One-dimensional TiO2 Nanotube Research Articles

Photocatalytic pollutant elimination and hydrogen production over TiO2 NTs/Bi2S3-MoS2 with Z-scheme configuration: Kinetics and mechanism

Functional hybrid materials with unique electron transport path could be fabricated by the construction of heterojunction semiconductors with matched energy band structures, which exhibit excellent photocatalytic capacity in environmental treatment and new energy exploitation. Herein, ternary photocatalysts with Z-scheme heteojunctions were fabricated by Bi2S3-MoS2 deposition on TiO2 nanotube arrays (TiO2 NTs/Bi2S3-MoS2) via a solvothermal method. The design included the advantages of one-dimensional TiO2 nanotubes and high visible light responsive Bi2S3-MoS2 sensitizers, enhancing the solar harvesting, electron transfer and photocatalytic performance. The TiO2 NTs/Bi2S3-MoS2 exhibited outstanding photocatalytic pollutant removal and H2 evolution, and it also showed high visible light-driven photoelectric conversion. Additionally, the photocatalytic mechanism and active species for pollutant degradation were also revealed. In particular, the photocatalyst displayed remarkably high photocatalytic stability after sustained experimental cycles. It is anticipated that the Z-scheme heterogeneous photocatalyst would exhibit glorious achievement in practical applications.

Investigating the thickness-effect of free-standing high aspect-ratio TiO2 nanotube layers on microwave-photoresponse using planar microwave resonators

One-dimensional TiO2 nanotube (TNT) layers are a promising candidate for UV detection due to their distinctive anisotropic geometry which is effective for light harvesting and rapid carrier transport. Here, the photosensitivity efficiency of TNT layers with various thicknesses of 15, 50, 80, and 110 µm was utilized at a microwave frequency regime by modeling and experimentally. A planar microwave split ring resonator (PMSRR) was designed and fabricated to operate at ∼8 GHz to study TNT layers by monitoring the scattering parameter (S21) of the PMSRR under a constant UV irradiation power of ∼96.4 µW/cm2. According to the results, the 80 µm thick TNT layers demonstrated the highest resonant amplitude variation for the customized PMSRR. The change of the resonant amplitude was mainly attributed to the conductivity variation contributed by perturbation of trapped electron concentration, as the dominant factor under UV illumination, and their electromagnetic wave interaction. The main advantage of the proposed method of PMSRR for microwave photosensitivity monitoring over the conventional direct current (DC) conductivity measurements is to eliminate the effect of contact resistance between the TNT layers and metal electrodes utilizing the contactless aspect of wave interactions with the TNT layers at microwave regime to perform electrode-less measurements.

Synergetic effect of CQD and oxygen vacancy to TiO2 photocatalyst for boosting visible photocatalytic NO removal

The wide band gap of TiO2 photocatalyst material limits its application in the field of visible photocatalysis. In this paper, oxygen vacancies and carbon quantum dots (CQD) with up-conversion character were proposed to improve the photocatalytic activity for NO removal of TiO2 under visible light irradiation. The one-dimensional TiO2 nanotube (TNs), TNs containing oxygen vacancies (OVTNs), TNs of composite CQD (CQD-TNs) and OVTNs of composite CQD (CQD-OVTNs) were prepared, respectively. Furthermore, the influence of oxygen vacancies and CQD on the removal of NOx by photocatalysis were explored. It is found that CQD-OVTNs exhibits the conspicuous synergetic effect of CQD and oxygen vacancy to boost visible photocatalytic NO removal, the NO removal efficiency was about 12, 2, and 2.6 times to TNs, OVTNs and CQD-TNs. Also, CQD-OVTNs exhibits the NO2-inhibited property during the process of photocatalytic NO removal. Finally, the synergetic mechanism of CQD and oxygen vacancies to TNs for boosting visible photocatalytic NO removal was revealed.

Improved photocatalytic activity of TiO2 with a regulated covalent organic framework thin film.

In this work, one-dimensional TiO2 nanotube arrays are coupled with a covalent organic framework (COF) thin film with a controlled thickness to form a three-dimensional heterojunction, which exhibits a 3.3-fold higher hydrogen evolution rate than that of TiO2, and becomes active for CO2 conversion, compared to the bare COF. Such high activity results from the large difference in Fermi levels forming an internal electric field at the interface.

A multi-species transport model for one-dimensional TiO2 nanotube array photoanode of photocatalytic fuel cell

One-dimensional TiO2 nanotube array photoanode used in photocatalytic fuel cells exhibits excellent performance. However, a fundamental understanding of multi-species transport coupled with photoelectrochemical reactions in this type of photoanode remains unclear. In this work, a multi-species transport model with the consideration of the electron/hole transport in the nanotube wall, the organics/hydroxyl ion transport in the pore space and the photoelectrochemical reaction at the semiconductor/electrolyte interface is developed for one-dimensional TiO2 nanotube array photoanode. The simulated result is in good agreement with the experimental data. Effects of operation conditions and nanotube structures are also studied. It is found that increasing the organics/hydroxyl ion concentrations improves the photoanode performance because of enhanced mass transport. The increase of illumination intensity allows for more electron-hole pairs to be generated, leading to the improved photoanode performance. Decreasing the wavelength results in the lowered photoanode performance. Thinner nanotube wall thickness yields better performance, while there exists an optimum nanotube layer thickness leading to the optimum performance. The results obtained not only shed light on the characteristics of multi-species transport coupled with photoelectrochemical reactions but also benefits the design and operation of one-dimensional TiO2 nanotube array photoanode.

TiO2 Nanotube Integrated Microwave Resonator UV Sensor

Ultraviolet (UV) irradiation is extensively utilized in numerous applications such as outer space communication, biological disinfection, memory storage, optoelectronic circuits, and biological analysis [1]. Excessive exposure to UV irradiation is deleterious and causes adverse health effects, for instance, premature aging and skin cancer. A rapid and highly sensitive device for the detection of UV is in great demand in various applications. Recently, planar microwave resonator sensors have demonstrated attractive and robust performance providing high sensitivity, real-time response, and low-cost fabrication process [2]. The planar microwave resonators can easily be integrated with nanostructured materials to make them sensitive to UV radiation via absorption and subsequent charge generation [3]. Among various wide bandgap metal oxides such as TiO2, ZnO, SnO2, one-dimensional TiO2 nanotubes (TNTs) are favorable in UV photodetectors as they possess, except intrinsic TiO2 properties high active surface area and their unique hollow geometry enables increased charge trapping and, a direct pathway for rapid transport of photogenerated carriers [4-6]. Therefore, the use of high aspect ratio (HAR) TNTs might offers superior sensing performance in the UV region.In this presentation, the impact of TNT thicknesses on the UV sensitivity of the planar microwave resonator’s response will be investigated. We will demonstrate the use of a high frequency microwave resonator integrated with different thicknesses (50, 80, 100 µm) of TNT membranes. The presented work will aid in selecting an optimized thickness of TNT membranes with a large active surface-to-volume ratio to provide the highest sensitivity to UV irradiation. We expect this investigation to act as basis for expanding the use of HAR TNTs to effectively develop low-cost, easy to use and robust microwave UV sensors in a wide range of applications. Experimental details and recent results will be presented and discussed.

Promoted charge separation and specific surface area via interlacing of N-doped titanium dioxide nanotubes on carbon nitride nanosheets for photocatalytic degradation of Rhodamine B

Abstract Titanium dioxide (TiO2) has been regarded as a promising catalyst owing to its superior charge transport properties in photocatalytic degradation of organic pollutants and photocatalytic hydrogen generation. However, a major bottleneck toward the utilization of TiO2 photocatalysts is inefficient exploitation of visible light and low adsorption behavior. To address this issue, we fabricated a hybrid nanocomposite composed of one-dimensional N-doped TiO2 nanotubes (N-TNTs) and two-dimensional graphitic carbon nitride nanosheets (g-CNNs) to improve photocatalytic behavior. Furthermore, photogenerated electron–hole pairs in the hybrid N-TNT/g-CNN composites were efficiently separated by introducing g-CNNs. In addition, the improved specific surface area provided many active sites, resulting in higher photocatalytic reactions in kinetics. Based on these features, the Rhodamine B photocatalytic degradation efficiency was the highest, ∼85%, under solar-light irradiation in the N-TNT/g-CNN composites (7 wt% of the g-CNN content), which is two times higher than that of the N-TNT. Moreover, excellent durability and stability were observed after four cycles, which can be attributed to the extended optical absorption range and enhanced separation of the photogenerated electron–hole pairs.

One-dimensional suboxide TiO2 nanotubes for electrodics applications

This mini-review summarizes the recent research on the applications of anodic one-dimensional suboxide titania nanostructures with enhanced electrodics properties by introducing atomic-scale defects into the titania systems. Due to the unique semiconductive properties of TiO2 nanostructures, so far, the main focus of research has been on photocatalytic applications of this semiconductor. The limited conductivity of pristine anodized titania restrains the application of one-dimensional (1D) nanostructured TiO2 as an electrode. However, recently published works present that suboxide TiO2 nanotubes, due to the enhanced conductivity, are promising electrodes for electrochemical applications. In this mini-review, we highlight the unique advantages of defective TiO2-x nanotubes as an electrode and address the recent electrodics applications of 1D anodic TiO2 nanotubes.

Gold and Platinum Nanoparticle-Functionalized TiO2 Nanotubes for Photoelectrochemical Glucose Sensing.

Developing stable photoelectrochemistry (PEC) glucose biosensors with high sensitivity and a low detection limit is highly desirable in the biosensor field. Herein, a highly sensitive and stable enzymatic glucose PEC biosensor is rationally designed and fabricated using a TiO2NTs/Au/Pt/GOx electrode. First, we prepared one-dimensional TiO2 nanotube arrays which could realize the orthogonalization of the light-incident direction and the carrier diffusion direction via anodization. Subsequently, we used the method of photoassisted deposition for anchoring Pt nanoparticles on TiO2NTs after electrodepositing Au nanoparticles. Among them, Au nanoparticles promote light absorption via the surface plasmon resonance effect and the separation of photogenerated carriers through forming a Schottky junction. Moreover, the Pt nanoparticles on the electrode surface can react with hydrogen peroxide (H2O2) generated from glucose (Glu) oxidation by glucose oxidase (GOx), accelerating the electron-transfer process during glucose oxidation and greatly improving the sensitivity of the glucose biosensor. As a result, TiO2NTs/Au/Pt/GOx exhibited excellent PEC performance, achieving a high sensitivity of 81.93 μA mM–1 cm–2 and a low detection limit (1.39 μM), far exceeding the performance of TiO2NTs/M/GOx (M = Au, Pt). Therefore, the introduction of Pt nanoparticles as active substances to promote enzymatic reactions is important for designing high-performance enzyme biosensors.

Controlling the geometric design of anodic 1D TiO2 nanotubes for the electrochemical reduction of 2,4,6-trinitrotoluene in ambient conditions

In this study, we investigated how the geometric design of anodic one-dimensional (1D) TiO2 nanotube arrays affected their performance in the electrochemical reduction of 2,4,6-trinitrotoluene (2,4,6-TNT) in aqueous solutions and ambient conditions. Two main geometric parameters were considered: the inner diameter and length of TiO2 nanotubes. The various TiO2 nanotubes were fabricated using an anodization technique performed in four types of fluoride-containing organic solvents by adjusting various anodization parameters. All as-prepared samples successfully detected 2,4,6-TNT in an aqueous solution using cyclic voltammetry and linear sweep voltammetry. Three peaks corresponding to the reduction of each nitro group in 2,4,6-TNT were observed between − 0.5 and − 1.0 V (versus Ag/AgCl, 3 M KCl). Further, we discovered that nanotube length affected 2,4,6-TNT reduction performance. We also performed experiments at various concentrations of 2,4,6-TNT and determined the sensitivity to be 1.78 × 10-3 mA cm−2 ppm−1, the limit of detection to be 6.49 ppm, and the limit of quantitation to be 21.6 ppm 2,4,6-TNT over a linear range of 20 – 100 ppm using an electrode with nanotube inner diameter of 75 nm and length of 6 µm. A simple and fast fabricated composite-free 1D TiO2 nanotube arrays electrode can form the basis for future studies in enhancing the detection sensitivity. Electrochemical detection of other nitro-based compounds, such as nitroglycerin, nitrocellulose, picric acid, and their mixtures, require further investigation.

Hydrothermal synthesis of rutile TiO2 nanotubes film on Ti foil for photocatalytic degradation

One-dimensional TiO2 nanotubes film was synthesized on the Ti foil oxidized at 800℃ by hydrothermal method in a highly alkaline solution. The crystal structure, morphology and chemical state of the samples were characterized by X-ray diffraction, scanning electron microscope, transmission electron microscope and X-ray photoelectron spectroscopy. The result indicates that the TiO2 nanotubes film still remains rutile phase structure after hydrothermal treatment. The growth mechanism of TiO2 nanotubes is also discussed that includes dissolution, nucleation and growing process. The photocatalytic activity of TiO2 film is remarkably improved after hydrothermal reaction. Due to the one-dimensional tubular structure, TiO2 nanotubes film hydrothermally synthesized for 6 hours displays the highest photocatalytic degradation rate of methylene blue

Multiple nano-filaments based efficient resistive switching in TiO2 nanotubes array influenced by thermally induced self-doping and anatase to rutile phase transformation

In this paper, the impact of thermally induced self-doping and phase transformation in TiO2 based resistive random-access memory (ReRAM) is discussed. Instead of a thin film, a vertically aligned one-dimensional TiO2 nanotube array (TNTA) was used as a switching element. Anodic oxidation method was employed to synthesize TNTA, which was thermally treated in the air at 350 °C followed by further annealing from 350 °C to 650 °C in argon. Au/TiO2 nanotube/Ti resistive switching devices were fabricated with porous gold (Au) top electrode. The x-ray diffraction results along with Raman spectra evidently demonstrate a change in phase of crystallinity from anatase to rutile, whereas photoluminescence spectra revealed the self-doping level in terms of oxygen vacancies (OV) and Ti interstitials (Tii) as the temperature of thermal treatment gets increased. The electrical characterizations establish the bipolar and electroforming free resistive switching in all the samples. Among those, the ReRAM sample S3 thermally treated at 550 °C displayed the most effective resistive switching properties with R OFF/R ON of 102 at a read voltage of −0.6 V and a SET voltage of −2.0 V. Moreover, the S3 sample showed excellent retention performance for over 106 s, where stable R OFF/R ON ≈ 107 was maintained throughout the experiment.

(Invited) Anodic TiO2 Nanostructures for Photoelectrochemical and Photocatalytic Applications

Since Zwelling et al. reported on anodized TiO2 nanotubes, many research groups have studied one-dimensional TiO2 nanotubes. This structure property can be controlled by pH, conductivity, fluoride concentration, base solution and applied potential and so on. TiO2 nanotubes are attracted attention owing to their variable function for use wide range applications such as photocatalyst, bio application, template and solar cell, etc. TiO2 nanotubes provide smooth electron pathway, less crystal defect to avoid electron recombination, high surface area for dye adsorption and the ability influence the adsorption and propagation of light.In this presentation we give an overview of the latest results on TiO2 nanotube and porous structures. We will discuss critical factors to grow highly aligned and high aspect ratio anodic TiO2 nanostructures and the morphological variation by anodization parameters and their advantage for photocatalytic and photoelectrochemical applications.

Black Phosphorus Quantum Dot-Sensitized TiO2 Nanotube Arrays with Enriched Oxygen Vacancies for Efficient Photoelectrochemical Water Splitting

Photon absorption, charge separation and transportation, and charge-induced reactions at the active sites are the main crucial factors involved in the photoelectrochemical (PEC) water splitting. Herein, a combination of black phosphorus quantum dot (BPQD) sensitization and defect engineering strategies is employed to optimize the PEC performance of one-dimensional TiO2 nanotube array (NTA) photoanodes. The as-prepared TiO2–x/BP electrode exhibits a strong photocurrent density under simulated solar light irradiation, which is almost ∼3 times higher than that of bare TiO2. Specifically, the photocurrent increment of TiO2–x/BP is even larger than the sum of TiO2–x and TiO2/BP, verifying the synergistic effect of oxygen vacancies and BPQD sensitization. The maximum photoconversion efficiency of TiO2–x/BP is as high as 0.35%, while the value of TiO2 NTAs is calculated to be 0.13%. The results reveal that oxygen vacancies and BPQDs in the TiO2–x/BP composite not only facilitate the charge separation and transportation but also enhance the activity and quantity of reactive sites for water oxidation. The present strategy might open new routes to develop high-performance photoelectrodes for water splitting.

Atomic Layer Deposition of SnO2-Coated Anodic One-Dimensional TiO2 Nanotube Layers for Low Concentration NO2 Sensing.

The continuous emission of nitrous oxides contributes to the overall air pollution and deterioration of air quality. In particular, an effective NO2 sensor capable of low concentration detection for continuous monitoring is demanded for safety, health, and wellbeing. The sensing performance of a metal oxide-based sensor is predominantly influenced by the availability of surface area for O2 adsorption and desorption, efficient charge transport, and size or thickness of the sensing layer. In this study, we utilized anodic one-dimensional (1D) TiO2 nanotube layers of 5 μm thick which offer large surface area and unidirectional electron transport pathway as a platform to accommodate thin SnO2 coatings as a sensing layer. Conformal and homogeneous SnO2 coatings across the entire inner and outer TiO2 nanotubes were achieved by atomic layer deposition with a controlled thickness of 4, 8, and 16 nm. The SnO2-coated TiO2 nanotube layers attained a higher sensing response than a reference Figaro SnO2 sensor. Specifically, the 8 nm SnO2-coated TiO2 nanotube layer has recorded up to ten-fold enhancement in response as compared to the blank nanotubes for the detection of 1 ppm NO2 at an operating temperature of 300 °C with 0.5 V applied bias. This is attributed to the SnO2/TiO2 heterojunction effect and controlled SnO2 thickness within the range of the Debye length. We demonstrated in this work, a tailored large surface area platform based on 1D nanotubes with thin active coatings as an efficient approach for sensing applications and beyond.

ALD growth of MoS2 nanosheets on TiO2 nanotube supports

Two-dimensional MoS2 nanostructures are highly interesting and effective in a number of energy-related applications. In this work, the synthesis of ultra-thin MoS2 nanosheets produced by the thermal Atomic Layer Deposition (ALD) process is reported for the first time using a previously unpublished set of precursors, namely bis(t-butylimido)bis(dimethylamino)molybdenum and hydrogen sulfide. These nanosheets are homogenously deposited within one-dimensional anodic TiO2 nanotube layers that act as a high surface area conductive support for the MoS2 nanosheets. The decoration of high aspect ratio TiO2 nanotube layers with MoS2 nanosheets over the entire nanotube layer thickness is shown for the first time. The homogeneous distribution of the MoS2 nanosheets is proved by STEM/EDX. This resulting new composite is employed as anode for Li-ion microbatteries. The MoS2-decorated TiO2 nanotube layers show a superior performance compared to their counterparts without MoS2. Compared to electrochemical performance of pristine TiO2 nanotube, a more than 50% higher areal capacity and a coulombic efficiency of 98% are obtained on the MoS2 decorated TiO2 nanotube layers, demonstrating clear synergic benefits of the new composite structure.

Ultrashort and short pulse nonlinear optical investigations in thiolated nine-atom silver quantum clusters embedded in one-dimensional TiO2 nanotube matrix

Nanostructured materials employed as passive nonlinear absorptive limiters offer protection to the human eyes and optical sensors from damage by pulsed laser radiation. However, such materials are prone to laser induced damage, which can be avoided or minimized by employing a robust shell layer. Here, we report the optical limiting properties of nine-atom silver quantum clusters grafted in TiO2 nanotube matrix (Ag9QCs-TNT). Nonlinear absorption revealed from open-aperture Z-scan measurements on Ag9QCs-TNT using both 5 ns (532 nm) and 100 fs (800 nm) laser pulses is larger than those observed from its pristine counterparts (Ag9QCs and TNT). The optical limiting threshold value obtained for Ag9QCs-TNT under fs excitation is smaller by a factor of five compared to that under ns excitation, indicating its ability to passively limit high intensity laser radiation for potential device applications.

Improved efficiency in front-side illuminated dye sensitized solar cells based on free-standing one-dimensional TiO2 nanotube array electrodes

Although morphological disorder of nanotube structure is further down than the nanoparticular electrode, its density of traps are the hindering effects in the charge transport. In this study, crack-free TiO2 nanotube membranes, which obtained through a re-anodizing process, are fixed on transparent fluorine–tin-oxide glass by applying a few drops of Titanium Isopropoxide without needing the TiO2 powder paste. Front-side illuminated dye sensitized solar cells fabricated by undoped, N-doped and blue TiO2 nanotube membranes are investigated. The electrical characteristics of TiO2 nanotube based dye sensitized solar cells are followed by theoretical analysis using simple one-diode model. Morphology improvement is utilized to remove top irregular layers and the efficiency can be increased up to 56%. Annealing temperature, dopant, and tube orientation effects are examined on the cell efficiencies. The results show that elevating annealing temperature from 480 to 520 °C improve the cell efficiency up to 70%. Also, in the N-doped TiO2 nanotube membranes, improvement in efficiency is observed up to 40% as compared to undoped samples. Finally, different orientations of N-doped TiO2 nanotubes are studied and it is found that the through-hole morphology of N-doped TiO2 nanotube membrane shows noteworthy enhancement and an efficiency of about 8%.

Al-Doped TiO2 Nanotube Arrays Achieved by Using Low Boiling AlCl3 as a Dopant Source for Supercapacitors

One-dimensional TiO2 nanotube arrays (TiO2 NTAs) fabricated by anodization of titanium have been considered as promising materials for many potential applications. However, TiO2 NTAs generally suffer from poor electrochemical properties due to limited electrical conductivity of TiO2. For this reason, various approaches to preparing nonmetal-doped and metal-doped TiO2 have been developed. Nevertheless, these reported methods were mainly appropriate for self-doping and nonmetal doping of TiO2. For the metal doping, some difficulties such as multiple steps, harsh reaction conditions, or expensive facilities were often encountered. Here, we propose a novel strategy to realize the metal doping of TiO2 NTAs by using a low boiling metal compound as a dopant source. As an example, through heating the as-anodized TiO2 NTAs together with AlCl3 in a confined space, the Al3+ ions are incorporated into the TiO2 lattice in the form of substitution of Ti4+ by Al3+. Moreover, the AlCl3 vapor treatment route can achieve adherent TiO2 NTA films on Ti substrate. The as-prepared Al-doped TiO2 NTA electrode delivers an average capacitance of 12.48 mF cm−2 at 100 mV s−1, which is ∼2.5 times higher than that of untreated TiO2 NTAs. Also, the Al-doped electrode shows an excellent rate capability and long-term cycling stability.

Exploring the photoelectrocatalytic behavior of free-standing TiO2 nanotube arrays on transparent conductive oxide electrodes: Irradiation direction vs. alignment direction

Although one-dimensional TiO2 nanotube arrays (TNA) grown on Ti substrates via electrochemical anodization are extensively studied in photoelectrochemistry, the photo(electro)catalytic activity of TNA detached from the Ti substrates remains unexplored. Herein, we synthesize TNA samples with various pore sizes (40–100 nm) and tube lengths (4–15 μm) via two-step electrochemical anodization, and transfer them to transparent conducting oxide (i.e. fluorine-doped tin oxide; FTO) substrates in normal (n) alignment (front plane outward) and reverse (r) alignment (backplane outward). The front and back planes of the as-fabricated TNA film are the same based on X-ray diffraction (anatase structure), X-ray photoelectron spectroscopy (Ti and O), and UV–vis transmittance data, though the tubes are open in the front and closed in the back. Regardless of the direction of irradiation (SE: FTO → TNA vs. EE: TNA → FTO), longer tubes generate a higher photocurrent (Iph) due to the large light absorption. However, for the same alignment of TNA (either n- or r-TNA), SE irradiation leads to a very large Iph (e.g., nSE >nEE), whereas n-TNA consistently generates a larger Iph than r-TNA for a given irradiation direction (i.e., n >r). The photocatalytic decomposition of phenol follows the same tendency (n >r); however, the Faraday efficiency (based on the photocharge) is higher with EE (nEE 28%, rEE 20%) than SE (rSE 11%, nSE 7%) irradiation. These photoelectrochemical and photocatalytic behaviors are explained in terms of charge carrier generation (FTO/TNA vs. TNA/solution), dissimilar charge carrier transfer pathways (e− transfer through tube framework vs. h+ transfer via radial direction), and charge injection at the tube (open vs. clogged tube mouth)/solution interface. The time-resolved photoluminescence (TRPL) emission and incident photon-to-current efficiency (IPCE) are also studied to gain insight into the charge transfer kinetics.