Photoelectrochemical and photocatalytic properties of Ag-loaded BaTiO3/TiO2 heterostructure nanotube arrays

BaTiO₃/TiO₂ (BT) heterostructure nanotube arrays were fabricated by in situ hydrothermal method using TiO₂ nanotubes as both template and reactant. Compared with pure TiO₂ nanotube arrays, the BT heterostructures exhibited enhanced photocurrent under UV light irradiation. For further improving the photoelectrochemical performance, Ag nanoparticles were loaded on the surface of BT heterostructure by two different photo-reduction (Ag/BT-P) and chemical reduction (Ag/BT-C) methods. The results showed that the Ag nanoparticles on Ag/BT-C were uniform and dispersed homogeneously, but the Ag nanoparticles on Ag/BT-P were very large, which resulted in the tailored and integrated nanotube structure destroyed. The electrochemical impedance spectra (EIS) indicated that the impedance arc radius of Ag/BT-C was much smaller than Ag/BT-P and the pure BT nanotube arrays, indicating that the enhanced charge carrier separation was achieved on Ag/BT-C. In addition, the Ag/BT-C nanotube arrays exhibited a higher photocatalytic activity for methylene blue (MB) degradation.     

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.     

Preparation of carbon dots/TiO2 electrodes and their photoelectrochemical activities for water splitting

Carbon dots with various functional groups can be employed as the potential sensitizer. In this study, carbon dots are obtained by electrochemical ablation of graphite rods in alkaline electrolyte. The better preparation condition is the applied potential of 40 V and the ablation time of 5 h. TiO₂ nanotube arrays and TiO₂ nanoparticles photoelectrodes are sensitized by the as-prepared carbon dots through using impregnation method. Carbon dots/TiO₂ nanotube arrays electrodes exhibit greater photoelectrochemical hydrogen production activities than carbon dots/TiO₂ nanoparticles electrodes. It is because more carbon dots can be well combined with TiO₂ nanotube arrays. Based on the IPCE values in visible light region, the role of carbon dots on TiO₂ nanotube arrays electrode depends on the up-converted PL behaviors from their surface states and the alkaline electrolyte. The results provide insight into carbon dots that serve as sensitizer of TiO₂ photoelectrode in water splitting system of alkaline solution.     

Flame-assisted pyrolysis formation of Cu2O/Cu/TiO2 nanotube arrays to boost superior photo-electrochemical response

Herein, a flame-assisted pyrolysis technique is developed for one-step in-situ construction of Cu₂O/Cu/TiO₂ nanotube arrays. The structure and morphology of the obtained samples are systematically characterized. Interestingly, UV–vis absorption spectroscopy reveals that the introduction of Cu₂O and Cu considerably enhance light response of TiO₂ nanotube arrays in the visible region. Moreover, the composited electrode shows enhanced photo-electrochemical activity. Compared with blank TiO₂ nanotube arrays, not only the photocurrent and photo-voltage of composited electrodes are improved but also the stability is also enhanced. And, the maximum photo-conversion efficiency for composited electrode presents 4.71 folds larger than that of blank TiO₂ electrode. This enhancement is due to the faster charge separation/transportation ability of Cu and visible light response of Cu₂O. This research develops a novel and facile method for the in-situ fabrication of Cu₂O/Cu/TiO₂ nanotube arrays and demonstrates their improved utilization of solar energy.     

Enhanced photoelectrocatalytic activity of Bi2S3–TiO2 nanotube arrays hetero-structure under visible light irradiation

Constructing heterosystems by sensitizing a wide band gap semiconductor with a narrow band gap semiconductor is an effective way to improve photocatalytic performance. Bismuth sulfide (Bi₂S₃) has a direct band gap of 1.38 eV and shows great potential in capturing visible light, which makes it a good candidate for photocatatlytic applications. In this work, Bi₂S₃ nanoparticles were efficiently deposited on TiO₂ nanotube arrays (Bi₂S₃-TNTAs) by sequential chemical bath deposition (CBD) method to enhance visible light response of the photocatalytic system. Notably, a high-throughput screening method of scanning photoelectrochemical microscopy (SPECM) was exploited to evaluate photoelectrochemical response of the as-prepared composites and to find out the optimized photocatatlytic system. The effects of Bi₂S₃ nanoparticles on visible light absorption and photoelectrocatalytic hydrogen production rate of the TiO₂-system were investigated in detail. When adopted as photoanode, the optimized heteroelectrode exhibited a more than 13-fold enhancement in hydrogen production rate. The result of electrochemical impedance spectroscopy (EIS) and photoluminescence (PL) shows that photo-generated charges excited under visible light in Bi₂S₃-TNTAs composites are efficiently separated, which gives rise to the superior photoelectrocatalytic performance of the Bi₂S₃-TNTAs photoanodes.     

AgCu/TiO2 hot-electron device for solar water splitting: the mechanisms of LSPR and time-domain characterization

More and more metal/semiconductor nanostructures have been served as a hot-electron device with the localized surface plasmonic resonance (LSPR) effect to boost hydrogen evolution from solar water splitting. In this work, bimetallic AgCu with optimal ratio are deposited onto TiO₂ nanopore/nanotube arrays to construct AgCu/TiO₂ photoanode for photoelectrochemical water splitting, a novel simulation characterization to visualize the LSPR process is proposed. The near electric field enhancement and plasmon resonance energy transfer mechanisms of single Ag and Cu are inferred by time-domain characterization, illustrating the contradictory photocurrent under AM 1.5 illumination with its LSPR effect based on the particle size. The variation of local electric field over time within the interfaces of AgCu bimetals and bimetal/TiO₂ models reveals the migration of hot electrons from Ag into Cu and the synergetic effect of different LSPR mechanisms. The resulting higher photoelectrochemical activities of AgCu/TiO₂ also verifies the positive roles of the coexistence of AgCu on electron generation and energy transfer to interband excitation of TiO₂.     

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.     

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.     

A photoelectrochemical investigation of the hydrogen-evolving doped TiO2 nanotube arrays electrode

The present work investigates the photoelectrochemical behavior of nanotubular N/C-TiO₂ electrode for hydrogen production. Via the sonoelectrochemical anodization process of 1 h, N-containing TiO₂ based nanotube arrays(N-TNT) with the length of about 650 nm were fabricated in fluoride aqueous solution added 0.25 M NH₄NO₃; C-containing TiO₂ based nanotube arrays(C-TNT) with the length of about 2 μm were prepared in fluoride ethylene glycol solution. In virtue of the longer tubes with the larger surface areas, C-TNT can harvest more light and produce more photoactive sites than N-TNT, which also made the charge transfer resistance in C-TNT larger than that in N-TNT. Considered the more negative flat band potential of C-TNT, C-TNT has the smaller energy barrier and the better photoelectrochemical activity. It may be attributed to the appropriate defect concentration gradient owing to the modification of C element. Under UV-vis light (320-780 nm) irradiation, the average hydrogen generation rate of C-TNT was 282 μL h⁻¹ cm⁻². The surface properties and near-surface properties of the resultant electrode were synthetically analyzed by using UV-vis diffuse reflectance spectra(DRS), field emission scanning electron microscopy (FESEM), I-t curves, and electrochemical impedance spectroscopy (EIS) techniques.     

Cu2O/Cu/TiO2 nanotube Ohmic heterojunction arrays with enhanced photocatalytic hydrogen production activity

Cu₂O/Cu/TiO₂ nanotube heterojunction arrays were prepared by assembling Cu@Cu₂O core-shell nanoparticles on TiO₂ nanotube arrays (NTAs) using a facile impregnation-reduction method. SEM and TEM results show that Cu@Cu₂O plate-like nanoparticles with tens of nanometers in size are confined inside TiO₂ NTAs. Only the outmost several nanometers of the nanoparticles are Cu₂O and the predominant inner of the nanoparticles are Cu metals. Cu L₃VV Auger spectra of Cu₂O/Cu/TiO₂ NTAs suggest that Cu metals are enveloped by at least several nanometers Cu₂O on the surface, which further confirms the Cu@Cu₂O core shell structure of Cu nanoparticles. The ability of light absorption of Cu₂O/Cu/TiO₂ NTAs is enhanced. The range of absorption wavelengths changes from 400 to 700 nm due to the surface plasmon response of Cu metals core and Cu₂O nanoparticles shell. The photocatalytic hydrogen production rate of Cu₂O/Cu/TiO₂ heterojunction arrays is enhanced when compared with those of Cu₂O/TiO₂ NTAs and TiO₂ NTAs under UV light. Moreover, a stable H₂ generation property was obtained under visible light (λ gt; 400 nm). The Cu metal core is believed to play a key role in the enhancement of photocatalytic properties of Cu₂O/Cu/TiO₂ nanotube heterojunction arrays.     

Photoelectrochemical hydrogen production at peak efficiency over 10% via PbSe QDs/TiO2 nanotube array photoanodes

Modulating the deposition of quantum dots onto TiO₂ nanotube arrays provides an effective approach for expansion of visible light response, enhancement of separation and interfacial transfer of photoinduced charge carries, and improvement of photoconversion efficiency. In this paper, titanium dioxide nanotube arrays (TNAs) were modified with PbSe quantum dots (QDs) through the successive ionic layer adsorption and reaction (SILAR) under nitrogen to prepare PbSe/TNAs photoanodes. XRD, SEM, HR-TEM, UV–Vis–NIR DRS, PL, XPS, EIS, Tafel plots, Mott–Schottky analyses and J–V characteristics were used to characterize the samples. The deposition of PbSe QDs was tuned by varying the Se/Pb atomic ratios during the SILAR process. Energy band configuration, interfacial contact characteristics and photoelectrochemical properties were dissected and optimized. At the Se/Pb ratio of 5, the sample (5)PbSe/TNAs demonstrated excellent photoelectrochemical performance with a short circuit photocurrent density of 15.45 mA cm^?2 and a corresponding photoconversion efficiency of 10.6%.     

Construction of CdSe@TiO2 core‐shell nanorod arrays by electrochemical deposition for efficient visible light photoelectrochemical performance

The CdSe@TiO₂ core‐shell nanorod arrays for photoelectrochemical (PEC) application were designed and constructed by a facile electrochemical deposition strategy. The CdSe@TiO₂ photoanodes exhibit highly efficient PEC performance under visible light irradiation, among which the CdSe shell layer thickness can be precisely adjusted by different electrodeposition time. In comparison with nude TiO₂ nanorods, the optimized CdSe@TiO₂ photoanode (TC‐500) shows a significant saturated photocurrent density of 2.1 mA/cm² at 0 V (vs Ag/AgCl), which is attributed to the good distribution of CdSe nanoparticles on TiO₂ nanorod arrays, the favorable band alignment, and the intimate interfacial interaction between CdSe nanoparticles and TiO₂ nanorods. The introduction of CdSe shell layer does not only improve light absorption ability but also enhances photogenerated charge carrier's transfer and separation. This current work systematically studies the accurate adjustment of CdSe shell layer thickness on TiO₂ nanorod arrays by electrochemical deposition strategy and provides a paradigm to design and fabricate heterostructure composite for PEC application.     

Deposition of heterojunction of ZnO on hydrogenated TiO2 nanotube arrays by atomic layer deposition for enhanced photoelectrochemical water splitting

The heterojunction of ZnO was deposited on hydrogenated TiO₂ nanotube arrays (H–TiO₂) by atomic layer deposition (ALD) with various cycles. The ZnO was uniformly wrapped with the H–TiO₂ samples and the thickness could be accurately controlled by the cycle numbers of ALD. The higher growth rate ~2.7 Å/cycle was obtained due to the surface amorphous layer, compared with the air-treated samples (A-TiO₂), ~2.3 Å/cycle. When the cycle numbers increased to 200, nanowire arrays appeared. Interestingly, the absorption in the visible light region improved more significantly when ALD ZnO was employed for the H–TiO₂ rather than the A-TiO₂ samples. The H–TiO₂ samples with 42 nm of ALD ZnO exhibited enhanced photoelectrochemical water splitting performances, compared with the A-TiO₂ with 42 nm of ALD ZnO. This was related to the higher degree of the electronic band bending and improved photo-response in the UV and visible light region, resulting from the oxygen vacancies.     

High photoelectrochemical water splitting performance on nitrogen doped double-wall TiO2 nanotube array electrodes

The nitrogen doped double-wall TiO₂ nanotube arrays (N-DW-TiO₂ NTs) have been prepared by a facile two steps electrochemical anodization method, and the nitrogen has been successfully incorporated into the nanotubes in situ anodization process. The unique double walls tubular surface morphology has been achieved by conducting higher anodic voltage in second anodization process than that in the first anodized step. The nitrogen doping and following annealed process in nitrogen atmosphere did not damage the unique, ordered, and vertically aligned structures. Under illumination of simulated solar light (AM 1.5, 100 mW/cm²), the N-DW-TiO₂ NTs presented a high photoelectrochemical water splitting performance, which mainly ascribed to the high surface areas and expended optical absorbance to visible light region. The high surface areas, ordered structure for facilitating electron transfer, and visible light absorbance present the new avenue for improving the solar light application in photoelectrochemical water splitting process for practical hydrogen generation. The N-DW-TiO₂ NTs can be one of promising prototype nanomaterials, and much higher photoconversion efficiency can be expected for the co-doped or sensitized on the N-DW-TiO₂ NTs.     

Enhanced photoelectrochemical-response in highly ordered TiO2 nanotube-arrays anodized in boric acid containing electrolyte

We examine the photoelectrochemical properties of highly ordered titanium dioxide nanotube-array photoanodes, fabricated by anodization of titanium in a nitric acid/hydrofluoric acid electrolyte, with and without the addition of boric acid. Under UV–Vis illumination the photocurrent densities achieved with TiO₂ nanotube-arrays fabricated in the H₃BO₃–HNO₃–HF electrolyte are a factor of seven greater than the TiO₂ nanotube-array samples obtained in the commonly used HNO₃–HF electrolyte, indicating the ability to control the photoelectrochemical response of the highly ordered nanotube arrays by tailoring the electrolyte composition. For 560 nm long boric-acid fabricated nanotube arrays, a photoconversion efficiency of 7.9% is achieved upon a 320–400 nm illumination at an intensity of 98 mW/cm², with hydrogen generated by water photoelectrolysis at the power-time normalized rate of 1708-μmol/h W (42 ml/h W). The resulting nanotube-arrays demonstrate excellent photocorrosion stability, with no detectable degradation in photoconversion properties over 6 months of testing. While the titania bandgap is not suitable for high visible spectrum efficiencies, the high photoconversion efficiency achieved under UV illumination indicates the suitability of the highly ordered nanotube-array architecture for hydrogen generation by water photoelectrolysis.     

Enhanced photo-electrochemical activity of ZnO/Cu2S nanotube arrays photocathodes

ZnO/Cu₂S nanotube arrays are fabricated firstly by a facile and capping-agent-free method, and the photo-electrochemical performance has been studied systematically. The results show that ZnO/Cu₂S nanotube arrays achieve enhanced photo-electrochemical water splitting performance and the photocurrent densities of ZnO/Cu₂S are 7.9 times than that of ZnO at 0 V versus Ag/AgCl. The performance of the ZnO/Cu₂S nanotube arrays can be adjusted by changing the amount of Cu₂S microcrystals. The results confirm that the enhanced photo-electrochemical performance of ZnO/Cu₂S is due to the significantly improved visible light absorption, effective separation of photo-induced carriers due to the well band energy match and the formed p-n junction between ZnO and Cu₂S.     

Effect of cathode material on the morphology and photoelectrochemical properties of vertically oriented TiO2 nanotube arrays

We report, for the first time, on the effect of the cathode material in controlling the morphology and properties of TiO₂ nanotube arrays fabricated by electrochemical anodization of Ti foil in both aqueous and ethylene glycol (EG) electrolytes. Some of the alternative less-expensive cathode materials result in TiO₂ nanotube architectures and photoelectrochemical properties similar to or in some cases superior to those obtained using a Pt cathode.     

Effect of anodization voltage on performance of TiO2 nanotube arrays for hydrogen generation in a two-compartment photoelectrochemical cell

Highly ordered TiO₂ nanotube arrays were prepared by anodic oxidation of Ti foil under different anodization voltages in ethylene glycol electrolyte. The morphology and photoelectrochemical performance of the TiO₂ nanotubes (NTs) samples were characterized by FESEM and electrochemical working station. Hydrogen production was measured by splitting water in the two-compartment photoelectrochemical (PEC) cell without any external applied voltage or sacrificial agent. The results indicated that anodization voltage significantly affects morphology structures, photoelectrochemical properties and hydrogen production of TiO₂ NTs. The pore diameter and layer thickness of TiO₂ samples increased linearly with the anodization voltage, which led to the enhancement of active surface area. Accordingly, the photocurrent response, photoconversion efficiency and hydrogen production of TiO₂ nanotubes were also linearly correlated with the anodization voltage.     

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.     

H2 evolution from a photoelectrochemical cell with n-Cu2O photoelectrode under visible light irradiation

H₂ evolution was observed for the first time from a photoelectrochemical cell using an n-type Cu₂O photoelectrode under visible light irradiation. Three-electrode configuration was used in the photoelectrochemical cell to observe H₂ evolution. AgCl/Ag calomel electrode and a platinum plate were used as the reference and counter electrodes, respectively. Fe²⁺/Fe³⁺ redox couple was used as the electrolyte. H₂ evolution was visible on the platinum electrode in the photoelectrochemical cell.