‐performance nanoscale‐devices in the telecommunication wavelength.

Mitigation of Edge and Surface States Effects in Two-Dimensional WS2 for Photocatalytic H2 Generation.

Large scale development of the 2D transition metal di-chalcogenides (TMDC) relies on landmark improvement in performance, which could emerge from nanostructuration. Using p-WS2 nanoflakes with different degrees of exfoliation and fracturing, perspectives were provided to develop high-surface-area 2D p-WS2 films for the photocatalytic hydrogen generation. The critical role of inter-nanoflakes contacts within high-surface-area 2D films was demonstrated, highlighting the benefit of plane/plane versus edge/plane contacts. Evidence of the high density of surface states displayed by these 2D films was provided through electrochemical measurements. In addition to operating as recombination centers, the surface states were shown to give rise to deleterious Fermi-level pinning (FLP), which dramatically decreased the efficiency of charge carrier separation. Lastly, promising strategies yielding FLP suppression via surface states modification were proposed. In particular, use of a multifunctional ultrathin film displaying healing, catalytic, and n-type semiconduction properties was shown to greatly enhance charge carrier separation and transport to the photo-electrode/electrolyte interface. When the 2D photoelectrodes were fabricated with the above prerequisites (i. e., a high proportion of plane/plane contacts and a successful surface states chemical modification), a photocurrent up to 4.5 mA cm-2 was achieved for the first time on 2D p-WS2 photocathodes for hydrogen generation.

(Sn,Ti,Nb)xO2 Solid Solution: An Innovative Nanostructured Material and Its Chemoresistive Properties

Introduction Recently, great attention has been paid to solid solutions and composite materials based on metal oxides in order to obtain nanostructures with enhanced sensing performance with respect to those of single-oxide counterparts [1]. Tin and titanium dioxides (SnO2 and TiO2) are wide-gap n-type semiconductors extensively investigated for the fabrication of solid-state devices for gas sensing applications. Albeit SnO2-based gas sensors exhibit high response to reducing gases, they suffer of poor selectivity and degradation of electrical properties at low oxygen partial pressure and high operating temperatures upon prolonged exposure to reducing gases [2]. Instead, TiO2 is more thermally stable than SnO2, but it has a lower sensitivity due to a higher density of surface states, that entails the pinning of the Fermi level [3]. SnO2 and TiO2 would easily form solid solutions because they can exhibit a rutile type structure where octahedrally coordinated Ti4+ and Sn4+ have similar ionic radii (i.e. 0.605 Å and 0.69 Å, respectively) [4,5].In an earlier investigation, the sensing properties of Sn1-x TixO2 (0.2<x<0.7) oxides prepared through sol-gel route from stoichiometric solutions of the two metal-alkoxides have been explored [6]. Sn1-xTixO2 solid solution showed better sensing performances than SnO2 and TiO2 under different target gases, with the Sn0.7Ti0.3O2 solid solution overperforming with respect to the other investigated compounds [6].Despite the Sn1-x Ti x O2 good sensing performances, a further improvement on the sensing aptitude of these materials has been attempted by means of a Nb doping. The incorporation of Nb5+ within the Sn0.7Ti0.3O2 lattice would increase the conductivity of the material, as niobium acts as donor dopant in n-type semiconductors and it can inhibit grain growth [7]. Scope and Methods The aim of this work was to study the sensing properties and the structural features of solid solutions based on Sn, Ti and Nb oxides. According to what reported in a previous investigation [6], different sample of (Sn,Ti,Nb)xO2 were synthesized keeping constant the Sn/Ti stoichiometric ratio (70/30) at different Nb doping of 2, 3.5 and 5% of the total metal amount, and hereafter labelled as STN 2, STN 3.5 and STN 5. As synthetized powders were heated at 650 and 850 °C for 2h (STN Nb% 650, STN Nb% 850) to investigate the possible influence of the temperature on grain size, Nb percolation and sensing properties. Observations at SEM microscopy revealed that the morphology of the samples consists of rounded nanoparticles. Samples heated at 650 °C are composed by grains with diameters from 10 to 30 nm, while those heated at 850 °C showed a more uniform grain size distribution (30-40 nm). Along with a small fraction of TiO2 nanocrystalline phase (about 2.5% on average), X-ray powder diffraction analyses confirm that the (Sn,Ti,Nb)xO2 with rutile-type structure is the main phase in all the investigated samples. On the base of the refined unit-cell parameters a 17% substitution of Ti4+ (i.r. 0.605Å) and Nb5+ (i.r.=0.64Å) for the bigger Sn4+ (i.r.=0.69Å) was estimated.The sensing performance of (Sn,Ti,Nb)xO2 sensors were investigated toward various gases (H2, CO, ethanol, NO2, CH4 and acetaldehyde) at their optimal working temperature of 450 °C. As shown in figure 1a, it was found that the STN 2 650 sample exhibit a response to hydrogen 6 to 8 times higher than STN 2 850 and SnO2 650, respectively. Moreover, STN 2 650 displays a better stability in wet conditions (Fig 1b). Despite the increase of relative humidity (RH%) affects the response to hydrogen, the dynamic response to H2 of the sample heated at 650 °C remains still high (Fig. 1c).Thanks to its outstanding characteristics, (Sn,Ti,Nb)xO2 solid solution is a promising candidate for hydrogen detection. The sensing properties towards other gases will be considered.

Controlling surface/interface states in GaN-based transistors: Surface model, insulated gate, and surface passivation

Gallium nitride (GaN) is one of the front-runner materials among the so-called wide bandgap semiconductors that can provide devices having high breakdown voltages and are capable of performing efficiently even at high temperatures. The wide bandgap, however, naturally leads to a high density of surface states on bare GaN-based devices or interface states along insulator/semiconductor interfaces distributed over a wide energy range. These electronic states can lead to instabilities and other problems when not appropriately managed. In this Tutorial, we intend to provide a pedagogical presentation of the models of electronic states, their effects on device performance, and the presently accepted approaches to minimize their effects such as surface passivation and insulated gate technologies. We also re-evaluate standard characterization methods and discuss their possible pitfalls and current limitations in probing electronic states located deep within the bandgap. We then introduce our own photo-assisted capacitance–voltage (C–V) technique, which is capable of identifying and examining near mid-gap interface states. Finally, we attempt to propose some directions to which some audience can venture for future development.

Influence of As+ Ion Implantation on Properties of MBE HgCdTe Near-Surface Layer Characterized by Metal–Insulator–Semiconductor Techniques

The effect of As+ ion implantation on the electrical properties of the near-surface layer of n-HgCdTe films grown by molecular beam epitaxy (MBE) on Si (310) substrates was experimentally studied. A specific feature of MBE n-Hg0.78Cd0.22Te films is the presence of near-surface graded-gap layers with a high CdTe content, formed during epitaxial growth. The properties of as-grown films and films after As+ ion implantation with ion energy of 200 keV and fluence of 1014 cm−2 were studied. Post-implantation activation annealing was not performed. Test metal–insulator–semiconductor (MIS) structures were created based on as-grown and as-implanted samples by plasma-enhanced atomic layer deposition of Al2O3 insulator films. The admittance of the fabricated MIS structures was measured over a wide range of frequencies and temperatures. When determining the parameters of MIS structures, we used techniques that take into account the presence of near-surface graded-gap layers and series resistance of the HgCdTe film bulk, as well as the high density of slow surface states. It was found that, in as-implanted samples, the donor center concentration in the near-surface layer exceeds 1017 cm−3 and increases with distance from the HgCdTe-Al2O3 interface (at least up to 90 nm). After implantation, the conductivity of MBE HgCdTe film bulk increases markedly. It was shown that, for as-implanted samples, the generation rate of minority charge carriers in the MBE HgCdTe surface layer is significantly reduced, which indicates the appearance of a low defect layer with a thickness of at least 90 nm.

Coupling effects of indium oxide layer on hematite enabling efficient photoelectrochemical water splitting

Hematite (α-Fe2O3) based nanomaterials are considered as the most promising photoelectrodes for solar water splitting. However, some inherent shortcomings like high density of surface states is still the critically challengeable issue. This paper reports the integration of In2O3 nanolayers (NLs) on the surface of worm-like Fe2O3 films to significantly improve the photoelectrochemical (PEC) water oxidation activity. The In2O3 NLs on the modified α-Fe2O3 photoelectrodes exhibit dual positive effects on PEC performance: i) acting as the passivation material to decrease surface electron-hole recombination losses; ii) constructing heterojunctions with Fe2O3 to facilitate efficient separation of surface charges and spatial transfer with extended lifetimes. The resulting treated Fe2O3 photoanode yields a current density of 3.4 mA cm―2 at 1.23 V vs. reversible hydrogen electrode and high incident photon-to-current efficiency of 80 % at 400 nm. These results may bring inspiration to introduce multifunctional modifiers on photoelectrodes that offer practically high-performance PEC applications.

Super-Planckian near-field heat transfer between hyperbolic metamaterials

Heat transfer between closely-spaced objects may be greatly enhanced with efficiency far beyond than the classic blackbody radiation limit under the participation of evanescent waves. Metamaterials with high density of optical surface states have been particularly explored to improve the heat flux upper limit but mostly confined in theoretical discussions due to the experimental challenges. In the near-field context, the effective medium theory (EMT) as widely used in the discussions is also lack of practical inspections. In this work, we managed to fabricate two 2 × 2 cm2 sized high-quality infrared hyperbolic metamaterials made of silicon nanorod array and studied their near-field thermophotonic interaction using home-made setup. At the 500 nm vacuum gap, a strong heat flux density of 830 W/m2 is observed, which is 4.7 times larger than the blackbody value. A local uniaxial model with parameters retrieved from the full-wave simulation was built to reproduce the measured results, which unambiguously validated the widely-adopted semi-empirical condition on the relationship between vacuum gap and lattice constant for the application of the EMT in the near-field regime. These results are believed essential for the future development of metamaterials for novel thermophotonic devices.

Physical Insights into Band Bending in Pristine and Co-Pi-Modified BiVO4 Photoanodes with Dramatically Enhanced Solar Water Splitting Efficiency.

Herein, a novel method is introduced to synthesize 3D hierarchically assembled BiVO4 nanosheet photoanodes. Despite the fact that the obtained photoanodes inherit the intrinsic properties of 2D and 3D structures, they generate low photocurrent under simulated solar light at 1.0 sun. Upon modification with the cobalt-phosphate (Co-Pi) cocatalyst, the photocurrent is dramatically enhanced from 0.41 to 3.32 mA cm-2 at 1.23 VRHE. Charge-transfer kinetic studies by intensity-modulated photocurrent spectroscopy indicated that the low photocurrent response is mainly due to the high density of surface states, which pin the Fermi level and suspend the band bending. The Co-Pi loading passivates these surface states, unpins the Fermi level, and thus resumes the band bending. It also greatly enhances the rate constant of charge transfer and the overall efficiency, evincing that Co-Pi exhibits a dual function (i.e., passivation and catalysis). The current results explicitly disclose the role of the Co-Pi cocatalyst in photoelectrochemical solar water splitting on BiVO4.

Interface‐Induced High Responsivity in Hybrid Graphene/GaAs Photodetector

AbstractPhotodetectors based on two‐dimensional (2D)/ three‐dimensional (3D) semiconductor heterojunction structures are emerging as appealing candidates for high‐sensitivity applications. The performances of these hybrid photodetectors are closely correlated with their current gain mechanism. Carrier recirculation is the most commonly reported mechanism. Recently, a Fermi level alignment mechanism was proposed for 2D graphene/0‐dimensional (0D) quantum dot heterostructures because of the easy Fermi level tunability of the quantum dot. In this article, an interface‐induced gain mechanism using this Fermi level alignment process is proposed and identified based on a 2D graphene/3D GaAs hybrid structure with comparative measurement configurations. Because of the high surface state density of GaAs, the photo‐excited holes tend to become trapped at the graphene/GaAs interface, which can easily lower the interface Fermi level and the Fermi level in graphene via an alignment process. When combined with the high carrier mobility characteristics of graphene, a maximum current gain of 2520 and responsivity of 1321 A W−1 are achieved in the devices. This study clarifies the role of the interface states in the gain characteristics of some 2D/3D hybrid devices, with results that are instructive for optimal device design.

Schotky Barrier Height and Calculation of Voltage–Current Characteristics of Al/n-(SiC)1–x(AlN)x Diodes And 4H–SiC Heterojunctions

The Schottky barrier heights in the М/n-(SiC)1–x(AlN)x systems are obtained on the assumption of a high density of surface states in the region of the metal (M) – SiC–AlN-solid solution contact. Current–voltage (I–V) characteristics of the Al/n-(SiC)1–x(AlN)x diodes are calculated. It is shown that at moderate concentrations of surface states (c ≈ 4–8), the Schottky barrier height ФxB(с) of these diodes is close to the heterojunction potential barrier Фxg, which is the reason for the known similarity in the behavior of the corresponding I–V characteristics. The role of the ideality factors in the behavior of the I–V characteristics is analyzed. The obtained values of the Schottky barrier heights are in accordance with experimental data.

Surface passivation of InGaN nanorods using H3PO4 treatment for enhanced photoelectrochemical performance

Due to the high density of surface states introduced in the nanorods growth process and subsequent exposure to air, electrons will be trapped in these states and recombine with photo-generated holes, which leads to a high loss of photo-generated carriers and creates a large onset potential for carrier transport. Here, we report a simple and low-cost surface treatment method, which utilizes H3PO4 to passivate surface states in the as-grown InGaN nanorods. Using optimized surface passivation condition, surface trapping states in as-grown InGaN nanorods can be removed by eliminating surface Indium aggregation, reducing the surface oxides, and saturating the dangling bonds. Hence, carrier recombination mediated by these surface states is weakened, as well as applied bias potential required for carrier transport is lowered. The optimized H3PO4-treated nanorods photoanode shows a larger photovoltage and a smaller charge transfer resistance to the electrolyte. Resultantly, a significant negative shift of onset potential from 0.7 to 0.3 V vs. RHE for the treated photoanode is achieved. Moreover, the optimized H3PO4-treated photoanode exhibits a high photocurrent density of 18 mA/cm2 at 1.23 V vs. RHE and a maximum applied bias photon-to-current efficiency value of 1.09%, whereas as-grown InGaN photoanode reaches only 0.64%.

Water reduction into hydrogen using Rh-doped SrTiO3 photoelectrodes surface-modified by minute amounts of Pt: Insights from heterogeneous kinetic analysis

In this paper, we report on the photoelectrochemical hydrogen evolution reaction (HER) from water using Rhodium-doped strontium titanate, surface-modified by the addition of minute amounts of platinum nanoparticles. Platinum has been used in an attempt to increase the charge transfer efficiency of the photoelectrode but also to investigate the balancing effect of surface co-catalysis on charge transfer versus recombination processes. The dynamics of the HER in pH-neutral aqueous solutions has been investigated by Photoelectrochemical Impedance Spectroscopy. Experimental impedance data has been analyzed using a model derived from considerations on heterogeneous kinetics. From the best experimental vs. calculation PEIS fits, microscopic rate parameters have been determined over an extended range of electrical potential. The charge transfer (kt) and recombination (kr) rate constants have been found to vary exponentially with the applied potential. The analysis of the potential dependence of kt led to a Butler-Volmer transfer coefficient of 0.21. The analysis of the potential dependence of the recombination rate constant kr led to the determination of a non-ideality factor β = 0.04 (β = 1 for ideal interfaces), demonstrating the strong non-ideal behavior of the interface. Such non-ideal behavior has been attributed to the presence of a high density of surface states. The space charge capacitance under inversion conditions, i.e. when a strong reversed bias is applied and when the surface of this p-semiconductor exhibits a n-type conductivity, has also been measured and analyzed.

Behavioral Features of MIS Memristors with a Si3N4 Nanolayer Fabricated on a Conductive Si Substrate

The effect of the material of the metal plates (Au, Ta, W) and exposure to a high-power blue laser on the memristive characteristics of metal–insulator–semiconductor (MIS) capacitors with a Si3N4 film 6 nm thick fabricated on the basis of n+-Si is studied. It is shown that bipolar switching by the current appears only in capacitors with Au. The causes of the absence of bipolar switching in capacitors with Ta and W are explained. The switching of capacitors with Ta by the current and light and the photomemory effect are detected. It is shown that, despite the high doping level of the semiconductor substrate, it decreases the MIS memristor response rate due to a high density of surface states localized at the Si3N4/n+-Si interface. However, illumination allows a significant increase in the response rate due to a decrease in the semiconductor resistivity. The surface state densities are determined. To improve the frequency characteristics of MIS memristors, it is necessary to achieve a low surface state density.

Optically modulated electric synapses realized with memristors based on ZnO nanorods

Artificial synapses based on compact nanodevices are indispensable building blocks for implementing artificial neural networks in hardware. In this work, we demonstrate optically modulated artificial synapses realized with ZnO nanorods. These artificial synapses advance preceding memristive synapses with regard to the ability of response to not only electric but also optical modulation. The high surface state densities of the ZnO nanorods endow the present device with high photoresponse, resulting in the optical shielding effect, while the subsequent deshielding can be realized by applying electric pulses. The optical shielding and electrical deshielding functions provide abilities to process massive data for artificial neural networks. Moreover, with the aid of the optical modulation, methods are opened up for future optically tuned synaptic devices for secured communications.

Effect of Post Nitridation Annealing on Thermally Grown Silicon Oxynitride Characteristics

The ability to block the direct current is one of the main requisite for the gate dielectric in a metal-oxide-field-effect-transistor. Since thickness of oxide is inversely related with oxide capacitance, therefore oxide capacitance can be increased as an alternative to oxide thickness to meet the scaling requirements. Oxynitrides and high-k materials as gate dielectric have been considered as promising candidates to overcome the above problems. At greater physical thickness, these materials have the similar capacitance value which avoids tunneling effect. SiON has been used as a substitute for SiO2 because of its high dielectric permittivity and low density of surface states. In this paper, the effect of post nitridation annealing on thermally grown silicon oxynitride characteristics have been investigated by analyzing high-frequency capacitance-voltage (C-V) and current- voltage characteristics (I-V) of n-Si/SiON/Ti-Pt MOS capacitor. Post nitridation annealing was carried out at 800°C for 15 min in Nitrogen (N2) ambient. Ellipsometery, FESEM, Atomic force microscopy (AFM) and Fourier transform infrared spectroscopy (FTIR) was used to study the effect of post nitridation annealing on the physical characteristics of deposited silicon oxynitride film. Cheung’s approach has been used for evaluating barrier height (ϕbo ) and ideality factor (η). We present a comparative study between the as-deposited and annealed film and it was found that the physical and electrical properties (k, η, ϕbo etc) improved by post nitridation annealing is attributed to the improvement in the interface properties of Si/SiON. Moreover, the value of positive fixed charge density (Q f ~1.8×1012 cm−2) is effective for field-effect passivation of n-type Si surfaces. For both as-deposited and annealed SiON film, the maximum current through the Si/SiON/Ti-Pt device never exceeded a value of 10–8A/cm2 which indicates that the film were very good insulators. These results indicate that the obtained SiON film is an appropriate gate insulator for advanced nano electronic devices.

Hematite photoanodes with size-controlled nanoparticles for enhanced photoelectrochemical water oxidation

The synthesis of hematite photoanodes with size-controlled nanoparticles is challenging due to the difficulty in finding the appropriate morphology-directing agents. In this paper, Ba2+ and Sr2+ ions were successfully employed to synthesize hematite photoanodes with size-controlled nanoparticles using a facile chemical bath deposition method. The synthesized electrodes were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), inductively coupled plasma-optical emission spectrometry (ICP-OES), (photo)electrochemical impedance spectroscopy ((P)EIS), X-ray photoelectron spectroscopy (XPS) and intensity modulated photocurrent spectroscopy (IMPS) measurements. It was found that the average diameter of hematite nanoparticles is decreased by a factor of 13% and 35% by the addition of Ba2+ and Sr2+ ions into the chemical bath, respectively. In agreement with these results, the electroactive surface area of Ba- and Sr-modified hematite electrodes increased by 2.4 and 3.2 times, respectively, in comparison with that of bare hematite. The photoelectrochemical measurements under the standard illumination conditions revealed that the generated photocurrent at 1.23 V vs. RHE on Ba- and Sr-modified hematite photoanodes is 2.6 and 3 times higher than that of bare hematite, respectively. Moreover, the photocurrent onset potential for water oxidation on Ba- and Sr-modified hematite photoanodes was shifted cathodically by about 150 and 220 mV, respectively. Based on these results, a good correlation between the photocurrent and the electroactive surface area was observed at high bias potential (i.e., 1.23 V vs. RHE) evincing that the photocurrent enhancement can be readily attributed to the enhanced electroactive surface area. The cathodic shift in the photocurrent onset potential of water oxidation was elucidated, however, by the higher density of surface states and the higher rate constant of charge transfer as proven by the PEIS and IMPS measurements, respectively.

Stable photoelectrochemical salt-water splitting using the n-ZnSe/n-Ag 8 SnS 6 photoanodes with the nanoscale surface state capacitances

Abstract Interest in the photoelectrochemical water splitting using the metal sulphide photoelectrodes increases due to their good photoelectrochemical performances in aqueous solutions under illumination. However, the accumulation of photo-excited holes at the surfaces of samples results in their poor stabilities for photoelectrochemical water splitting during light illumination. We report the systemic investigations of the physical properties, photoelectrochemical performances and the influence of surface states for the silver-tin-sulphides (Ag8SnS6) photoelectrode in order to find out the reason about its poor stability for photoelectrochemical salt-water splitting. From the electrical impedance spectra of samples in the salt-water solution with various applied voltages, the Fermi-level pinning effect occurs in the Ag8SnS6 photoelectrode due to its high density of surface states. The accumulation of holes in the surface states results in the photocorrosion of the photoanode rather than the photoelectrochemical salt-water splitting. With the ZnSe/Ag8SnS6 heterogeneous junction as the photoelectrode, these surface states at the Ag8SnS6 sample act as the capacitances that can extract photo-excited electrons from the ZnSe layer and holes from the Ag8SnS6 sample. They make the increase in the charge separation rate for photo-excited carriers in the samples, which enhances its photoelectrochemical performance and stability in the salt-water solution.