Turn-on Potential Research Articles

Annealing modulated microstructural and electrical properties of plasma-enhanced atomic layer deposition-derived HfO2/SiO2 nanolaminates on AlGaN/GaN

In current work, HfO2/SiO2 nanolaminates and HfO2 films were grown on AlGaN/GaN substrates via plasma-enhanced atomic layer deposition. A comparative study of how rapid thermal annealing modulates the microstructural and electrical properties of both films has been presented. It is found that the HfO2/SiO2 nanolaminate keeps an amorphous structure when thermally treated below 600 °C, whereas crystal grains appear within the 800 °C annealed sample. High-temperature annealing facilitates the transformation from Hf–O and Si–O to Hf–O–Si in the HfO2/SiO2 nanolaminates, forming an HfSiO4 composite structure simultaneously. The 800 °C annealed HfO2/SiO2 shows a low k value and large leakage current density. While the 600 °C annealed HfO2/SiO2 possesses an effective dielectric constant of 18.3, a turn-on potential of 9.0 V, as well as a leakage density of 10−2 μA/cm2 at gate biases of both −10 and 2 V, revealing good potential in fabricating high electron mobility transistors.

Fe Substitutions Improve Spectral Response of Bi2WO6-Based Photoanodes

The quest for a scalable solar fuel technology has stimulated a concerted effort to develop a metal oxide semiconductor for solar-driven photoelectrochemical water oxidation (oxygen evolution reaction) in an efficient and durable manner. So far, the search for such a metal oxide photoanode has highlighted the promise of Bi-based oxides, which have also been extensively studied for other photocatalyst applications. Bi2WO6 is a durable photocatalyst whose primary shortcoming is a 2.8 eV band gap that limits utilization of the solar spectrum. Improvements in visible photoresponse upon incorporation of Fe have been reported in the photocatalysis literature, motivating our use of high-throughput synthesis and photoelectrochemistry to determine the spectral photoresponse for Bi–W–Fe oxides synthesized under nonequilibrium conditions based on thermal oxidation of metallic films. Photoactivity down to 2 eV was achieved over a broad range of compositions, with detailed characterization of optimal compositions revealing that Fe incorporation increases the valence band position by 0.75 eV. Density functional theory calculations of Fe substitutions on W sites in Bi2WO6 are consistent with this valence band shift, providing a plausible explanation for the experiments. This Fe-mediated band tuning yields a ca. 2 eV band gap while retaining a turn-on potential for photoanodic current near 0.4 V versus RHE, which combined with the operational durability motivates continuous study and development of this promising class of metal oxide photoanodes.

Dual co-catalysts activated hematite nanorods with low turn-on potential and enhanced charge collection for efficient solar water oxidation

Hematite (α-Fe2O3) photoanode suffers from significant photocarrier recombination and sluggish water oxidation kinetics for photoelectrochemical water splitting. To address these challenges, this work demonstrates the construction of dual co-catalysts modified Fe2O3 nanorods photoanode by strategically incorporating CoPi and Co(OH) x for photoelectrochemical water oxidation. The Fe2O3/CoPi/Co(OH) x nanorods photoanode exhibits the lowest ever turn-on potential of 0.4 V RHE (versus reversible hydrogen electrode) and a photocurrent density of 0.55 mA cm−2 at 1.23 V RHE, 358% higher than that of pristine Fe2O3 nanorods. The dual co-catalysts modification enhances the light-harvesting efficiency, surface photovoltage and hole transfer kinetics of the hybrid photoanode. The dual co-catalyst coupling also increases the carrier density and significantly reduces the depletion width (1.9 nm), resulting in improved conductivity and favorable band bending, boosting photogenerated hole transfer efficiency for water oxidation.

Understanding temporal evolution of microstructures on metal-assisted chemically etched Ge surface and its applications

We report on metal-assisted chemical etching (MaCE) of single crystalline p-Ge(100) substrates at an elevated temperature, using dilute H2O2 solution, for various etching times in the range of 30–1200 s. We carry out atomic force microscopic and scanning electron microscopic studies to investigate the temporal evolution of microstructures on Ge substrates. It is observed that at lower etching times rough surfaces evolve, whereas higher etching times lead to the formation of pyramidally textured surfaces. In order to understand the present observations numerical simulation studies, based on continuum theory of stress-induced morphological instability, are carried out. Further, in an attempt to demonstrate the tunable multifunctional properties of pyramidally textured-Ge surfaces, we study light trapping and cold cathode electron emission properties of the same as a function of etching time. Interestingly, the optical reflectance of pyramidally textured surfaces reduces systematically with increasing etching time and the one evolved under an etching time of 1200 s can go down to an unprecedented low value of 0.23% over a broad spectral range (600–3000 nm). In addition, the turn-on potential for cold cathode electron emission from the etched-Ge surfaces also reduces steadily with increasing etching time with the lowest one being 1.7 V µm−1 (with the field-enhancement factor of 15,171) for the highest etching time of 1200 s. These are by far the best known results for any chemically etched-Ge surface and offer tremendous potential to fabricate various Ge-based optoelectronic devices.

Direct growth of hematite film on p+n-silicon micro-pyramid arrays for low-bias water splitting

Solar water splitting based on photoelectrochemical (PEC) cells has been intensively demonstrated to be promising for hydrogen energy, but it is usually difficult for one pure PEC system to achieve overall solar-driven water splitting. To solve this problem, integrating one PEC cell with one photovoltaic (PV) cell is widely employed, however, the preparation of an unabridged PV cell and the electrical connection between them are complicated. Herein, we directly synthesized hematite (α-Fe2O3) film on the as-prepared p+n-Si micro-pyramid arrays with the modifications of SnO2 interlayer and Co-Pi catalyst overlayer, and the turn-on potential achieved a substantial decrease, which is 0.34 V smaller than the Sn-doped hematite film grown on the FTO substrate (FTO/Sn@α-Fe2O3). The influences of interlayer, overlayer and α-Fe2O3 preparation conditions are systemically investigated. Besides, we comparatively studied the FTO/Sn@α-Fe2O3 photoanode in tandem with the complete p+n-Si micro-pyramid PV cell, and observed the turn-on potential as low as 0.32 V vs. RHE. The performance improvement is ascribed to a combination of the additional photovoltage from the embedded p+n-Si junction, the enhancement both in optical absorption and surface area of α-Fe2O3 film, and the promoted collection efficiency of photogenerated carriers from the interface and surface modifications. This work fills the gap of combining α-Fe2O3 photoanode and p+n-Si junction with the aim of overall solar-driven water splitting, and provides a new route to simplify the PEC/PV tandem system.

In Situ Construction of Hierarchical Diamond Supported on Carbon Nanowalls/Diamond for Enhanced Electron Field Emission.

The integration of sp2-/sp3-bonded carbon has aroused increasing attention on attaining a great electron field emission (EFE) performance. Herein, a novel hierarchical diamond@carbon nanowalls/diamond (D@C/D) architecture is facilely prepared through the growth of the hybrid carbon nanowalls/diamond (C/D) film followed by the in situ hydrogen plasma treatment using microwave plasma chemical vapor deposition. The hierarchical D@C/D architecture is composed of thin diamond nanoplatelets sandwiched into carbon nanowalls (CNWs) as the bottom layer and the thickened nanoplatelets constituted by diamond nanograins as the upper layer. The hydrogen plasma plays an effective role in the transformation of sacrificial sp2-bonded CNWs to sp3-bonded diamond, eventually leading to the template thickening of diamond nanoplatelets in the upper layer. Impressively, the D@C/D-90 film demonstrates much better EFE behaviors of low turn-on potential (Eo = 4.3 V μm-1), high current density (Je@8 V μm-1 = 20.81 mA cm-1), and superior long-term stability, in comparison with the pristine C/D film (Eo = 6 V μm-1, Je@8 V μm-1 = 0.33 mA cm-1). The enhanced EFE performance of the hierarchical D@C/D film is ascribed to the well-established graphite pathway for electrons transported from the bottom to the top and the increased diamond emitting sites with negative electron-affinity and robust nature at the top. This work will promote the development of the high-performance cathode EFE material based on hybrid sp2/sp3-bonded carbon, and the method proposed here also provides an effective strategy to construct a diamond nanostructure for various applications.

Zipping Up NiFe(OH)x-Encapsulated Hematite To Achieve an Ultralow Turn-On Potential for Water Oxidation

The addition of oxygen evolution reaction (OER) catalysts improves photoelectrochemical (PEC) performance of photoanodes via facilitating charge transfer at the electrode/electrolyte interface and/or passivating surface hole-trap states. Yet, the poor interface and coating quality of the catalyst overlayer often plagues the performance enhancement due to the interference of hole/electron transport across the photoanode/catalyst interface. Here we address these challenges using a citrate-assisted method that allows conformal deposition of Ni–Fe hydroxide [NiFe(OH)x] overlayers on hematite nanowire photoanodes. These NiFe(OH)x overlayers significantly enhance the surface hole transfer kinetics and passivates the surface states. The NiFe(OH)x-encapsulated hematite nanowires exhibit an early turn-on potential of 0.53 V vs RHE. The findings highlight not only the importance of uniform OER catalyst coating but also the critical role of molecular linkers in boosting the performance of photoanodes.

Influence of grain size on local work function and optoelectronic properties of n-ZTO/p-Si heterostructures

In this work, we investigate on tunability of the local work function, morphological, structural, electrical, and optical properties of ZTO (zinc tin oxide) thin films. Using Kelvin probe force microscopy, we demonstrate that local work function of ZTO films can be tuned from 5.04 to 4.94 eV by varying the film thickness from 60 to 240 nm. X-ray diffraction studies reveal the amorphous nature of ZTO films, whereas AFM measurements confirm their granular nature. It is observed that the average grain size increases (from 24 to 58 nm) with increasing film thickness. In addition, a systematic red shift in the band gap as well as a decrease in the resistivity takes place with increasing thickness. Moreover, current-voltage characteristics show a systematic reduction in the turn-on potential and an enhancement in the leakage current with increasing ZTO film thickness. Above observations are explained in the framework of ZTO thickness-dependent variation in the grain size. This study will be of importance to make a transparent conductive oxide-based electrode in photovoltaic applications and ZTO-based optoelectronic devices.

Z-scheme design of Ag@g-C3N4/ZnS photoanode device for efficient solar water oxidation: An organic-inorganic electronic interface

Abstract Herein, we demonstrate a design of a composite photoanode comprising a very thin layer of ZnS NPs onto a dense layer of Ag@g-C3N4 to form a dual absorber tandem device based on organic-inorganic electronic interface for photo water-splitting. The XRD pattern of Ag@g-C3N4 showed both the diffraction peaks of cubic Ag and main peak of g-C3N3. The XPS survey spectrum confirms the existence of the C, N, Ag, Zn, and S elements on the surface of the Ag@g-C3N4/ZnS photoelectrode. With incorporating Ag in g-C3N4 structure, the band gap decreased from 2.90 for g-C3N4 to 2.55 eV for Ag@g-C3N4 and also light absorption ability increased. The device based on the architecture of TiO2/Ag@g-C3N4/ZnS/Ni(OH)2 showed a photocurrent of about 0.1 and 0.2 mA cm−2 at potential of 1.23 and 1.7 V vs. RHE. The Ag@g-C3N4/ZnS photoanode exhibited a photocurrent turn-on potential of 0.45 V vs. RHE. In this photoanode device, Ag@g-C3N4 as a second absorber layer could absorb the photons of visible light which are transparent to ZnS layer and transforms into additional photovoltage. This architecture introduces a successful band alignment for high efficiency water-splitting through organic-inorganic junction.

Vertically aligned MoS2 nanosheets on graphene for highly stable electrocatalytic hydrogen evolution reactions.

Conducting an efficient hydrogen evolution reaction (HER) using two-dimensional molybdenum disulphide as electrocatalysts remains a challenging task due to the insufficient active edge sites. In this regard, herein, molybdenum disulphide nanosheets with rich active sulphur sites were vertically grown on the graphene surface via a chemical vapour deposition process. The direct integration of vertically aligned MoS2 nanosheets on graphene forms a van der Waals (vdW) heterojunction, which facilitates a barrier-free charge transport towards the electrolyte as a result of unique and well-matched energy band alignment at the interface. The prospective combination of Ohmic graphene/MoS2 heterostructure and the high electrocatalytic edge activity of sulphur delivers an incredibly and small turn-on potential of 0.14 V vs. RHE in the acid electrolyte solution. Most importantly, the use of a vertical vdW device architecture exhibits nearly 8× improvement in HER than that of its layered counterpart. Moreover, the HER reaction is highly stable over 50 hours of continuous operation even after 150 days. The combined analysis of our study makes it certain that the graphene/MoS2 heterostructure will be an efficient alternative electrode for low-cost and large-scale electrochemical applications.

Three-Dimensional Bicontinuous BiVO4/ZnO Photoanodes for High Solar Water-Splitting Performance at Low Bias Potential.

A photoanode capable of high-efficiency water oxidation at low bias potential is essential for its practical application for photocathode-coupled tandem systems. To address this issue, a photoanode with low turn-on voltage for water oxidation and high charge separation efficiency at low bias potential is essential. In this study, we demonstrate the photoanode of the BiVO4/ZnO three-dimensional (3D) bicontinuous (BC) structure. ZnO has a relatively cathodic flat-band potential, which leads to low turn-on potential; the BiVO4/ZnO 3D BC photoanode shows an onset potential of 0.09 V versus the reversible hydrogen electrode ( VRHE). Moreover, we achieve remarkably high charge separation efficiency at low bias potential (78% at 0.6 VRHE); this is attributed to the application of thin-film BiVO4 shells by high light-scattering properties of the 3D BC structure. As a result, the BiVO4/ZnO 3D BC photoanode generates a high water oxidation photocurrent of up to 3.4 ± 0.2 mA cm-2 (with CoPi catalyst coating). This photocurrent value is reproducible, and the photocurrent-to-O2 conversion efficiency is over 90%. To the best of our knowledge, this is the highest value among the values of the photocurrent at 0.6 VRHE in previous BiVO4-based heterojunction photoanodes.

Si/Mo4O11 nanowire arrays with enhanced photoelectrochemical performance

To utilize Si nanostructures as photocathode for high-efficiency solar water splitting, a catalyst with low cost and high activity is inevitable to modify Si. In this study, employing the electrochemical deposition method, the surfaces of Si nanowire arrays were decorated with a novel non-noble metal electrocatalyst, Mo4O11. The morphology, chemical composition and structure of the as-synthesized Si/Mo4O11 nanowire arrays were characterized in details. As photocathodes for photoelectrochemical water splitting, the Si/Mo4O11 nanowire arrays exhibited more positive turn-on potential and larger photocurrent compared to Si nanowire arrays. The results demonstrate that the Si/Mo4O11 nanowire arrays could be a promising candidate for solar water splitting.

Hematite-based photoelectrode for solar water splitting with very high photovoltage

Hematite photoelectrode is a promising candidate for photoelectrochemical (PEC) water splitting. However, the high density of surface defects and slow kinetics for oxygen evolution have restricted the PEC performance of hematite photoelectrodes, mainly their low photovoltage. This study provides the hematite surface modification combining two strategies, annealing at high temperatures (800°C) and coating with RuO2 and IrO2/RuO2 co-catalysts. IrO2/RuO2-coated hematite thin film exhibited a low turn-on potential of 0.48 VRHE and a final photovoltage of 1.20V, which is the highest photovoltage reported for hematite-based photoelectrodes. A photocurrent density of ca. 1.52mAcm−2 was obtained at 1.23 VRHE, corresponding to ca. 140% increase compared to bare hematite; the photocurrent obtained at 0.60 VRHE was 0.34mAcm−2, the highest value ever reported. The IrO2/RuO2-coated hematite photoelectrode showed a quite stable performance at 0.80 VRHE for over 72h. The water oxidation efficiency was therefore enhanced with values above 90% for potentials higher than 1.00 VRHE due to faster charge transfer and reduced electron-hole recombination at its surface.

Tunable light emission by exciplex state formation between hybrid halide perovskite and core/shell quantum dots: Implications in advanced LEDs and photovoltaics.

We report the first observation of exciplex state electroluminescence due to carrier injection between the hybrid lead halide perovskite (MAPbI3-xClx) and quantum dots (core/shell PbS/CdS). Single layers of perovskite (PS) and quantum dots (QDs) have been produced by solution processing methods, and their photoluminescent properties are compared to those of bilayer samples in both PS/QD and QD/PS configurations. Exciplex emission at lower energies than the band gap of both PS and QD has been detected. The exciplex emission wavelength of this mixed system can be simply tuned by controlling the QD size. Light-emitting diodes (LEDs) have been fabricated using those configurations, which provide light emission with considerably low turn-on potential. The "color" of the LED can also be tuned by controlling the applied bias. The presence of the exciplex state PS and QDs opens up a broad range of possibilities with important implications not only in tunable LEDs but also in the preparation of intermediate band gap photovoltaic devices with the potentiality of surpassing the Shockley-Queisser limit.

Thickness-controlled photoresponsivity of ZnO:Al/Si heterostructures: Role of junction barrier height

Thickness-dependent enhancement in photoresponsivity of conformally grown n-type aluminium-doped zinc oxide (AZO) on ion-beam fabricated nanofaceted p-type silicon has been investigated in the spectral range of 300–800nm. Current–voltage characteristics of n-AZO/p-Si heterojunctions show rectifying behaviour with systematic decrease (by a factor of 2) in turn-on potential as a function of increasing AZO thickness. Enhancement in the photoresponsivity (by a factor of 5) and reduction in the turn-on potential, with increasing AZO thickness, are attributed to the reduced potential barrier across the n-AZO/p-Si junction due to the use of nanofaceted-Si templates and corroborate well with the capacitance–voltage measurements.

Branched Nanowire Heterostructures for Efficient and Durable Solar-Driven Hydrogen Production

Branched nanowire (NW) heterostructures have recently been attracted considerable attention for solar water splitting and clean hydrogen production due to their unique properties such as nanoscale integration of different functional materials, greatly enhanced junction and surface area, enhanced gas evolution efficiency, broadband light absorption, etc. Moreover, branched NWs can be fabricated using facile and scalable fabrication methods such as hydrothermal or solvothermal growth methods. In this presentation, we show branched NWs of different compositions for core (or trunk) and branch NWs which were fabricated with facile and low-cost synthesis methods using cheap, non-toxic, and earth abundant materials including Si, CuO, Cu2O, ZnO, TiO2, and Fe2O3. The branched NW structures and the heterostructures’ interfaces are investigated in detail using different characterization techniques such as SEM/HRSEM, TEM/HRTEM, STEM/HRSETM, etc. The photoelectrochemical (PEC) performances including photocurrent turn-on potential, photocurrent, solar conversion efficiency, and incident photon-to-current efficiency (IPCE) are studied systematically and optimized, based on different core and branch NW dimensions, for each specific branched NW heterostructure to provide efficient water splitting in a neutral medium. The electrode stability of different branched NWs is also investigated and long-term stability of over one day or several hours using a thin passivation layer or robust branched NWs are presented. The achieved results pave the way for accomplishing spontaneous overall solar water splitting for clean, efficient, cost-effective and durable solar hydrogen generation at large scales.

Hematite‐Based Water Splitting with Low Turn‐On Voltages

Power up: The photoelectrochemical performance of hematite in a water-splitting device was improved significantly through surface modification with amorphous NiFeOx (see graph showing the large cathodic shift observed; FTO=fluorine-doped tin oxide). The measured photovoltage increased from 0.24 to 0.61 V, which yielded a record-low turn-on potential of 0.62 V (versus the reversible hydrogen electrode, RHE).

3D Branched Nanowire Photoelectrochemical Electrodes for Efficient Solar Water Splitting

We report the systematic study of 3D ZnO/Si branched nanowire (b-NW) photoelectrodes and their application in solar water splitting. We focus our study on the correlation between the electrode design and structures (including Si NW doping, dimension of the trunk Si and branch ZnO NWs, and b-NW pitch size) and their photoelectrochemical (PEC) performances (efficiency and stability) under neutral conditions. Specifically, we show that for b-NW electrodes with lightly doped p-Si NW core, larger ZnO NW branches and longer Si NW cores give a higher photocathodic current, while for b-NWs with heavily doped p-Si NW trunks smaller ZnO NWs and shorter Si NWs provide a higher photoanodic current. Interestingly, the photocurrent turn-on potential decreases with longer p-Si NW trunks and larger ZnO NW branches resulting in a significant photocathodic turn-on potential shift of ~600 mV for the optimized ZnO/p-Si b-NWs compared to that of the bare p-Si NWs. A photocathode energy conversion efficiency of greater than 2% at -1 V versus Pt counter electrode and in neutral solution is achieved for the optimized ZnO/p-Si b-NW electrodes. The PEC performances or incident photon-to-current efficiency are further improved using Si NW cores with smaller pitch size. The photoelectrode stability is dramatically improved by coating a thin TiO2 protection layer using atomic-layer deposition method. These results provide very useful guidelines in designing photoelectrodes for selective solar water oxidation/reduction and overall spontaneous solar fuel generation using low cost earth-abundant materials for practical clean solar fuel production.

Effect of grain-boundaries on electrical properties of n-ZnO:Al/p-Si heterojunction diodes

We report on room temperature diode characteristics of ZnO:Al (AZO)/Si heterostructures by current-voltage measurements. In this study, with increasing AZO film thickness, systematic reduction in the turn-on potential (from 3.16 to 1.80 V) and the film stress are observed. Complementary capacitance-voltage studies reveal a decreasing trend in barrier height at the junction with increasing AZO film thickness. A gradual decrease in resistivity takes place with increasing AZO film thickness. Above observations are explained in the framework of AZO thickness dependent variation in grain size and in turn trap density at the grain boundaries influencing carrier transport across the adjacent grains.

Three-dimensional ZnO/Si broom-like nanowire heterostructures as photoelectrochemical anodes for solar energy conversion

We report a low-cost solution fabrication of three-dimensional (3D) ZnO/Si broom-like nanowire (NW, “nanobroom”) heterostructures, consisting of Si NW “backbones” and ZnO NW “stalls”, and their application as photoelectrochemical anodes for solar water splitting and energy conversion. The nanobroom morphology and atomic structure are characterized using the scanning, transmission, and scanning transmission electron microscopies. Both Si NW backbones and ZnO NW stalls are defect-free, single-crystalline, and their surfaces are smooth. The optical absorption and photocurrents from nanobroom array electrodes with different Si and ZnO NW dimensions are studied. The longer Si NW backbones and smaller ZnO NW stalls lead to better light absorption and larger photoanodic current. The ZnO/Si nanobrooms show much higher photoanodic current than the bare Si NWs due to the effective Si/ZnO junction and increased surface area. The nanobroom electrode stability is also investigated and using a thin TiO2 coating layer protecting the NWs against dissolution, long-term stability is obtained without any change in shape and morphology of nanobrooms. Finally, the effect of catalyst to improve the oxygen evolution reaction (OER) at the electrode surface is studied resulting in large enhancement in photoanodic current and significant reduction in anodic turn-on potential. This study reveals the promise of the use of simply fabricated and low-cost 3D heterostructured NW photoelectrodes for clean solar energy harvesting and conversion.