A ternary Ag/TiO2/CNT photoanode for efficient photoelectrochemical water splitting under visible light irradiation

A ternary Ag/TiO₂/CNT photoanode was prepared by grafting Ag nanoparticles on the surface of as-synthesized TiO₂/CNT nanocomposite for the photoelectrochemical (PEC) water splitting under visible light irradiance. The ternary composite photoanode was observed to generate four times higher photocurrent density compared to binary TiO₂/CNT nanocomposite under visible light irradiance. The Ag nanoparticles on the surface of nanocomposite act as a surface plasmon resonance (SPR) photosensitizer under visible light. The enhanced photocurrent density of Ag/TiO₂/CNT ternary photoanode is attributed to the increased light absorption in the visible region, decrease in band-bending and effective interfacial electron transfer due to the synergetic effect of Ag nanoparticles and CNTs. The enhanced charge transfer within the Ag/TiO₂/CNT was also confirmed by the electrochemical impedance spectroscopy. This work demonstrates a feasible route to improve the PEC performance of TiO₂ towards water splitting under sunlight irradiation.     

In situ synthesis of MoO3/Ag/TiO2 nanotube arrays for enhancement of visible-light photoelectrochemical performance

A ternary composites of MoO₃/Ag/TiO₂ nanotube arrays were synthesized by in-situ annealing of TiO₂ nanotube arrays impregnated with AgNO₃ over MoO₃ powders. During the annealing process, the crystallization of the TiO₂ nanotubes, the thermo-decomposition of AgNO₃ to Ag nanoparticles, and the sublimation of MoO₃ occur jointly. The photoelectrochemical measurements of the resultants indicate that MoO₃/Ag/TiO₂ nanotube arrays present better photoelectrochemical properties compared with Ag/TiO₂ nanotube arrays and pristine TiO₂ nanotube arrays. Especially, the highest photocurrent and open circuit voltage are up to 21.29 μA/cm² and 0.058 V under visible light irradiation, whereas 1.77 and 3.87 times larger than those of TiO₂ nanotube arrays, respectively. Superior photoelectrochemical stability and larger photo-conversion efficiency of the ternary composites are also demonstrated. The improved photoelectrochemical properties are related to the close interfacial contact among MoO₃, Ag, and TiO₂ as well as the surface plasma resonance of Ag in the ternary composites, which broaden the range of light response and enhance the efficiency of charge separation. This study provides a skillful solution to construct TiO₂-based composite materials and demonstrates it is an unique architecture to promote the visible light driven photocatalytic application of TiO₂.     

In2O3:Sn/TiO2/CdS heterojunction nanowire array photoanode in photoelectrochemical cells

The design of photoanode with highly efficient light harvesting and charge collection properties is important in photoelectrochemical (PEC) cell performance for hydrogen production. Here, we report the hierarchical In₂O₃:Sn/TiO₂/CdS heterojunction nanowire array photoanode (ITO/TiO₂/CdS-nanowire array photoanode) as it provides a short travel distance for charge carrier and long light absorption pathway by scattering effect. In addition, optical properties and device performance of the ITO/TiO₂/CdS-nanowire array photoanode were compared with the TiO₂ nanoparticle/CdS photoanode. The photocatalytic properties for water splitting were measured in the presence of sacrificial agent such as SO₃²⁻ and S²⁻ ions. Under illumination (AM 1.5G, 100 mW/cm²), ITO/TiO₂/CdS-nanowire array photoanode exhibits a photocurrent density of 8.36 mA/cm² at 0 V versus Ag/AgCl, which is four times higher than the TiO₂ nanoparticle/CdS photoanode. The maximum applied bias photon-to-current efficiency for the ITO/TiO₂/CdS-nanowire array and the TiO₂ nanoparticle/CdS photoanode were 3.33% and 2.09%, respectively. The improved light harvesting and the charge collection properties due to the increased light absorption pathway and reduced electron travel distance by ITO nanowire lead to enhancement of PEC performance.     

Enhanced photoelectrochemical water splitting using gadolinium doped titanium dioxide nanorod array photoanodes

In this study highly oriented, rutile phase one dimensional Titania nanorod array (TiO₂ NRA) modified by gadolinium doping were synthesized on the conductive glass substrate (FTO) by the hydrothermal method. The effect of Gd doping on the photoelectrochemical performance of TiO₂ NRA was investigated. Crystal phase, structural, morphological and composition characteristics of these synthesized photoelectrodes were analyzed by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), energy dispersive spectroscopy (EDS), and atomic force microscopy (AFM). FE-SEM images clearly show that some of the Gd dopant is uniformly distributed on the surface of TiO₂ NRA in the form Gadolinia (Gd₂O₃) microsphere. These gadolinia microsphere play an important role in reducing the surface recombination of electron and hole supported by photoluminescence's studies. Linear sweep voltammetry results show that Gd doping results in a two-fold increase in photocurrent density as compared to pristine TiO₂ NRA. UV–visible spectra, and Mott-Schotty measurements show that Gd doping shift the flat-band potential of TiO₂ NRA more toward negative potential that results in effective charge separation and transportation in the Gd doped TiO₂ NRA (Gd@TiO₂ NRA). Applied biased photon to current efficiency (ABPE) equation was used to find solar to hydrogen efficiency (STH). Gd@TiO₂ NRA show optimum conversion efficiency of ~0.64% at 0.03 V vs Ag/AgCl, while pristine TiO₂ NRA display ~0.33% at −0.21 V vs Ag/AgCl.     

TiO2 nanoparticles modified with ZnIn2S4 nanosheets and Co-Pi groups: Type II heterojunction and cocatalysts coexisted photoanode for efficient photoelectrochemical water splitting

The optimization of photoelectrode is the key issue for the efficient photoelectrochemical water splitting process. In this work, the TiO₂ photoanode is synthesized and modified with ZnIn₂S₄ nanosheets and Co-Pi cocatalyst (TiO₂/ZnIn₂S₄/Co-Pi) for a favorable photoelectrochemical performance. The synthesis and modification process of the TiO₂ photoanode are optimized. The physical and chemical characterizations indicate that the TiO₂ has a nano-cauliflower-like structure and rutile crystal form modified with a network hexagonal ZnIn₂S₄ nanosheets and amorphous Co-Pi groups. After optimization of the hydrothermal and annealing process, the optimized TiO₂ photoanode manifests a photocurrent density of 1.82 mA cm^?2, 1.73-fold of the pristine TiO₂ photoanode (1.05 mA cm^?2). With the surficial ZnIn₂S₄ and Co-Pi modification, the photocurrent density of the TiO₂/ZnIn₂S₄/Co-Pi photoanode is raised to 5.05 mA cm^?2, 5.32-fold of the optimized TiO₂ photoanode (1.82 mA cm^?2). The applied bias photon-to-current efficiency, the charge separation and injection efficiencies of the TiO₂/ZnIn₂S₄/Co-Pi photoanodes are 8.79, 3.40, and 1.64-folds of the optimized TiO₂ photoanode. Combined the Tauc plot, valence band XPS spectra, EIS and Mott-Schottky analysis, the PEC water splitting mechanism could be that: (i) the type II heterojunction formed by the TiO₂ and ZnIn₂S₄ semiconductors improves the charge separation/injection efficiencies; (ii) the Co-Pi groups facilitate the oxygen evolution kinetics; (iii) the Co-Pi groups and 2D ZnIn₂S₄ nanosheets synergistically enhance the charge separation efficiency. This investigation could offer a prospect of practical implementation for photoelectrochemical water splitting.     

Simultaneously efficient light absorption and charge transport of CdS/TiO2 nanotube array toward improved photoelectrochemical performance

CdS has been widely used to modify TiO₂-based photoanodes for photoelectrochemical (PEC) water splitting. Due to the poor interface contact between chalcogenides and oxides, however, such CdS modified TiO₂ materials usually exhibit inefficient separation and transport of charges, leading to an unsatisfactory efficiency during the PEC water splitting process. Addressing this issue, we herein report a CdS/TiO₂ nanotube array (CdS/TNA) photoanode that was fabricated through a successive ion layer absorption and reaction (SILAR) method with an additional subsequent annealing. This post-annealing process is essential to enhance the interface contact between the CdS and the TNAs, resulting in an accelerated transfer of photogenerated electrons from the CdS to the TNAs. In addition, the post-annealing also improves the light absorption capability of the CdS/TNA photoanode. The simultaneous enhancement of charge transport and light absorption provided by the post-annealing is essential for improving the PEC performance of the CdS/TNA photoanode. The CdS/TNA photoanode obtained by this strategy exhibits a much enhanced PEC performance in water splitting, and its photocurrent density and solar-to-hydrogen conversion efficiency could reach 4.56 mA cm⁻² at 1.23 V vs. reversible hydrogen electrode and 5.61%, respectively. This simple but effective route can provide a general strategy for obtaining high-performance oxide-based photoelectrodes.     

An efficient tandem photoelectrochemical cell composed of FeOOH/TiO2/BiVO4 and Cu2O for self-driven solar water splitting

An integrated solar water splitting tandem cell without external bias was designed using a FeOOH modified TiO₂/BiVO₄ photoanode as a photoanode and p-Cu₂O as a photocathode in this study. An apparent photocurrent (0.37 mA/cm² at operating voltage of +0.36 V_RHE) for the tandem cell without applied bias was measured, which is corresponding to a photoconversion efficiency of 0.46%. Besides, the photocurrent of FeOOH modified TiO₂/BiVO₄–Cu₂O is much higher than the operating point given by pure BiVO₄ and Cu₂O photocathode (∼0.07 mA/cm² at +0.42 V_RHE). Then we established a FeOOH modified TiO₂/BiVO₄–Cu₂O two-electrode system and measured the current density-voltage curves under AM 1.5G illumination. The unassisted photocurrent density is 0.12 mA/cm⁻² and the corresponding amounts of hydrogen and oxygen evolved by the tandem PEC cell without bias are 2.36 μmol/cm² and 1.09 μmol/cm² after testing for 2.5 h. The photoelectrochemical (PEC) properties of the FeOOH modified TiO₂/BiVO₄ photoanode were further studied to demonstrate the electrons transport process of solar water splitting. This aspect provides a fundamental challenge to establish an unbiased and stabilized photoelectrochemical (PEC) solar water splitting tandem cell with higher solar-to-hydrogen efficiency.     

A molybdenum dithiolene complex as a potential photosensitiser for photoelectrochemical cells

Molybdenum dithiolene complexes with the general formula [MoTp*(NO)(L)], where Tp* = tris(3,5-dimethylpyrazolyl)hydroborate and L = toluene-3,4-dithiolate (L1), 1,2-benzenedithiolate (L2), or 3,6-dichloro-1,2-benzenedithiolate (L3), were found to exhibit the chemical and physical properties required for a photosensitiser in a photoelectrochemical cell. These complexes were characterised using micro-elemental, spectroscopic (IR and UV–vis) and electrochemical analyses. Cyclic voltammetry (CV) was used to determine the oxidation/reduction potentials and to calculate the energy band gap. All of the complexes had an energy band gap in the range 1.45–1.48 eV, which extends far into the visible light region. A TiO₂ thin film to be used as a photoanode for photoelectrochemical cells was prepared using the paste technique on a Fluorine-doped Tin Oxide (FTO) plate and characterised using scanning electron microscope (SEM) and X-ray diffractometer (XRD). The [MoTp*(NO)(L)]-doped TiO₂ photoanodes were analysed photochemically in a 1.0 M NaOH electrolyte solution using SCE reference and platinum counterelectrodes. The [MoTp*(NO)(L3)]-doped TiO₂ photoanode exhibited an increased photoinduced current compared with the undoped TiO₂ photoanode. The Cl atoms on the dithiolene group offered a better interaction between the photosensitiser molecule and the TiO₂ photocatalyst by providing a means for halogen atom-induced chemical bonding. Based on the band edge calculations and the subsequent photocurrent results, these complexes may be potential photosensitisers for use in the preparation of photoelectrodes for photoelectrochemical cells.     

Nickel-doped TiO2 and thiophene-naphthalenediimide copolymer based inorganic/organic nano-heterostructure for the enhanced photoelectrochemical urea oxidation reaction

To overcome the global challenges of energy crises and environmental threats, urea oxidation is a hopeful route to utilize urea-rich wastewater as an energy source for hydrogen production. Herein, we report an inorganic/organic type of nano-heterostructure (NHs–Ni-TiO₂/p-NDIHBT) as a photoanode with excellent urea oxidation efficiency driven by visible light. This heterostructured photoanode consists of nickel (Ni)-doped TiO₂ nanorods (NRs) arrays as an inorganic part and a D-A-D type organic polymer i.e p-NDIHBT as an organic part. The as-prepared photoanode was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The morphological studies of TEM confirm the coating of p-NDIHBT on Ni–TiO₂ NPs (～1 μm). The consequence of heterostructure formation on optical and photoelectrochemical (PEC) properties of photoanode were explored through photoelectrochemical responses under visible light irradiation. The photoelectrochemical activity of Ni–TiO₂ and Ni–TiO₂/p-NDIHBT photoanode from linear sweep voltammetry (LSV) shows the ultrahigh photocurrent density of 0.36 mA/cm² and 2.21 mA/cm², respectively measured at 1.965 V_RHE. Electrochemical impedance spectroscopy (EIS) of both photoanodes shows a highly sensitive nature toward the urea oxidation reaction. The hybrid photoanode also exhibits high photostability, good solar-to-hydrogen conversion efficiency, and high faradaic efficiency for urea oxidation.     

The co-decorated TiO2 nanorod array photoanodes by CdS/CdSe to promote photoelectrochemical water splitting

A cascade structure of TiO₂/CdS/CdSe semiconductor heterojunction is synthesized using a three-step technique of facile hydrothermal growth for the enhancement of the photoelectrochemical performances. The optical and photoelectrochemical properties controlled by the deposition processing parameters have been investigated. It is shown that the patterns of semiconductor heterojunction enlarge the absorption range of solar spectra, and improve the properties of the photogenerated charge carriers describing separation and transportation, and reduce the interface resistance between the photoelectrode and electrolyte comparing with the pure TiO₂ and CdS-decorated TiO₂ nanorod array photoanodes. The higher photocurrent density and photoconversion efficiency are up to 4.23 mA cm⁻² and 4.2%, which are the 4.1 and 25.3 times superior than that of the pure TiO₂ photoanode. The hydrothermal growth time increment of CdSe yields greater photoelectrochemical water splitting performances. The underlying physics mechanisms have been discussed based on forming a type-Ⅱ energy band alignment structure.     

Enhanced photoelectrochemical performance of 2D core-shell WO3/CuWO4 uniform heterojunction via in situ synthesis and modification of Co-Pi co-catalyst

In order to enhance the photoelectrochemical (PEC) performance of tungsten oxide (WO₃), it is critical to overcome the problems of narrow visible light absorption range and low carrier separation efficiency. In this work, we firstly prepared the 2D plate-like WO₃/CuWO₄ uniform core-shell heterojunction through in-situ synthesis method. After modification with the amorphous Co-Pi co-catalyst, the ternary uniform core-shell structure photoanode achieved a photocurrent of 1.4 mA/cm² at 1.23 V vs. RHE, which was about 6.67 and 1.75 times higher than that of pristine WO₃ and 2D uniform core-shell heterojunction, respectively. Furthermore, the onset potential of 2D WO₃/CuWO₄/Co-Pi core-shell heterojunction occurred a negatively shifts of about 20 mV. Experiments illuminated that the enhanced PEC performance of WO₃/CuWO₄/Co-Pi photoanode was attributed to the broader light absorption, reduced carrier transfer barrier and increased carrier separation efficiency. The work provides a strategy of maximizing the advantages of core-shell heterojunction and co-catalyst to achieve effective PEC performance.     

Construction of g-C3N4/BCN two-dimensional heterojunction photoanode for enhanced photoelectrochemical water splitting

Two-dimensional heterojunction g-C₃N₄/BCN was constructed via thermal polymerization process. The formed two-dimensional heterostructure could enhance the interfacial contact area between BCN and porous g-C₃N₄ as well as shorten the photogenerated charge carriers transfer time and distance. The two-dimensional g-C₃N₄/BCN heterojunction photoanode shows enhanced photoelectrochemical (PEC) performance for water splitting under visible-light irradiation, which primarily originates from the improved charge transfer and separation, and prolonged lifetime of electrons. Under the visible light irradiation, the g-C₃N₄/BCN heterojunction sample yields a photocurrent density of ∼0.62 mA cm⁻² at 1.23 V vs. RHE, which is about eight times as many as that of CN (0.08 mA cm⁻²) electrode at the same conditions. In addition, the possible electron transfer model and mechanism of PEC water splitting for H₂ evolution have been discussed.     

Synergistic effect of titanium oxide underlayer and interlayer on zirconium-doped zinc ferrite photoanode for photoelectrochemical water splitting

Here, we report the synergistic effect of dual TiO₂ layers to enhance the PEC performance of Zirconium-doped zinc ferrite (ZZFO) photoanode by improving the charge carrier density and suppressing the photogenerated charge recombination. The TiO₂ underlayer works as a blocking layer to remarkably suppress the back-injection of electrons from the fluorine-doped tin oxide (FTO) leading to reducing the bulk charge recombination. While interlayer TiO₂ improves the bulk charge transfer property of ZZFO photoanodes. The optimal TiO₂ double-layer modified ZZFO photoanode exhibits an enhanced photocurrent of 0.435 mA/cm² at 1.23 V vs. reversible hydrogen electrode (RHE), which is 2.5 times higher than that of the ZZFO photoanode. The effect of each layer was deeply investigated by electrochemical impedance spectroscopy (EIS), intensity-modulated photocurrent spectroscopy (IMPS) and time-resolved photoluminescence studies (TRPL) with the aim of gaining a clear picture of the interface modifications and their impact on the efficiency of the ZZFO photoanode.     

Plasmonic Ag nanoparticles decorated Bi2S3 nanorods and nanoflowers: Their comparative assessment for photoelectrochemical water splitting

In the present work, nanostructures comprising of plasmonic silver (Ag) nanoparticles (NPs) decorated on the hydrothermally grown bismuth sulfide nanorods (Bi₂S₃–NR) and hierarchical nanoflowers (Bi₂S₃NF) have been successfully synthesized by chemical route. We have fabricated the nanostructured photoanodes and demonstrated their enhanced photoelectrochemical (PEC) performance for water splitting applications. Bismuth sulfide nanoflowers have shown appreciable increased PEC activity due to its hierarchical structure as compared to the nanorods facilitated by the faster charge transport due to the availability of more interfacial sites. Among all, Ag/Bi₂S₃NF photoanode has exhibited highest photocurrent density (12.34 mA/cm² at 2 V vs. Ag/AgCl) which is ∼2 fold higher as compared to Bi₂S₃ nanoflowers. A ∼3 fold enhancement in the incident photon-to-current conversion efficiency is achieved by Ag/Bi₂S₃NF photoanode as compared to Bi₂S₃NF photoanode. The remarkable enhancement in the photocurrent density of Bi₂S₃ after grafting metal plasmons is attributed to its enhanced photoresponse, faster transfer of plasmon mediated hot electrons from excited state of Ag NPs to Bi₂S₃ surface and higher separation efficiency of photogenerated charge carriers facilitated by the metal-semiconductor interface. A plausible mechanism is also proposed for the improved PEC water splitting over Ag/Bi₂S₃NF photoanode.     

Synthesis of CuTi-LDH supported on g-C3N4 for electrochemical and photoelectrochemical oxygen evolution reactions

A nanocomposite CuTi layered double hydroxide (LDH) supported on g-C₃N₄ (15 wt% of g-C₃N₄) is facilely synthesized by hydrothermal method. There are electrostatic interactions between positive layers of CuTi-LDH and negatively charged inner g-C₃N₄ sheets. The nanocomposite and its precursors are characterized through various analytical techniques, which affirmed the presence of both g-C₃N₄ and CuTi-LDH characteristic features. The pore-enriched hybrid geometry of CuTi-LDH@g-C₃N₄ with high specific surface area (146 m²/g), and suitable band gap of 2.46 eV enables the nanocomposite to act as both an electrocatalyst and photoelectrocatalyst for oxygen evolution reaction (OER). Both the electrochemical and photoelectrochemical studies are done using 1 M KOH (pH = 13.6) with applied potential of ?0.2 V to 1.5 V vs. Ag/AgCl. The onset potential of CuTi-LDH@g-C₃N₄ for OER appears at η = 0.36 V in dark and η = 0.32 V under visible light illumination of 30 min. Also, Mott-Schottky analysis shows n-type semiconductor behaviour for CuTi-LDH@g-C₃N₄ and its precursors. The photoelectrochemical water oxidation proceeds by charge transfer across a Type II heterojunction formed between the CuTi-LDH and g-C₃N₄ materials.     

Enhanced photoelectrochemical performance of TiO2 through controlled Ar+ ion irradiation: A combined experimental and theoretical study

Here we demonstrate that the performance of TiO₂ electrodes for photoelectrochemical water oxidation can be effectively enhanced through oxygen vacancy doping. The ion irradiation is a simple method to introduce oxygen vacancies on the surface and into the interior of TiO₂ for making oxygen-deficient titania (TiO_2-x). The TiO_2-x thin films exhibit obvious increase in the performance of photoelectrochemical water splitting under light irradiation. The photoconversion efficiency η of the TiO_2-x photoanode is 0.5-fold higher than that of the pristine TiO₂ photoanode, which benefits from the introducing of oxygen vacancies produced by ion irradiation. While the irradiation-induced titanium vacancies act as trapping centers for charge carriers and decrease the photoelectrochemical performance of the samples. Positron annihilation spectroscopy (PAS) was used to study the type of the formed vacancies. Combining experimental with theoretical study, this study demonstrates that ion irradiation technique combing with thermal annealing could be an effective way to enhance the performance of photoanode for water splitting.     

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

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.     

Effect of ferroelectric poling on the photoelectrochemical activity of hematite-BaTiO3 nanowire arrays

Ferroelectric α-Fe₂O₃/BaTiO₃ photoanodes (hematite/BT) were fabricated on FTO and FTO/TiO₂ substrates using a hydrothermal process and spin coating along with thermal treatments. The prepared hematite nanowires had length under 1 μm and the BT film was about 18 nm thick. SEM, TEM and XPS investigations prove the formation of α-Fe₂O₃/BaTiO₃ heterojunction structure. The ferroelectric poling of hematite/BT heterojunction was conducted both in propylene carbonate and in air. The photoelectrochemical performance of hematite/BT photoanodes is strongly influenced by the direction of ferroelectric polarization. The positive poling of the hematite/BT prepared on FTO/TiO₂ substrate produces a 40.4% photocurrent density enhancement, in comparison with not poled version of the sample. Electrochemical impedance spectroscopy measurements provided usefull information regarding the effect of ferroelectric polarization on the charge transfer kinetics at the photoanode/electrolyte interface.     

Core-shell hetero-phase reduced Ti–Ni–O nanotubes photoanode with enhanced optical absorption and charge transport for boosted photoelectrochemical water splitting

Bulk-phase doping and surface oxygen-defective engineering of TiO₂-based nanostructures are identified as effective routes for enhanced photoelectrochemical (PEC) water splitting. Here, we reported a reduced Ti–Ni–O nanotubes photoanode with anatase-rutile crystalline-core and oxygen vavancies amorphous-shell for boosted PEC water splitting. The core-shell hetero-phase reduced Ti–Ni–O nanotubes were fabricated through phase-structure modulation by a thermal treatment of anodized Ti–Ni–O nanotubes on Ti–Ni alloy and with one-step electrochemical reduction. Microstructure, optical and PEC measurement results confirmed effective bulk-phase Ni-doping and surface oxygen vacancies self-doping into the reduced mixed-phase Ti–Ni–O nanotubes, which enabled high capability of optical-absorption and simultaneously favored charge separation-transfer for remarkably improved the PEC water splitting. A higher photocurrent density of 1.66 mA/cm² at 0 V vs. Ag/AgCl and solar-to-hydrogen efficiency of 0.79% were achieved for the reduced Ti–Ni–O system, which was 5.35 and 5.27 times that of undoped TiO₂, respectively. This work may shed an insight view on fabricating high-performance Ti-based nano-photoanodes with enhanced light harvesting and carrier kinetics for efficient PEC water splitting, through synergistic strategy of bulk-phase elements doping and surface oxygen vacancies self-doping.     

Novel CoOx/CdS/TiO2 with multi-shelled porous heterostructure for enhanced charge separation efficiency of photoelectrochemical water splitting

Hydrogen production by solar energy is an efficient and clean approach to fulfill the future energy demand. Herein, a novel multi-shelled porous heterostructure CoO_x/CdS/TiO₂ photoanode was fabricated by the hydrothermal and chemical method. There were more active sites, suitable surface defects and heterojunction structures in the homogeneous-porous-multi-shelled CoO_x/CdS/TiO₂ photoanode. It showed a photocurrent density of 2.89 mA/cm² at 1.23V vs. RHE, which is 2.22 fold of the original TiO₂ photoanode. The heterostructure fabrication of the CdS/TiO₂ could broaden the visible light absorption and enhance the charge separation efficiency. The multi-shelled homogeneous porous structure of the CoO_x/CdS/TiO₂ further enhanced the charge separation efficiency and accelerated the interfacial oxygen evolution kinetics. The mechanism for the enhanced photoelectrochemical water splitting of favorable CoO_x/CdS/TiO₂ photoanode is proposed.