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.     

Large scale compatible fabrication of gold capped titanium dioxide nanoantennas using a shadowing effect for photoelectrochemical water splitting

In this paper, a visible light driven plasmonic based photoelectrochemical water splitting (PEC-WS) cell is designed with an elegant two-step fabrication route. First, titanium dioxide (TiO₂) nanowires (NWs) were synthesized using the hydrothermal method. Then, angled deposition was used to selectively coat the tips of the NWs yielding Au-capped TiO₂ NWs with multiple sizes and shapes. The provided randomness leads to a multi-resonant system where the superposition of these resonance modes leads to an overall broadband absorption. The excited localized surface resonance (LSPR) modes contribute to the performance enhancement of the cell via near field effects and a hot electron injection mechanism. Moreover, these nanotips can trigger the formation of Fabry-Pérot (FP) cavity modes. The combination of the above-mentioned mechanisms leads to a high performance visible light driven plasmonic cell. At an applied potential of 1.23 V vs RHE, a photocurrent value as high as 82 μA/cm² is acquired for the plasmonic based photoanode. The proposed design strategy is a large scale compatible route with no material restriction. Therefore, vast variety of semiconductor-metal pairs can be fabricated to obtain highly efficient water splitting cell for hydrogen generation.     

Optimization hydrogen production over visible light-driven titania-supported bimetallic photocatalyst from water photosplitting in tandem photoelectrochemical cell

Solar hydrogen production was investigated over a Cu-Ni doped TiO₂ photocatalyst from water photosplitting in a tandem photoelectrochemical cell, which was made up by connecting a modified photoelectrochemical cell to dye solar cell in a series. A mathematical representation for preparation parameters for hydrogen production was successfully generated. Optimization of hydrogen production was conducted with varying preparation parameters of Cu-Ni doped TiO₂ photocatalyst including molar ratios of water, acetic acid and Cu to titanium tetraisopropoxide. The optimum preparation parameters of photocatalyst was obtained at molar ratios of water, acetic acid and Cu to titanium tetraisopropoxide of 32, 4.9, and 5.9, respectively. Physical and photoelectrochemical characterization revealed that low content of water and Cu decreased the charge transfer resistance and charge carrier recombination rate on Cu-Ni/TiO₂ surface. This is attributed to the better crystallinity and less degree of agglomeration which led to obtain optimum particle size at this condition. Maximum hydrogen production rate of 2.12 mL/cm². h was achieved under the optimum condition using the tandem photoelectrochemical cell in the aqueous KOH and glycerol solution under visible light irradiation (λ > 400 nm).     

Probing plasmonic Ag nanoparticles on TiO2 nanotube arrays electrode for efficient solar water splitting

Decoration of semiconductors with plasmonic nanoparticles provides a new direction for efficient solar water splitting for hydrogen production. Herein, Ag nanoparticles as the plasmonic metal were electrodeposited on a TiO₂ nanotube arrays (TNTA) photoelectrode by controlling deposition-charge density. The resulting Ag/TNTA electrode with 10–50 nm diameter Ag nanoparticles exhibited a higher photocurrent density and hydrogen production rate than a bare TNTA electrode under AM 1.5 irradiation. The origins of this enhancement were explored by analyzing the photoelectrochemical behaviors with the relevant optical properties. The absorption of these Ag/TNTA electrodes in the visible light region increased owing to the surface plasmon resonance (SPR) effect of the Ag nanoparticles, and only low photocurrent densities under visible light irradiation were observed. The overall enhancement is owing to the greater incident photon-to-electron conversion efficiency in the 300–400 nm range that is more dependent on the interfacial charge transfer from TNTA to Ag nanoparticles. The localized electronic field of the SPR also reduces the electron transport time in the Ag/TNTA electrodes.     

Ag-doped TiO2 photocatalysts with effective charge transfer for highly efficient hydrogen production through water splitting

The development of efficient metal doped semiconductors for solar energy harvesting to produce hydrogen has attracted significant attention. Herein, the H₂ generation over Ag-doped TiO₂ photocatalyst, synthesized using a simple and cost-effective method based on chemical reduction, was reported. The Ag/TiO₂ exhibited an absorption peak in the visible region and the reduction of the bandgap to 2.5 eV due to surface plasmonic resonance (SPR). X-ray photoelectron spectroscopy revealed the presence of oxygen vacancies and 11% of Ag in Ti–Ag–O phase. The effect of reaction time and photocatalyst loading in the absence and presence of sacrificial reagents (alcohols and sulfur) on water splitting was studied and compared the activity of Ag/TiO₂ with that of bare TiO₂. The H₂ production rate of 23.5 mmol g⁻¹ h⁻¹ (with an apparent quantum yield of 19%), over 1.5Ag/TiO₂, was the highest ever reported so far. The observed higher activity could mainly be attributed to the existence of oxygen vacancies and the Ti–Ag–O phase. The photocatalyst was stable for three consecutive cycles in both the presence and absence of sacrificial reagents. This study offers new insights into the rational design of metal-support hybrid structures for hydrogen production through photocatalytic water splitting.     

Fabrication and photoelectrochemical study of Ag@TiO2 nanoparticle thin film electrode

Ag@TiO₂ nanoparticle thin film was fabricated for photoelectrochemical water splitting in the visible light region. Under the irradiation of UV light, positive photocurrent was enhanced in both electrolytes of 0.1 M HNO₃ and 0.1 M NaOH owing to the excitation of photoelectrons within the TiO₂ shells. However, under the irradiation of visible light, the enhancement of positive photocurrent was observed only in 0.1 M HNO₃ because of the formation of a Schottky barrier band bending at the Ag-TiO₂ core-shell interface and the generation of photoelectrons resulted from the surface plasmon resonance of Ag cores. In 0.1 M NaOH, significant negative photocurrent was enhanced due to the influences of higher pH on the surface state and energy level of TiO₂ shells. Such a visible light-induced photoresponse enhancement and photocurrent direction switching made the Ag@TiO₂ nanoparticle thin film useful not only as a photoelectrode for water splitting but also as a photo-switch in a basic electrolyte.     

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.     

Oxygen vacancies mediated in-situ growth of noble-metal (Ag,Au, Pt) nanoparticles on 3D TiO2 hierarchical spheres for efficient photocatalytic hydrogen evolution from water splitting

Defect engineering is effective to extend the light absorption range of TiO₂. However, the oxygen vacancy defects in TiO₂ may serve as recombination centers, hampering the separation and transfer of photo-generated charges. Here, we present a strategy of in-situ depositing noble-metal (M = Ag, Au or Pt) nanoparticles (NPs) on defective 3D TiO₂ hierarchical spheres (THS) with large surface area through the redox reaction between metal ions in solution and the electrons trapped at oxygen vacancies in THS. The oxygen vacancies at the THS surface are consumed, resulting in direct contact between TiO₂ and noble-metal NPs, while the other oxygen vacancies in the bulk are retained to promote visible light absorption. The noble-metal NPs with well-controlled size and distribution throughout the porous hierarchical structure not only facilitate the generation of electron-hole pairs in THS due to the effect of surface plasmon-induced resonance energy transfer (SPRET) from noble-metal NPs to TiO₂, but also expediate the electron transfer from TiO₂ to noble-metal NPs due to the Schottky junction at the TiO₂/M interface. Therefore, THS-M shows improved photocatalytic performance in water splitting compared to THS. The optimum performance is achieved on THS-Pt (13.16 mmol h⁻¹g⁻¹) under full-spectrum (UV–Vis) irradiation but on THS-Au (1.49 mmol h⁻¹g⁻¹) under visible-light irradiation. The underlying mechanisms are proposed from the surface plasmon resonance of noble-metal NPs as well as the Schottky junction at the TiO₂/M interface.     

NiO decorated Mo:BiVO4 photoanode with enhanced visible-light photoelectrochemical activity

Photoelectrochemical water splitting has attracted increasing attention recently in the perspective of clean and sustainable energy economy. Herein, we reported the synthesis of NiO functionalized Mo doped BiVO₄ (denoted as NiO/Mo:BiVO₄) nanobelts and their enhanced photoelectrochemical activity for efficient water oxidation. The prepared NiO/Mo:BiVO₄ p–n junction structure showed much higher water splitting activity than the pristine BiVO₄ and Mo:BiVO₄. Such obvious enhancement are due to the increased donor density by doping with Mo and fast separation of the photoexcited electron–hole pairs by the novel p–n junction composite structure. Under light irradiation, photoexcited holes in the conduction band (CB) of Mo:BiVO₄ will conveniently transfer to the p-type NiO with the effect of the inner electric field. Meanwhile, the holes would oxidize the water into oxygen and the electrons transfer to the counter electrode (Pt electrode) to produce hydrogen. This novel p–n junction structure could open up new opportunities to develop high-performance photoanode for water splitting.     

Silver/titania nanocomposite-modified photoelectrodes for photoelectrocatalytic methanol oxidation

Silver deposited titania (Ag/TiO₂) nanocomposite thin films were fabricated by the simple sonochemical deposition of Ag on preformed aerosol-assisted chemical vapor deposited TiO₂ thin films. The photelectrocatalytic performance of a newly fabricated Ag/TiO₂-modified photoelectrode was studied for methanol oxidation under simulated solar AM 1.5G irradiation (100 mW/cm²). The Ag/TiO₂-modified photoelectrode showed a photocurrent density of 1 mA/cm², which is four times that of an unmodified TiO₂ photoelectrode. The modification of Ag on the TiO₂ surface significantly enhanced the photoelectrocatalytic performance by improving the interfacial charge transfer processes, which minimized the charge recombination. Density functional theory (DFT) calculation studies revealed that methanol could be easily adsorbed onto the Ag surfaces of Ag/TiO₂ via a partial electron transfer from Ag to methanol. The newly fabricated Ag/TiO₂-modified photoelectrode could be a promising candidate for photoelectrochemical applications.     

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.     

Three-dimensional ZnO@TiO2 core-shell nanostructures decorated with plasmonic Au nanoparticles for promoting photoelectrochemical water splitting

A novel three-dimensional (3D) core-shell nanostructure decorated with plasmonic Au nanoparticles (NPs) was prepared for photoelectrochemical water splitting. In the new nanostructure, ZnO nanorods (NRs) are perpendicular to ZnO nanosheets (NSs), and the ZnO NSs-NRs are coated with a thin TiO₂ shell formed by liquid phase deposition. The plasmonic Au NPs were formed in situ on the surface of ZnO NSs-NRs@TiO₂ by thermal reduction. A thin TiO₂ shell and uniformly distributed Au NPs were successfully obtained. The photoconversion efficiency and photocurrent density of the 3D ZnO NSs-NRs@TiO₂-Au nanostructure respectively reached 0.48% and 1.73 mA cm⁻² at 1.23 V vs. reversible hydrogen electrode, 4.80 and 4.33 times higher than those of ZnO NSs, respectively. The thin TiO₂ shell effectively promoted charge separation, while the surface plasmon resonance effects of the Au NPs improved the photocurrent density. The findings suggest that the 3D ZnO NSs-NRs@TiO₂-Au nanostructure is a promising photoanode for photoelectrochemical water splitting.     

Effects of silver particles on the photovoltaic properties of dye-sensitized TiO2 thin films

To evaluate the possibility of using the plasmon resonance effect to enhance the efficiency of photochemical cells, cis-(SCN)₂Bis(2,2′-bipyridyl-4,4′-dicarboxylate) ruthenium (II) dye-sensitized cells were used to measure the photoresponse of TiO₂ film electrodes before and after deposition of Ag particles. The deposited Ag particles created a film with Ag islands. We found that the photoresponse in the visible region increased as the mass-equivalent Ag-island film thickness, t_Ag, increased to 3.3 nm, but decreased when t_Ag was further increased to 6 nm. On the other hand, compared with bare TiO₂ films, the photoresponse in the UV region decreased for any level of Ag islands. These results suggest that under proper conditions, enhancement of the optical absorption of the dye by the Ag plasmon resonance effect contributes to the photocurrent, and indicates the possibility of improving the energy conversion efficiency of photoelectrochemical cells with Ag-island films.     

Investigation of plasmonic Cu with controlled diameter over TiO2 photoelectrode for solar-to-hydrogen conversion

Plasmonic metal nanoparticles (NPs) have been used to improve the solar-to-hydrogen conversion efficiency. Relative to Au and Ag, Cu is cheaper and more abundant. In the present work, Cu NPs with the controlled diameter were deposited on TiO₂ nanotube arrays (TNTAs) by using a pulse electrochemical deposition method. When the deposition was cycled 3600 times, the size of Cu NPs can be tuned to approximately 30 nm with the most uniform distribution, resulting in the remarkable characteristic peak of surface plasmon resonance and higher photocurrent density. The hydrogen production rates remained unchanged during irradiation (AM 1.5, 100 mW/cm²) of 2 h, indicating a good stability of the resultant Cu/TNTAs electrode. The photoelectrochemical performances of as-prepared Cu/TNTAs can also be comparable to those of Ag/TNTAs electrode fabricated by the same method.     

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.     

Boosting the visible-light photoelectrochemical performance of C3N4 by coupling with TiO2 and carbon nanotubes: An organic/inorganic hybrid photocatalyst nanocomposite for photoelectrochemical water spitting

Developing photoanode with proficient sunlight harvesting, stability as well as enhancing the electron injection across the interface remains a major challenge in the photoelectrochemical water splitting strategy to generate hydrogen. Herein, we design and fabricate an organic/inorganic TiO₂/C₃N₄/CNT photoanode by a hydrothermal technique which exhibits much enhanced photoelectrochemical properties. The TiO₂/C₃N₄/CNT photoanode exhibits a photocurrent density of 2.94 mA/cm², which is ~6.4 time higher than pristine graphitic carbon nitride (C₃N₄) at an applied bias potential of 0.6 V vs. Ag/AgCl. The excellent photoelectrochemical performance benefits from the impactful migration of photo-induced electrons at the TiO₂/C₃N₄ interface from C₃N₄ to TiO₂ and their intimate interface contact with CNT. Kelvin probe force microscopy result shows a smaller interface barrier height (~10 meV) between TiO₂ and C₃N₄, suggesting that electrons transport is favored through TiO₂/C₃N₄ interfaces in a ternary photoanode. The TiO₂/C₃N₄/CNT photoanode exhibited an onset potential of 0.25 V vs. Ag/AgCl which is much lower compared to pristine C₃N_4. The electrochemical impedance spectroscopy results also confirmed the enhanced electron injection across the interface in a ternary photoanode. These results demonstrate a promising approach to develop a highly proficient and visible light active photoanode with excellent stability for renewable energy applications.     

Photoelectrochemical application of mesoporous TiO2/WO3 nanohoneycomb prepared by sol–gel method

A mesoporous TiO₂/WO₃ nanohoneycomb at a molar ratio of 3:1 was prepared by sol–gel method for photoelectrochemical splitting of water. In order to create a highly porous structure, the composite TiO₂/WO₃ with a block copolymer internal template was deposited on the substrate covered with polystyrene (PS) nanospheres. A mesoporous TiO₂/WO₃ composite nanohoneycomb was obtained after removing the PS spheres and copolymer by thermal treatment. It exhibited a lower band gap energy than TiO₂ so that the optical absorption edge was shifted toward the visible light region. It also showed a better photoelectrochemical efficiency of water splitting and higher production of hydrogen due to lower energy gap, higher reactive surface area, and better charge separation efficiency.     

Photoelectrochemical water splitting with black Ni/Si-doped TiO2 nanostructures

Black Ni/Si-doped TiO₂ nanostructures were successfully fabricated through electrochemical anodization of Ti–1Ni–5Si alloy and Sn-reduction treatment. After Sn-reduction treatment, the band gap of Ni/Si-doped TiO₂ became much smaller compared to other samples, which can be attributed to the synergetic effect of Ni/Si doping and the large quantity of Ti³⁺/oxygen vacancy species induced by Sn-reduction treatment. The black Ni/Si-doped TiO₂ nanostructures exhibited a remarkable enhancement in the photoelectrochemical (PEC) water splitting in comparison with the pure TiO₂ and Ni/Si-doped TiO₂. The highest photocurrent reached 2.15 mA/cm² (at 0 V versus Ag/AgCl), corresponding to a conversion efficiency (~1.10%) which was 5.8 times that of the pure TiO₂ nanotubes. The first-principles calculations using density-functional theory (DFT) showed that ion doping and self-doped Ti³⁺ defect levels in the forbidden gap induced through Sn-reduction treatment could improve the mobility of photogenerated carriers and suppress charge recombination, which was in well agreement with the experimental results.     

Plasmonic Au nanoparticle-decorated Bi2Se3 nanoflowers with outstanding electrocatalytic performance for hydrogen evolution

Hydrogen production from water splitting through electrocatalytic or photoelectrochemical route shows great potential for renewable energy conversion. Herein, the plasmon-enhanced photoelectrical nanocatalysts (NCs) have been successfully developed by Au nanoparticle-decorated Bi₂Se₃ nanoflowers (Au@Bi₂Se₃ NFs) as catalysts for hydrogen evolution reaction (HER), leading to a more than 3-fold increase of current under excitation of Au localized surface plasmon resonance (LSPR) and affording a markedly decreased overpotential of 375 mV at a current density of 10 mA cm⁻². The HER enhancement can be largely attributed to effective electron-charge separation and the increase of carrier density in Bi₂Se₃ induced by the injection of hot electrons of Au nanoparticles. Meanwhile, Bi₂Se₃ nanoflowers (NFs), a kind of topological insulators, possess gapless edges on boundary and show metallic character on surface, providing a path for the flow of electrons in the electrocatalytic system. This study opens up a new avenue towards the design of higher energy conversion catalytic water splitting systems with the assistance of light energy, which could increase of HER catalysis efficiency by plasmonic excitation.     

Hetero-nanostructured metal oxide-based hybrid photocatalysts for enhanced photoelectrochemical water splitting – A review

This review is mainly focused on nanostructured metal oxide-based efficient photocatalysts for photoelectrochemical (PEC) water splitting applications. Owing to their distinctive physical and chemical properties, metal-oxide nanostructures have attracted a wide research interest for solar power-stimulated water splitting applications. Hydrogen generation by solar energy-assisted water splitting is a clean and eco-friendly route that can solve the energy crisis and play a significant role in efforts to save the environment. In this review, synthesis strategies, control of morphology, band-gap properties, and photocatalytic application of solar water splitting using hierarchical hetero-nanostructured metal oxide-based photocatalysts, such as titanium dioxide (TiO₂), zinc oxide (ZnO), and tungsten/wolfram trioxide (WO₃), are discussed.