Surface Engineering of Anodic WO3 Layers by In Situ Doping for Light-Assisted Water Splitting.

This study presents a novel approach to fabricating anodic Co-F-WO3 layers via a single-step electrochemical synthesis, utilizing cobalt fluoride as a dopant source in the electrolyte. The proposed in situ doping technique capitalizes on the high electronegativity of fluorine, ensuring the stability of CoF2 throughout the synthesis process. The nanoporous layer formation, resulting from anodic oxide dissolution in the presence of fluoride ions, is expected to facilitate the effective incorporation of cobalt compounds into the film. The research explores the impact of dopant concentration in the electrolyte, conducting a comprehensive characterization of the resulting materials, including morphology, composition, optical, electrochemical, and photoelectrochemical properties. The successful doping of WO3 was confirmed by energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), Raman spectroscopy, photoluminescence measurements, X-ray photoelectron spectroscopy (XPS), and Mott-Schottky analysis. Optical studies reveal lower absorption in Co-doped materials, with a slight shift in band gap energies. Photoelectrochemical (PEC) analysis demonstrates improved PEC activity for Co-doped layers, with the observed shift in photocurrent onset potential attributed to both cobalt and fluoride ions catalytic effects. The study includes an in-depth discussion of the observed phenomena and their implications for applications in solar water splitting, emphasizing the potential of the anodic Co-F-WO3 layers as efficient photoelectrodes. In addition, the research presents a comprehensive exploration of the electrochemical synthesis and characterization of anodic Co-F-WO3, emphasizing their photocatalytic properties for the oxygen evolution reaction (OER). It was found that Co-doped WO3 materials exhibited higher PEC activity, with a maximum 5-fold enhancement compared to pristine materials. Furthermore, the studies demonstrated that these photoanodes can be effectively reused for PEC water-splitting experiments.

Scanning with Laser Beam over the TiO2 Nanotubes Covered with Thin Chromium Layers towards the Activation of the Material under the Visible Light.

This work presents pulsed UV laser treatment (355 nm, 2 Hz) of TiO2 nanotubes decorated with chromium oxides. The modification was performed in a system equipped with a beam homogenizer, and during the irradiation, the samples were mounted onto the moving motorized table. In such a system, both precisely selected areas and any large area of the sample can be modified. Photoelectrochemical tests revealed photoresponse of laser-treated samples up to 1.37- and 18-fold under the illumination with ultraviolet-visible and visible light, respectively, in comparison to bare titania. Optimal beam energy fluence regarding sample photoresponse has been established. Scanning electron microscopy images, X-ray diffraction patterns, along with Raman and X-ray photoelectron spectra, suggest that the enhanced photoresponse results from changes solely induced in the layer of chromium oxides. It is believed that the results of the present work will contribute to a wider interest in laser modification of semiconductors exhibiting improved photoelectrochemical activity.

Fabrication of ZnO Ceramics with Defects by Spark Plasma Sintering Method and Investigations of Their Photoelectrochemical Properties.

ZnO, as an important semiconductor material, has attracted much attention due to its excellent physical properties, which can be widely used in many fields. Notably, the defects concentration and type greatly affect the intrinsic properties of ZnO. Thus, controllable adjustment of ZnO defects is particularly important for studying its photoelectric properties. In this work, we fabricated ZnO ceramics (ZnO(C)) with different defects through spark plasma sintering (SPS) process by varying sintering temperature and using reduction environment. The experimental results indicate that the changes of color and light absorption in as-prepared ZnO originate from the different kinds of defects, i.e., oxygen vacancies (VO), interstitial zinc (Zni), and Zinc vacancies (VZn). Moreover, with the increase in calcination temperature, the concentration of oxygen defects and interstitial zinc defects in the ceramics increases gradually, and the conductivity of the ceramics is also improved. However, too many defects are harmful to the photoelectrochemical properties of the ceramics, and the appropriate oxygen defects can improve the utilization of visible light.

Scaling Up the Process of Titanium Dioxide Nanotube Synthesis and Its Effect on Photoelectrochemical Properties.

In this work, for the first time, the influence of scaling up the process of titanium dioxide nanotube (TiO2NT) synthesis on the photoelectrochemical properties of TiO2 nanotubes is presented. Titanium dioxide nanotubes were obtained on substrates of various sizes: 2 × 2, 4 × 4, 5 × 5, 6 × 6, and 8 × 8 cm2. The electrode material was characterized using scanning electron microscopy as well as Raman and UV-vis spectroscopy in order to investigate their morphology, crystallinity, and absorbance ability, respectively. The obtained electrodes were used as photoanodes for the photoelectrochemical water splitting. The surface analysis was performed, and photocurrent values were determined depending on their place on the sample. Interestingly, the values of the obtained photocurrent densities in the center of each sample were similar and were about 80 µA·cm2. The results of our work show evidence of a significant contribution to wider applications of materials based on TiO2 nanotubes not only in photoelectrochemistry but also in medicine, supercapacitors, and sensors.

Structural, photocatalytic and photoelectrochemical properties of porous ZnO nanosheets prepared by air cold plasma.

Porous ZnO nanosheets with different thickness were prepared on zinc substrate by air cold plasma for photocatalytic degradation and photoelectrochemical water splitting. The ZnO nanosheets consisted of nanocrystallines with high-density oxygen-related defects characterized by the strong red luminescence. The UV absorption tended to be saturated as the thickness increased, and the saturation occurred at a thickness of about 2.3µm. Under UV irradiation (365 nm), the 2.3µm thick sample with higher content of oxygen vacancies and oxygen interstitials showed the highest photocatalytic activity (and higher than P25 TiO2) in degradation of gaseous ethyl acetate. Due to the excellent UV-vis absorption ability and the effective transfer of photogenerated carriers, the ZnO nanosheets with thickness of 3.3µm showed a photocurrent density as high as 0.22 mA cm-2at -0.28 V (versus Ag/AgCl) under AM 1.5 G 100 mW cm-2.

Role of Interfacial Defects in Photoelectrochemical Properties of BiVO4 Coated on ZnO Nanodendrites: X-ray Spectroscopic and Microscopic Investigation.

Synchrotron-based X-ray spectroscopic and microscopic techniques are used to identify the origin of enhancement of the photoelectrochemical (PEC) properties of BiVO4 (BVO) that is coated on ZnO nanodendrites (hereafter referred to as BVO/ZnO). The atomic and electronic structures of core-shell BVO/ZnO nanodendrites have been well-characterized, and the heterojunction has been determined to favor the migration of charge carriers under the PEC condition. The variation of charge density between ZnO and BVO in core-shell BVO/ZnO nanodendrites with many unpaired O 2p-derived states at the interface forms interfacial oxygen defects and yields a band gap of approximately 2.6 eV in BVO/ZnO nanocomposites. Atomic structural distortions at the interface of BVO/ZnO nanodendrites, which support the fact that there are many interfacial oxygen defects, affect the O 2p-V 3d hybridization and reduce the crystal field energy 10Dq ∼2.1 eV. Such an interfacial atomic/electronic structure and band gap modulation increase the efficiency of absorption of solar light and electron-hole separation. This study provides evidence that the interfacial oxygen defects act as a trapping center and are critical for the charge transfer, retarding electron-hole recombination, and high absorption of visible light, which can result in favorable PEC properties of a nanostructured core-shell BVO/ZnO heterojunction. Insights into the local atomic and electronic structures of the BVO/ZnO heterojunction support the fabrication of semiconductor heterojunctions with optimal compositions and an optimal interface, which are sought to maximize solar light utilization and the transportation of charge carriers for PEC water splitting and related applications.

Enhanced photocatalytic performance of Cu2O/MoS2/ZnO composites on Cu mesh substrate for nitrogen reduction.

Cu2O nanoparticles and MoS2nanoflowers decorated with ZnO nanospheres were successfully co-deposited on Cu mesh via a mild electrodeposition method to build a dual direct Z-scheme heterostructure. The prepared materials can effectively synthesize ammonia with N2and H2O in the liquid membrane reactor under simulated visible light. The results indicate that 3D nanomaterials exhibit better performance compared to a pure semiconductor due to the synergistic effect of enhanced visible light absorption, longer photogenerated carrier lifetime and the specific charge transfer path of dual direct Z-scheme structure. Meanwhile, the hydrophilicity of Cu2O/MoS2/ZnO rapidly makes the surface of the catalyst wet when it participates in the photo reaction, which promotes the contact between the reactant and exciton. This work proposes the electron transfer and possible reaction mechanism corresponding to the designed catalyst, which can provide a reference for other photocatalytic applications using a semiconductor heterojunction as a catalyst.

p-Cu2O/n-ZnO heterojunction thin films with enhanced photoelectrochemical properties and photocatalytic activities for norfloxacin.

A two-step electrochemical deposition technique was applied to fabricate p-Cu2O/n-ZnO heterojunction thin films. The influence of the deposition potential upon photoelectric performance of the prepared samples was examined utilizing XRD, XPS, SEM, UV-Vis, and electrochemical tests. The results show that the deposition potential has a substantial influence on the properties of the prepared samples. When the deposition potential is -0.45 V, the peak intensity of the (111) crystal plane of the prepared heterojunction is the highest, the band gap increased, and the morphology changes obviously compared to those of Cu2O. The transient photocurrent value is three times that of pure Cu2O, and the charge transfer resistance significantly reduced. The p-Cu2O/n-ZnO heterojunction has a high carrier concentration. Photocatalytic degradation experiments show that degradation rate of norfloxacin increases by 14.4%-76.6%. The enhanced photocatalytic performance of Cu2O is mainly due to the formation of a high-quality heterojunction and the change in the energy band structure, which promotes the transfer rate of the carrier and the separation of photogenic electron hole pairs, thus effectively improving the catalytic efficiency of photocatalysts. Active species detection experiments reveal that positive hole and superoxide anion radical play leading roles in norfloxacin molecule decomposition. In addition, a possible mechanism for the photocatalytic performance of p-Cu2O enhanced by n-ZnO is proposed according to the analysis of the bandgap of p-Cu2O and n-ZnO, along with the built-in electric field formed in the p-n heterojunction. This study provides an effective and alternative method for removing norfloxacin residues in wastewater.

Improving Photoelectrochemical Properties of Anodic WO3 Layers by Optimizing Electrosynthesis Conditions.

Although anodic tungsten oxide has attracted increasing attention in recent years, there is still a lack of detailed studies on the photoelectrochemical (PEC) properties of such kind of materials grown in different electrolytes under various sets of conditions. In addition, the morphology of photoanode is not a single factor responsible for its PEC performance. Therefore, the attempt was to correlate different anodizing conditions (especially electrolyte composition) with the surface morphology, oxide thickness, semiconducting, and photoelectrochemical properties of anodized oxide layers. As expected, the surface morphology of WO3 depends strongly on anodizing conditions. Annealing of as-synthesized tungsten oxide layers at 500 °C for 2 h leads to obtaining a monoclinic WO3 phase in all cases. From the Mott-Schottky analysis, it has been confirmed that all as prepared anodic oxide samples are n-type semiconductors. Band gap energy values estimated from incident photon-to-current efficiency (IPCE) measurements neither differ significantly for as-synthesized WO3 layers nor depend on anodizing conditions such as electrolyte composition, time and applied potential. Although the estimated band gaps are similar, photoelectrochemical properties are different because of many different reasons, including the layer morphology (homogeneity, porosity, pore size, active surface area), oxide layer thickness, and semiconducting properties of the material, which depend on the electrolyte composition used for anodization.

Blue TiO2 nanotube arrays as semimetallic materials with enhanced photoelectrochemical activity towards water splitting.

In the past years there has been a great interest in self-doped TiO2 nanotubes (blue TiO2 nanotubes) compared to undoped ones owing to their high carrier density and conductivity. In this study, blue TiO2 nanotubes are investigated as photoanode materials for photoelectrochemical water splitting. Blue TiO2 nanotubes were fabricated with enhanced photoresponse behavior through electrochemical cathodic polarization on undoped and annealed TiO2 nanotubes. The annealing temperature of undoped TiO2 nanotubes was tuned before cathodic polarization, revealing that annealing at 500 °C improved the photoresponse of the nanotubes significantly. Further optimization of the blue TiO2 nanotubes was achieved by adjusting the cathodic polarization parameters. Blue TiO2 nanotubes obtained at the potential of -1.4 V (vs. SCE) with a duration of 10 min exhibited twice more photocurrent response (0.39 mA cm-2) compared to the undoped TiO2 nanotube arrays (0.19 mA cm-2). Oxygen vacancies formed through the cathodic polarization decreased charge recombination and enhanced charge transfer rate; therefore, a high photoelectrochemical activity under visible light irradiation could be achieved.

Enhanced Photoelectrochemical Properties of Ti3+ Self-Doped Branched TiO2 Nanorod Arrays with Visible Light Absorption.

A novel Ti3+ self-doped branched rutile TiO2 nanorod arrays (NRAs) was successfully grown on an F-doped tin oxide (FTO) transparent conductive glass by a combined hydrothermal and magnetron sputtering method. Surface morphology, structure, optical properties, and photoelectrochemical behavior of the branched TiO2 NRAs are determined. Using TiO2 nanoparticles (NPs) deposited on the top of the nanorods as seeds, TiO2 nanobranches can easily grow on the top of the nanorods. Moreover, the Ti3+ defects in the TiO2 NPs and associated oxygen vacancies, and the nanobranches expend the optical absorption edge of the TiO2 NRAs from 400 nm to 510 nm. Branched TiO2 NRAs exhibit excellent photoelectrochemical properties compared to the pure TiO2 NRAs, as revealed by photoelectrochemical measurements. This enhanced photoelectrochemical properties is induced by the increased surface area and expanded optical absorption range. Due to their favorable characteristics, these novel branched TiO2 NRAs will provide a new path to the fabrication of hierarchical nanostructured materials.

Efficient and Stable Silicon Photocathodes Coated with Vertically Standing Nano-MoS2 Films for Solar Hydrogen Production.

Water splitting in a photoelectrochemical cell, which converts sunlight into hydrogen energy, has recently received intense research. Silicon is suitable as a viable light-harvesting material for constructing such cell; however, there is a need to improve its stability and explore a cheap and efficient cocatalyst. Here we fabricate highly efficient and stable photocathodes by integrating crystalline MoS2 catalyst with ∼2 nm Al2O3 protected n+p-Si. Al2O3 acts as a protective and passivative layer of the Si surface, while the sputtering method using a MoS2 target along with a postannealing leads to a vertically standing, conformal, and crystalline nano-MoS2 layer on Al2O3/n+p-Si photocathode. Efficient (0.4 V vs RHE onset potential and 35.6 mA/cm2 saturated photocurrent measured under 100 mA/cm2 Xe lamp illumination) and stable (above 120 h continuous water splitting) photocathode was obtained, which opens the door for the MoS2 catalyst to be applied in photoelectrochemical hydrogen evolution in a facile and scalable way.

Improved growth rate of anodized TiO2 nanotube arrays under reduced pressure field and light illumination.

The one-dimensional titanium oxide (TiO2) nanotubes (TONT) can be rationally fabricated in the fluoride-containing electrolyte by electrochemical anodization. The high-speed growth of TONT for elongated nanotubes is highly desirable because the undesirable chemical etching will induce "nanograss" on the top of nanotubes and restrain the continued elongation of nanotubes. Herein, the external fields were employed to accelerate the growth of TONTs and obtain the elongated TONT arrays. A growth rate up to 18µm/h was achieved under the presence of reduced pressure (0.07MPa) and UV light (365nm) stimulation. The generation of longer nanotube arrays could be attributed to the applied fields, which facilitate timely gas pumping out and induce chemical equilibrium shift forward. The TONT films obtained under different parameters were subsequently employed as anodes for photoelectrochemical (PEC) water splitting. The photocurrent (at 0V vs Ag/AgCl) of TONT electrode obtained under external fields represented a 50% enhancement compared with the photoanode produced by the conventional method.

The interlace of Bi2S3 nanowires with TiO2 nanorods: An effective strategy for high photoelectrochemical performance.

A netlike heterostructure is constructed by interlacing the Bi2S3 nanowires with well-aligned TiO2 nanorod arrays via a facile and effective solvothermal method. The winding Bi2S3 nanowires with several hundred nanometers long and 20-30nm wide are distributed in the interspace of TiO2 nanorods and cross-linked with these nanorods reducing the isolation of nanorods. The photoelectrochemical characterizations show that in addition to the high stability in air without any encapsulation, the netlike heterostructure exhibits an enhanced photoelectrochemical performance compared with TiO2 nanorods and controlled Bi2S3/TiO2 nanoparticle structure. The dual roles of Bi2S3 nanowires (1) as sensitizer for the enlargement of photoresponse range and (2) as multiple electron transport channels facilitating the fast separation of photogenerated electron-hole pairs are considered as key factors for the high energy conversion efficiency of 2.96%. This facile synthesis method offers an attractive strategy to further improve the photoelectrochemical performance of semiconductors and undoubtedly shows promising applications in solar conversion and storage devices.

One-step hydrothermal synthesis of a porous Cu2O film and its photoelectrochemical properties.

A cuprous oxide (Cu2O) film with a novel porous structure is successfully synthesized on Cu foil by using a simple hydrothermal method. A redox reaction occurs between Cu and Cu(2+) in aqueous solution to form the Cu2O film. The porous structures are formed as a result of the Ostwald ripening mechanism. In addition, photoluminescence measurements indicate that the porous Cu2O film may reduce the recombination of photogenerated electrons and holes. The UV/Vis absorption property of the porous Cu2O film is better than that of the granular Cu2O film, and its high photocurrent is expected to make the porous Cu2O film more suitable for solar energy applications.