Visible-light responsive BiNbO4 nanosheet photoanodes for stable and efficient solar-driven water oxidation.

Herein, we report bismuth niobate (BiNbO4), which is regarded as an emerging photoanode material for sustainable photoelectrochemical (PEC) solar energy conversion. BiNbO4 possesses a direct bandgap (Eg) of ∼2.6 eV, and shows an appropriate band alignment for the water oxidation/reduction reaction. In this study, a simple sol-gel route followed by a spin coating method was applied to develop BiNbO4 nanosheets under the optimum annealing conditions. It is known that the annealing temperatures of 500 and 550 °C influence the crystallinity and PEC properties of BiNbO4 films. In particular, the 550 °C annealed film exhibited sharply improved crystalline properties, and rapidly enhanced PEC performance, which were accompanied by a photocurrent density of 0.45 mA cm-2 at 1.23 V vs. the reversible hydrogen electrode (RHE) (briefly abbreviated as 1.23 VRHE) in a strong alkaline solution of 1 M NaOH, compared with 0.26 mA cm-2 at 1.23 VRHE of the 500 °C annealed film. This may be attributed to the main increase of the crystallinity, as well as the improvement of the electronic properties. In addition, the BiNbO4 (550 °C) film showed an incident photon-to-current efficiency of 20% at 425 nm, and produced a stable photoresponse under light illumination in a strong alkaline solution over 5 h, compared with a BiVO4 electrode.

Facile Synthesis of Microsphere-like Co0.85Se Structures on Nickel Foam for a Highly Efficient Hydrogen Evolution Reaction.

Microsphere-shaped cobalt selenide (Co0.85Se) structures were efficiently synthesized via a two-step hydrothermal process. Initially, cobalt hydroxide fluoride (Co(OH)F) microcrystals were prepared using a hydrothermal method. Subsequently, Co0.85Se microsphere-like structures were obtained through selenization. Compared to Co(OH)F, the microsphere-like Co0.85Se structure exhibited outstanding catalytic activity for the hydrogen evolution reaction (HER) in a 1.0 M KOH solution. Electrocatalytic experiments demonstrated an exceptional HER performance by the Co0.85Se microspheres, characterized by a low overpotential of 148 mV and a Tafel slope of 55.7 mV dec-1. Furthermore, the Co0.85Se electrocatalyst displayed remarkable long-term stability, maintaining its activity for over 24 h. This remarkable performance is attributed to the excellent electrical conductivity of selenides and the highly electroactive sites present in the Co0.85Se structure compared to Co(OH)F, emphasizing its promise for advanced electrocatalytic applications.

Improving the photoelectrochemical water splitting performance of CuO photocathodes using a protective CuBi2O4 layer.

A heterojunction photocathode of CuO and CuBi2O4 grown on an FTO substrate (FTO/CuO/CuBi2O4) was synthesized using hydrothermal method followed by spin coating and annealing to overcome the bottlenecks encountered by CuO in photoelectrochemical (PEC) water splitting application. The synthesis methods, morphological, structural properties, and composition of each sample under each synthesis condition are discussed in detail. The photocathode with 15 coating layers annealed at 450 °C exhibited the best PEC performance. Moreover, its current density reached 1.23 mA/cm2 under an applied voltage of - 0.6 V versus Ag/AgCl in a neutral electrolyte. Additionally, it exhibited higher stability than the bare CuO thin film. The bonding of CuBi2O4 on CuO resulted in close contact between the two semiconductors, helping the semiconductors support each other to increase the PEC efficiency of the photocathode. CuO acted as the electron-generating layer, and the CuBi2O4 layer helped minimize photocorrosion as well as transport the carriers to the electrode/electrolyte interface to accomplish the hydrogen evolution reaction.

Formation of CuInSe2 absorber layers formed using co-electrodeposition combined with selenization.

Stoichiometric CuInSe2 absorber layers were formed using co-electrodeposition coupled with selenization. We investigated the influence of the metal ion ratio, supporting electrolyte, and deposition voltages on the structural and chemical properties of Cu-In alloys. The increases in deposition voltage and metal ion concentration helped to form In-rich Cu-In alloy with dendrite structure composed of a long central trunk with secondary branches. In addition, on increasing the concentration of the supporting electrolyte, the ratio of In to Cu in the Cu-In alloy increased, and surface morphology improved. Finally, based on an optimized co-electrodeposition process, the selenization of Cu-In alloys using the evaporation of the Se element was employed to form high quality CuInSe2 absorber layers.

Enhanced electron lifetime on nitrogen-doped TiO2 films for dye-sensitized solar cells.

Nitrogen-doped TiO2 crystallites were prepared via the hydrolysis of TiCl4 using an ammonia medium in an aqueous solution for DSSC photoelectrodes. The optimized photoelectrode for the DSSC was prepared with 9.4 nm sized N-doped TiO2 crystal (BET; 200 m2/g), which provides a relatively high short circuit current and energy conversion efficiency in the DSSC. The photovoltaic performance of the N-doped TiO2 electrode was confirmed using incident photon-to-current efficient spectra, impedance analyses, and Bode-phase plots which proved that the N-doped TiO2 electrode has a significantly enhanced electron lifetime compared with that of the P25 electrode.

Photovoltaic performance of dye-sensitized solar cell low temperature growth of ZnO nanorods using chemical bath deposition.

Nanostructured ZnO photoelectrodes were synthesized on fluorine-doped tin oxide (FTO) glass substrates that were spin-coated with a sol-gel based ZnO seed layer via a chemical bath deposition (CBD) method at varying times of 1, 2, 4, and 8 h. Then, TiO2 nanoparticulate electrodes were prepared on ZnO nanorods using the doctor blade technique. The uniformly grown ZnO nanorod layer had a length of approximately 710 nm on the FTO glass substrate with wurtzite structures which was confirmed through X-ray diffraction patterns. The length and diameter of the ZnO nanorods increased with an increase in the deposition time. The DSSCs fabricated with TiO2 nanoparticulate/grown ZnO nanorods and grown for 8 h showed the maximum efficiency (5.51%) with a short circuit current density (J(sc)) of 12.21 mA/cm2 and an open circuit voltage (V(oc)) of 0.70 at 100 mW/cm2 light intensity.

Reflow of phosphorous silicate glass layer formed on textured Si surface in crystalline Si solar cells.

We have investigated the reflow behavior of phosphorus silicate glass (PSG) layer formed on textured Si surface using transmission electron microscopy and simulation. For conventional wet oxidation process, stress-dependent surface reaction and stress-dependent oxidant diffusion led to the oxidation retardation in both convex and concave regions of the textured Si surface, respectively. However, PSG film formed by POCl3-diffusion underwent reflow, resulting in the formations of thinner and thicker PSG films in convex and concave regions, respectively. Simulation results showed that the reflow of PSG films causes lateral thermal mismatch stresses to increase and decrease in convex and concave regions, respectively.

Enhanced electron transport in mesoporous TiO2 films modified by sol-gel necking for dye-sensitized solar cells.

Mesoporous TiO2 films modified via sol-gel necking were fabricated by dispersing Ti tetra-isopropoxide (TTIP; 8 to 16 wt% over TiO2) with TiO2 nanoparticles in isopropyl alcohol. The dye-sensitized solar cells (DSSCs) with 13 wt% TTIP-modified TiO2 film exhibited significantly improved overall energy conversion efficiency, despite having less adsorbed dye when compared with DSSCs with untreated and TiCl4 post-treated TiO2 films. The improvement can be attributed to the sol-gel necking (or interconnection) between the nanoparticles which leads to a much faster electron transport and a suppression of the recombination (or back electron transfer) between the TiO2 and electrolyte.

Rationally Designed Bimetallic Co-Ni Sulfide Microspheres as High-Performance Battery-Type Electrode for Hybrid Supercapacitors.

Rational designing of electrode materials is of great interest for improving the performance of battery-type supercapacitors. The bimetallic NiCo2S4 (NCS) and CoNi2S4 (CNS) electrode materials have received much attention for supercapacitors due to their rich electrochemical characteristics. However, the comparative electrochemical performances of NCS and CNS electrodes were never studied for supercapacitor application. In this work, microsphere-like bimetallic NCS and CNS structures were synthesized via a facile one-step hydrothermal method by controlling the molar ratio of Ni and Co precursors. The physico-chemical results confirmed that microsphere-like structures with cubic spinel-type NCS and CNS materials were successfully fabricated by this method. When tested as the supercapacitor electrode materials, both NCS and CNS electrodes exhibited battery-type behavior in a three-electrode configuration with outstanding electrochemical performances such as specific capacity, rate performance and cycle stability. Impressively, the CNS electrode delivered a high specific capacity of 430.1 C g-1 at 1 A g-1, which is higher than 345.9 C g-1 of the NCS electrode. Furthermore, the NCS and CNS electrodes showed a decent cycling stability with 75.70 and 84.70% capacity retention after 10,000 cycles. Benefiting from the electrochemical advantage of CNS microspheres, we fabricated a hybrid supercapacitor (HSC) device based on CNS microspheres (positive electrode) and activated carbon (AC, negative electrode), which is named as CNS//AC. The assembled CNS//AC HSC device showed a large energy density of 41.98 Wh kg-1 at a power density of 800.04 W kg-1 and displayed a remarkable cycling stability with a capacity retention of 91.79% after 15,000 cycles. These excellent electrochemical performances demonstrate that both bimetallic NCS and CNS microspheres may provide potential electrode materials for high performance battery-type supercapacitors.

Nanoporous Ta3N5via electrochemical anodization followed by nitridation for solar water oxidation.

Nanoporous tantalum nitride (Ta3N5) is a promising visible-light-driven photoanode for photoelectrochemical (PEC) water splitting with a narrow band gap of approximately 2.0 eV. It can utilize a large portion of the solar spectrum up to 600 nm to improve the activity of photooxidation reactions because of enhanced light scattering and an overall increase of the surface area with high light absorption and carrier collection. Herein, we synthesized a new n-type nanoporous tantalum nitride film on Ta foil by electrochemical anodization with a fluorinated electrolyte. Post-annealing in a nitrogen/ammonia mixture gas environment then transformed amorphous TaOx to crystalline Ta3N5. Effects of annealing temperature on the microstructure, optical properties, and PEC properties of samples were then investigated under changeable stoichiometry of Ta and N elements in the Ta-based nitride film. Results showed that the film annealed at 1000 °C showed high crystallinity, high visible light absorption, and a highly conductive interlayer between the substrates, resulting in the highest photocurrent density (JSC) of ∼0.25 mA cm-2 at 1.23 VRHE in PEC water splitting. In addition, depending on the annealing temperature, it is possible to engineer band alignment in the nanoporous Ta3N5 layer, allowing a beneficial charge transfer process.

Electrochemically Deposited Polypyrrole for Counter Electrode of Quasi-Solid-State Dye-Sensitized Solar Cell.

In this study, quasi-solid-state dye-sensitized solar cells (QSS-DSSCs) were fabricated by employing polypyrrole (PPy)-functionalized counter electrodes and an iodine-based gel electrolyte. The PPy film was fabricated on a fluorine-doped tin oxide substrate by simple potentiostatic electrodeposition in an aqueous solution using a pyrrole monomer as the precursor. The prepared polypyrrole counter electrode was used in a QSS-DSSC and characterized by scanning electron microscopy and electrochemical impedance spectroscopy. The use of PPy counter electrodes was judged to be acceptable for practical applications because they were only 21% less efficient than counter electrodes fabricated with Pt.

Monolithic Inorganic ZnO/GaN Semiconductors Heterojunction White Light-Emitting Diodes.

Monolithic light-emitting diodes (LEDs) that can generate white color at the one-chip level without the wavelength conversion through packaged phosphors or chip integration for photon recycling are of particular importance to produce compact, cost-competitive, and smart lighting sources. In this study, monolithic white LEDs were developed based on ZnO/GaN semiconductor heterojunctions. The electroluminescence (EL) wavelength of the ZnO/GaN heterojunction could be tuned by a post-thermal annealing process, causing the generation of an interfacial Ga2O3 layer. Ultraviolet, violet-bluish, and greenish-yellow broad bands were observed from n-ZnO/p-GaN without an interfacial layer, whereas a strong greenish-yellow band emission was the only one observed from that with an interfacial layer. By controlled integration of ZnO/GaN heterojunctions with different postannealing conditions, monolithic white LED was demonstrated with color coordinates in the range (0.3534, 0.3710)-(0.4197, 0.4080) and color temperatures of 4778-3349 K in the Commission Internationale de l'Eclairage 1931 chromaticity diagram. Furthermore, the monolithic white LED produced approximately 2.1 times higher optical output power than a conventional ZnO/GaN heterojunction due to the carrier confinement effect at the Ga2O3/n-ZnO interface.

Efficient Dye-Sensitized Solar Cells with Highly Catalytic Pt-Deposited ZnO/FTO Counter Electrode.

The development of novel cathode is essential for developing high performance dye sensitized solar cells (DSSCs). Here, Pt-coated ZnO nanostructures are used as cathodes of DSSCs to increase their photovoltaic performances. The ZnO nanostructures are grown by chemical bath deposition method, and then Pt is deposited on the nanostructured ZnO substrates. The scanning electron microscopy analyses show that nanostrcutrured Pt-deposited ZnO/FTO is well-formed on fluorinated tin oxide (FTO) substrate with yielding high surface area. The increase in the surface area of Pt-deposited ZnO/FTO leads to the high electrochemical kinetics for reduction of I3- to I-. As a result, the optimized Pt-deposited ZnO/FTO cathode shows the high photovoltaic performances of DSSCs about 6.70% of overall power conversion efficiency, which is 52% higher than the DSSCs using typical Pt-deposited FTO cathode.

Joint Effects of Photoactive TiO2 and Fluoride-Doping on SnO2 Inverse Opal Nanoarchitecture for Solar Water Splitting.

Inverse opal (IO) films of tin dioxide (SnO2) were fabricated on polystyrene (PS) beads (diameter=350 nm (±20 nm) with a spin coating method. To compensate for the large band gap (Eg=3.8 eV), a thin TiO2 shell was deposited on the SnO2-IO films with atomic layer deposition (ALD), which produced shells with thicknesses of 10-40 nm. The morphological changes and crystalline properties of the SnO2 and TiO2-coated SnO2 (herein after referred to as TiO2/SnO2) IO films were investigated with field-emission scanning electron microscopy and X-ray diffraction, respectively. The photoelectrochemical (PEC) behavior of the samples was tested in a 0.1 M KOH solution under 1 sun illumination (100 mW/cm2 with an AM 1.5 filter). The highest PEC performance was obtained with the TiO2(10 nm)/SnO2 IO films, which produced a photocurrent density (Jsc) of 4.67 mA/cm2 at 0.5 V (vs NHE) and was sequentially followed by the TiO2(20 nm)/SnO2-IO, TiO2(30 nm)/SnO2-IO, TiO2 (40 nm)/SnO2-IO and SnO2 IO films. Overall, the thin TiO2 shell covered on the SnO2-IO core enhanced Jsc by 3 orders of magnitude, which in turn the PEC activity. This is mainly ascribed to the extremely low charge-transfer resistance (Rct) in the photoelectrode/electrolyte and at the TiO2/SnO2 interface, as well as the contribution of the photoactive TiO2 layer, which has an Eg of 3.2 eV. Moreover, to improve the electrical conductivity of the core SnO2 IO film, the films were doped with 10 mol % of F. The F- doped films were labeled as the FTO IO film. The Rct of the FTO-IO films decreased because of the improved electronic conductivity, enhancing the PEC performance of the TiO2(10 nm)/FTO-IO films by approximately 20%. The core-shell nanowire mesh nanoarchitecture is therefore suggested to provide an insight for designing the peculiar structure based on the material's properties and the engineering of their band gap energy for highly efficient PEC performance.

Reactively sputtered nickel nitride as electrocatalytic counter electrode for dye- and quantum dot-sensitized solar cells.

Nickel nitride electrodes were prepared by reactive sputtering of nickel under a N2 atmosphere at room temperature for application in mesoscopic dye- or quantum dot- sensitized solar cells. This facile and reliable method led to the formation of a Ni2N film with a cauliflower-like nanostructure and tetrahedral crystal lattice. The prepared nickel nitride electrodes exhibited an excellent chemical stability toward both iodide and polysulfide redox electrolytes. Compared to conventional Pt electrodes, the nickel nitride electrodes showed an inferior electrocatalytic activity for the iodide redox electrolyte; however, it displayed a considerably superior electrocatalytic activity for the polysulfide redox electrolyte. As a result, compared to dye-sensitized solar cells (DSCs), with a conversion efficiency (η) = 7.62%, and CdSe-based quantum dot-sensitized solar cells (QDSCs, η = 2.01%) employing Pt counter electrodes (CEs), the nickel nitride CEs exhibited a lower conversion efficiency (η = 3.75%) when applied to DSCs, but an enhanced conversion efficiency (η = 2.80%) when applied to CdSe-based QDSCs.

Microstructural and chemical properties of ZnO films formed using electrodeposition.

We investigated the effect of bath temperature and electrodeposition potential on the microstructural and chemical properties of ZnO films formed on Mo-coated soda-lime glass substrates using electrodeposition. The electrodeposition was performed using an electrolytic solution containing 0.05 M Zn(NO3)2 for 6 min. The ZnO islands grew larger to impinge with other islands until the bath temperature was increased up to 40 degrees C, above which continuous ZnO film was eventually formed. An increase in the electrodeposition potential resulted in enhancement of the growth rate of the electrodeposited ZnO film with the facilitation of film texturation. The c-axis was perpendicular to surface, which could be associated with the preferential orientation along the (002) direction. At the electrodeposition potential of -1.3 V (vs. a saturated calomel electrode), significant amounts of hydrogen bubbles that electrochemically evolved near the surface of the working electrode hampered the homogenous growth of the ZnO film, which could be responsible for morphological degradation of the ZnO film.

Optimization of the TiO2-surface modification temperature for performance enhancement of dye-sensitized solar cells.

A nanoporous TiO2 electrode was modified with magnesium salts (MSs), MgCO3 and Mg(CH3COO)2, by simple dip coating process at varied temperatures, and then applied to dye-sensitized solar cells (DSSCs). When the surface treatment was conducted at 40 °C, the DSSC with MS-modified TiO2 layer showed an increase in short circuit current (JSC) and open circuit voltage (VOC), resulting in a power conversion efficiency of 8.52%, compared to that (7.02%) of reference device with bare TiO2. The improved JSC value was attributed to the increased dye adsorption. Electrochemical impedance spectroscopy and dark current-voltage studies revealed that the VOC enhancement was caused by the suppression of charge recombination between injected electrons and I3(-) ions.