Reduced TiO2 Nanotube Arrays Research Articles

Superior long service life of a porous Ti-foam/TiOxHy-NTs/SnO2-Sb anode for highly efficient degradation of the pretreatment polyacrylamide-containing fracturing flowback fluid

Doped-SnO2 electrodes have shown outstanding performances in electrochemical oxidation (EO), but limitations in long service life and high durability have necessitated further advances in electrode design. Herein, this study proposes a porous Ti-foam/TiOxHy-NTs/SnO2-Sb anode with superior long service life for efficient degradation of polyacrylamide (PAM) - containing fracturing flowback fluid. The Ti-foam/TiOxHy-NTs/SnO2-Sb anode was designed by preferably choosing reduced TiO2 nanotube arrays grown in situ on porous Ti foam (Ti-foam/TiOxHy-NTs) as substrate coated with SnO2-Sb coating through the sol-gel method. The superiority of the design concept is verified by achieving a superior long service life of 735.2 h (0.5 M H2SO4) and 53.2 h (0.5 M HCl) at a current density of 1.0 A cm−2 by the tests of accelerated service life. The superior long service life originates from multidimensional and hierarchical architecture for lowering electrical resistance by shorting the transport path of electrolyte ions and electrons, and loading more active material without aggregation. Furthermore, the Ti-foam/TiOxHy-NTs/SnO2-Sb electrode displays excellent ability in efficiently degrading the real and simulated PAM-containing fracturing flowback fluid, achieving a chemical oxygen demand decomposition of above 90.0 % after 120 min of EO process. Therefore, the synthesized Ti-foam/TiOxHy-NTs/SnO2-Sb electrode exhibits great potential industrial application for efficient decomposition of PAM-containing fracturing flowback fluid using in EO process.

Reduced TiO2 nanotube arrays as environmental catalysts that enable advanced oxidation processes: A mini review

Advanced oxidation processes (AOPs) that utilize heterogeneous catalysts are an engaging and compelling technology for treating emerging contaminants in water and wastewater. Substantial efforts have been devoted to developing cost-effective and environmentally benign materials for application in AOPs. Reduced TiO2 nanotube arrays (r-TNAs) have recently emerged as promising environmental materials for efficiently mitigating persistent organic pollutants (POPs) resistant to conventional biological treatment. r-TNAs possess distinct electrical, optical, and catalytic features, including superior conductivity and solar light absorption due to structural defects involving oxygen vacancies and Ti3+ states. Moreover, their well-organized open channel structures provide a substantial surface area, facilitating the efficient mass transfer of reactants and products. Notably, r-TNAs can be grown directly on the substrate without using organic adhesives or binders, which improves their mechanical and chemical durability and facilitates their repeated use. This article represents the pioneering comprehensive review of r-TNAs in AOP applications. We introduce an electrochemical anodization technique to fabricate TNAs and summarizes the reduction methods employed in r-TNA synthesis. We also elucidate the appealing attributes of r-TNAs and explore their applications in emerging AOPs, including persulfate activation, electrocatalysis, and photo(electro)catalysis. Finally, we outline critical challenges and provide insights into future prospects for the development of AOP systems based on r-TNAs.

Defective TiO2 Nanotube Arrays for Efficient Photoelectrochemical Degradation of Organic Pollutants.

Oxygen vacancies (OVs) are one of the most critical factors that enhance the electrical and catalytic characteristics of metal oxide-based photoelectrodes. In this work, a simple procedure was applied to prepare reduced TiO2 nanotube arrays (NTAs) (TiO2-x) via a one-step reduction method using NaBH4. A series of characterization techniques were used to study the structural, optical, and electronic properties of TiO2-x NTAs. X-ray photoelectron spectroscopy confirmed the presence of defects in TiO2-x NTAs. Photoacoustic measurements were used to estimate the electron-trap density in the NTAs. Photoelectrochemical studies show that the photocurrent density of TiO2-x NTAs was nearly 3 times higher than that of pristine TiO2. It was found that increasing OVs in TiO2 affects the surface recombination centers, enhances electrical conductivity, and improves charge transport. For the first time, a TiO2-x photoanode was used in the photoelectrochemical (PEC) degradation of a textile dye (basic blue 41, B41) and ibuprofen (IBF) pharmaceutical using in situ generated reactive chlorine species (RCS). Liquid chromatography coupled with mass spectrometry was used to study the mechanisms for the degradation of B41 and IBF. Phytotoxicity tests of B41 and IBF solutions were performed using Lepidium sativum L. to evaluate the potential acute toxicity before and after the PEC treatment. The present work provides efficient PEC degradation of the B41 dye and IBF in the presence of RCS without generating harmful products.

Environment-friendly and efficient electrochemical degradation of sulfamethoxazole using reduced TiO2 nanotube arrays-based Ti membrane coated with Sb-SnO2

This study focused on the preparation, characterization, and sulfamethoxazole (SMX) removal performance of the SnO2-coated reactive electrochemical membrane (REM). This REM was fabricated by loading SnO2 on the reduced TiO2 nanotube arrays (RTNA)-based Ti membrane (TM). Regarding the dopant for SnO2, Sb was more effective in boosting the electrocatalytic activity than Bi, and the energy consumption for Sb-SnO2-coated REM (TM/RTNA/ATO) was lower than Bi-SnO2-coated REM (TM/RTNA/BTO). As for the internal layer, RTNA provided TM/RTNA/ATO with more electroactive surface areas and prolonged the service lifetime. Compared with batch mode, the SMX removal efficiency in flow-through mode was increased up to 8.4-fold. The SMX degradation performances were also affected by fluid velocity, current density, initial SMX concentration, and electrolyte concentration. The synergistic effects of •OH oxidation and direct electron transfer were responsible for the effective removal of SMX. TM/RTNA/ATO was proved to be stable and durable by multi-cycle and accelerated lifetime tests. Its extensive applicability was verified with high removal efficiencies of SMX in the surface water and wastewater effluent. These results demonstrate the promise of TM/RTNA/ATO for water treatment applications.

Efficient electrochemical oxidation of sulfamethoxazole by a novel reduced TiO2 nanotube arrays-based flow-through electrocatalytic membrane

The reduced TiO2 nanotube arrays (RTNA) electrode is a cost-effective anode, but its limited electrocatalytic activity is an obstacle for the application. To solve this problem, a reduced TiO2 nanotube arrays-based titanium membrane (RTNA/TM) was fabricated by a facile electrochemical method to promote the electrocatalytic performance. The one-dimensional nanostructure and reduction treatment increased the electrocatalytic capacity and reduced the energy consumption, resulting in the sulfamethoxazole (SMX) removal rate of 86.1% and electrical efficiency per order of 0.55 kWh/m3 for RTNA/TM. Compared with the batch mode, the SMX removal efficiency of the flow-through mode was significantly increased by 71%, verifying the superiority of a porous flow-through anode. The SMX degradation performances of RTNA/TM were strongly affected by the fluid velocity, current density and initial SMX concentration, except for pH. Both hydroxyl radicals and direct electron transfer were responsible for the SMX removal, while the contribution of sulfate radicals could be ignored. Furthermore, RTNA/TM displayed stable performances for five experimental cycles and showed remarkable degradation efficiencies of SMX spiked into the natural water (76.5%) and wastewater effluent (75.4%). Therefore, RTNA/TM has been proven to be a promising flow-through anode for electrochemical oxidation.

Effects of chloride and other anions on electrochemical chlorine evolution over self-doped TiO2 nanotube array

Electrochemically reduced TiO2 nanotube arrays (r-TiO2 NTA) have emerged as an alternative that can replace the dimensionally stable anode (DSA®) due to comparable performance for chlorine evolution reaction (ClER). However, previous studies have reported applications of r-TiO2 NTA for ClER only under limited conditions (concentrated NaCl solution without other anions). Thus, the potential of r-TiO2 NTA for CIER has not yet been fully demonstrated. Therefore, this study focused on investigating ClER of r-TiO2 NTA under various parameters such chloride concentration (5–1,000 mM) and the presence of other anions (i.e., SO 4 2− , HPO 4 2− , and CO 3 2− ). The results suggest that, at low chloride concentration (5–50 mM NaCl), the r-TiO2 NTA exhibited higher performance for CIER (production rate of 3.35–9.82 mg l−1 min−1, current efficiency of 14.43–42.04%, energy consumption of 69.24–11.02 Wh g(Cl2)−1) than RuO2 (2.55–7.88 mg l−1 min−1, 11.07–33.85% and 77.29–6.84 Wh g(Cl2)−1, respectively). Additionally, other anions did not affect the ClER of r-TiO2 NTA more than RuO2. These can be explained by the indirect pathway of ClER in r-TiO2 NTA while the direct pathway of RuO2 was negatively affected by dilute chloride and other anions.

High chlorine evolution performance of electrochemically reduced TiO2 nanotube array coated with a thin RuO2 layer by the self-synthetic method.

Recently, reduced TiO2 nanotube arrays via electrochemical self-doping (r-TiO2) are emerging as a good alternative to conventional dimensionally stable anodes (DSAs) due to their comparable performance and low-cost. However, compared with conventional DSAs, they suffer from poor stability, low current efficiency, and high energy consumption. Therefore, this study aims to advance the electrochemical performances in the chlorine evolution of r-TiO2 with a thin RuO2 layer coating on the nanotube structure (RuO2@r-TiO2). The RuO2 thin layer was successfully coated on the surface of r-TiO2. This was accomplished with a self-synthesized layer of ruthenium precursor originating from a spontaneous redox reaction between Ti3+ and metal ions on the r-TiO2 surface and thermal treatment. The thickness of the thin RuO2 layer was approximately 30 nm on the nanotube surface of RuO2@r-TiO2 without severe pore blocking. In chlorine production, RuO2@r-TiO2 exhibited higher current efficiency (∼81.0%) and lower energy consumption (∼3.0 W h g−1) than the r-TiO2 (current efficiency of ∼64.7% of and energy consumption of ∼5.2 W h g−1). In addition, the stability (ca. 22 h) was around 20-fold enhancement in RuO2@r-TiO2 compared with r-TiO2 (ca. 1.2 h). The results suggest a new route to provide a thin layer coating on r-TiO2 and to synthesize a high performance oxidant-generating anode.

Reprint of "Electrochemical oxidation of lignin at electrochemically reduced TiO2 nanotubes"

For this study, the electrochemical oxidation of lignin was investigated at the surfaces of electrochemically (EC) reduced TiO2 nanotube arrays. The effects of nanotube lengths on the lignin oxidation were explored by growing the nanotubes with different lengths through anodization. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDS), and electron paramagnetic resonance (EPR) techniques were employed to characterize the fabricated TiO2 nanotubes. The electrochemical behaviors of the nanotubes before and after the EC treatment were assessed by various electrochemical methods, including cyclic voltammetry (CV), chronopotentiometry (CP), and impedance spectroscopy. The TiO2 nanotubes were treated by applying a cathodic current (5 mA cm−2) for 10 min, which significantly increased its electrocatalytic activity. It was also determined that the nanotube length was linearly increased with the increase of the anodization time, and that the longer the nanotubes and the larger the double layer capacitance. The length of the nanotubes had a significant effect on the level of lignin oxidation, and an optimal TiO2 nanotube length that enabled the most efficient oxidation of lignin was determined. The efficiency of the TiO2 electrodes towards the oxidation of lignin was also compared to a Pt electrode. The TiO2 nanotubes that were grown for 16 h by anodization with ~13.5 μm length exhibited the lowest impedance and the highest lignin oxidation efficacy. The total organic carbon (TOC) of the lignin solution under different oxidation times was also measured to further evaluate the efficiency of the electrochemical degradation of lignin, and 70% TOC removal was achieved in 3 h. The TiO2 electrodes were shown to outperform the Pt electrode in all the lignin oxidation studies. Moreover, the activation energy required for the electrochemical oxidation of lignin was investigated by performing the oxidation under various temperatures and found to be 21.0 kJ mol−1. The EC reduced TiO2 nanotube electrode showed high activity and stability, promising for environmental applications.

The electrowetting-on-dielectric enhancement of the reduced and bamboo-like layered TiO2 nanotube array and the visual sensor application

Based on the space charge polarization theory, the reduced and bamboo-like layered TiO2 nanotube arrays were designed and prepared by the voltage pulse anodization technique including reduction process of negative voltage. The reduction process synchronously introduces amount of oxygen vacancies, and the bamboo-like layered structure provides a plenty of interfaces to accumulate large amounts of free charges in the TiO2 nanotube arrays, which greatly enhance the space charge polarization in the TiO2 nanotube arrays. Thus, the capacitances of the reduced and layered TiO2 nanotube arrays are clearly increased, which further lead to the obvious enhancement of the electrowetting-on-dielectric (EWOD) responses. Furthermore, the reduced TiO2 nanotube arrays with bamboo-like layered structure were successfully used as the visual sensor to detect the NaCl and glucose concentration. Especially, it has higher sensitivity for the detection of the NaCl and glucose solution with low concentration.

Electrochemical oxidation of lignin at electrochemically reduced TiO2 nanotubes

For this study, the electrochemical oxidation of lignin was investigated at the surfaces of electrochemically (EC) reduced TiO2 nanotube arrays. The effects of nanotube lengths on the lignin oxidation were explored by growing the nanotubes with different lengths through anodization. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDS), and electron paramagnetic resonance (EPR) techniques were employed to characterize the fabricated TiO2 nanotubes. The electrochemical behaviors of the nanotubes before and after the EC treatment were assessed by various electrochemical methods, including cyclic voltammetry (CV), chronopotentiometry (CP), and impedance spectroscopy. The TiO2 nanotubes were treated by applying a cathodic current (5 mA cm−2) for 10 min, which significantly increased its electrocatalytic activity. It was also determined that the nanotube length was linearly increased with the increase of the anodization time, and that the longer the nanotubes and the larger the double layer capacitance. The length of the nanotubes had a significant effect on the level of lignin oxidation, and an optimal TiO2 nanotube length that enabled the most efficient oxidation of lignin was determined. The efficiency of the TiO2 electrodes towards the oxidation of lignin was also compared to a Pt electrode. The TiO2 nanotubes that were grown for 16 h by anodization with ~13.5 μm length exhibited the lowest impedance and the highest lignin oxidation efficacy. The total organic carbon (TOC) of the lignin solution under different oxidation times was also measured to further evaluate the efficiency of the electrochemical degradation of lignin, and 70% TOC removal was achieved in 3 h. The TiO2 electrodes were shown to outperform the Pt electrode in all the lignin oxidation studies. Moreover, the activation energy required for the electrochemical oxidation of lignin was investigated by performing the oxidation under various temperatures and found to be 21.0 kJ mol−1. The EC reduced TiO2 nanotube electrode showed high activity and stability, promising for environmental applications.

Plasmon-assisted partially reduced TiO2 nanotube arrays for photoelectrochemical water splitting

The partially reduced TiO2 nanotube arrays (R-TNTs) has been extensively investigated for photoelectrochemical (PEC) water splitting. However, the severe charge recombination and limited light harvesting hinder their photocurrent output. Herein, we present a strategy for prepared R-TNTs photoanode which can facilitate the charge separation and transfer simultaneously via formation of homojunction induced by the Ti3+ self-doping and oxygen vacancies. Additionally, we deposited plasmonic Au nanoparticles on the R-TNTs surface to extend the light absorption in visible region and accelerate the charge transfer at the solid-liquid interface via plasmonic effect. As a result, in PEC water splitting experiments, the optimized Au modified R-TNTs (Au@R-TNTs) photoanode exhibits a photocurrent density of 1.64 mA·cm−2 with solar-to-hydrogen conversion efficiency up to 0.55% under AM 1.5 G illumination, 4-fold and 2-fold higher than that of the pristine TNTs, respectively. The photocatalytic activity in the visible region was dramatically improved. The exploration of interfacial carrier dynamics in homojuntion and the investigation of the synergistic effects of plasmonic enhanced photocatalyst provides guideline for designing solar energy harvesting devices.

A short review on electrochemically self-doped TiO2 nanotube arrays: Synthesis and applications

Electrochemically self-doped TiO2 nanotube arrays (known as reduced TiO2 nanotube arrays, r-TiO2 NTAs) are currently drawing great attention as emerging and promising materials for energy and environmental applications as they exhibit highly enhanced electrochemical properties, such as good capacitive properties and electro- and photocatalytic activity when compared to pristine TiO2 NTAs. Such enhanced properties are attributed to the introduction of trivalent titanium (Ti(III)) as a self-dopant in the lattice of pristine TiO2 NTAs through simple electrochemical reduction. However, in spite of the great interest in, and potential of this material, there is no comprehensive review on the synthesis and applications of r-TiO2 NTAs. Therefore, in this review, we critically and briefly review r-TiO2 NTAs in terms of the electrochemical self-doping mechanism, their functional features, and various applications including photolysis, dye-sensitized solar cells (DSSCs), biomedical coatings and drug delivery. In addition, to better understanding r-TiO2 NTAs, pristine TiO2 NTAs are briefly introduced. Furthermore, this review proposes future research directions with major challenges to be overcome for the successful development of r-TiO2 NTAs, such as to standardize matrices for performance evaluation, to confirm the organic degradation performance as anode, and to improve mechanical stability.

Structures and photoelectrochemical performances of reduced TiO2 NTAs obtained by hydrogen thermal and electrochemical reduction methods

Reduced TiO2 nanotube arrays were obtained by hydrogen-thermal and electrochemical reduction methods respectively, and the photoelectrochemical performances were studied. Phase structures, elemental compositions, and surficial morphologies were characterized with X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) to investigate the structural differences between as-prepared TiO2 NTAs and reduced TiO2 NTAs, including two different reducing products. The photoelectrochemical performances of TiO2 NTAs were found to be enhanced by both two reducing methods. The different mechanisms of hydrogen-thermal reduction and electrochemical reduction were investigated by comparing optical absorption, charge transport, separation efficiency of charge carriers, and surficial reactions during the photoelectrochemical processes. For hydrogen-thermal-reduced TiO2 NTAs, the improved photoelectrochemical performances are induced by high optical absorption and low recombination of charge carries, whereas for electrochemical reduced TiO2 NTAs, the enhanced performances are attributed to low charge transport resistance.

Electrochemically Faceted Bamboo‐type TiO2 Nanotubes Provide Enhanced Open‐Circuit Hydrogen Evolution

AbstractIn this paper we prepare facetted “black” TiO2 nanotube arrays (NTAs) with a bamboo‐type structure which exhibits an enhanced performance for open‐circuit H2 generation without the use of any noble metal co‐catalyst. The TiO2 nanotubes were formed by applying alternating positive and negative voltages. During the electrochemical formation, not only a bamboo structure is formed but the tubes are reduced in the cathodic cycle. After a rapid annealing treatment of these tubes in air at 500 °C for 10 s, the bamboo rings (knots) provide (001) facets preferentially located at the knot, while (101) planes prevail on the smooth part of tube between knots, as shown by TEM. The short annealing time keeps Ti3+/oxygen vacancies (VO) as co‐catalytically active centers still preserved in the TiO2 NTAs. The results of intensity modulated photocurrent spectroscopy (IMPS) and open‐circuit potential decay measurements (VOC) indicate that these faceted and reduced TiO2 nanotube arrays provide an efficient separation and reaction of photo‐generated charge carriers, leading to an improved photocatalytic H2 evolution performance.

A facile strategy to fabricate reduced TiO2 nano-tube arrays photoelectrode and its high visible light photocatalytic performance for detoxification of trichlorophenol solution

Abstract In the research, reduced TiO2 nano-tube arrays (denoted as R-TiO2 NTAs) photoelectrode was successfully fabricated by potassium borohydride (PBH) reduction treatment. Afterwards, the as-fabricated photoelectrode was characterized by scanning electron microscope, X-ray diffraction, Raman spectra and electron spin resonance. Meanwhile, the optical and photoelectrochemical properties of R-TiO2 NTAs photoelectrode were also studied through ultraviolet-visible diffuse reflectance spectroscopy and transient photocurrent response, respectively. The photocatalytic (PC) activity of R-TiO2 NTAs photoelectrode was measured by degradation of trichlorophenol (TCP). Moreover, the change of toxic intermediates in the process of degradation TCP was further evaluated by photobacterium inhibition tests. Results suggested that an impurity level can be induced in the TiO2 NTAs band gap by solution reduction treatment due to the generation of Ti3+ and oxygen vacancies (Ov). Furthermore, R-TiO2 NTAs photoelectrode exhibited higher PC activities than that of pristine TiO2 NTAs owing to the enhancement of visible light harvesting between 450 and 800 nm and separation efficiency of photogenerated electrons-holes (e/h+) pairs. The possible pathway for TCP degradation and photocatalytic mechanism were also proposed and confirmed. Furthermore, R-TiO2 NTAs photoelectrode displayed good stability and reusability.

Microwave assisted construction of Ag-AgBr/reduced TiO2 nano-tube arrays photoelectrode and its enhanced visible light photocatalytic performance for degradation of 4-chlorphenol

Abstract There is an increasingly significant issue on decomposition of contaminants in modern chemical industry and environmental protection. Hence, in the study, to improve the catalytic of semiconductor materials, the silver-silver bromide nanoparticles decorated reduced TiO2 nano-tube arrays photoelectrode (Ag-AgBr/r-TiO2 NTAs), which exhibits better photocatalytic (PC) performance in the wide range of solar spectra, was successfully fabricated by anodization process, followed by microwave reduction strategy. The partly reduced AgBr nanoparticles are decorated and both Ti3+and Ov states are simultaneously introduced in the TiO2 NTAs photoelectrode inducing the formation of impurity energy level to form Ag-AgBr/r-TiO2 NTAs photoelectrode. Moreover, PC activity of Ag-AgBr/r-TiO2 NTAs photoelelctrode was measured by degradation of 4-chlorphenol (4-CP). Results suggest that Ag-AgBr/r-TiO2 NTAs photoelectrode exhibits higher PC activity (99.9%) than that of other samples within 120 min illumination. The well combination of surface plasmons Ag-AgBr nanoparticles and r-TiO2 NTAs was responsible for the enhancing of PC, and the recombination of photo-generated electrons and holes can simultaneously be inhibited. Thus, Ag-AgBr nanoparticles and Ti3+ exert huge influence on the PC and photoelectrochemical properties of Ag-AgBr/r-TiO2 NTAs photoelectrode.

Electrochemical Reductive Doping and Interfacial Impedance of TiO2 Nanotube Arrays in Aqueous and Aprotic Solvents

Electrochemical reductive doping is considered to be a simple and effective method to improve the electric properties of TiO2 nanotube arrays (NTAs). We report on electrochemical tests of the reductive reaction of anatase TiO2 NTAs in aqueous and aprotic solvents. It is confirmed that hydrogen is not necessary for the electrochemical reduction of TiO2, and the reaction can be performed in either aqueous or aprotic solvents, where the former is more effective. Three equivalent circuit models were also built to understand the electrochemical behavior, via electrochemical impedance, of pristine, reduced, and post-oxidized TiO2 NTAs in contact with electrolyte. Ti4+ is reduced to Ti3+ by electrochemical treatment to form self-doped TiO2 with metallic properties at the initial stages, and the reduced TiO2 NTAs can be easily re-oxidized to form a stable semiconductor when exposed in air. Two different reaction mechanisms performed in aqueous and aprotic solvents are discussed. These findings help to understand the complexity of the processes involved in the electrochemical reductive doping of TiO2 NTAs and suggest a way to improve the electronic properties of TiO2 nanomaterials.

Low-temperature liquid phase reduced TiO2 nanotube arrays: synergy of morphology manipulation and oxygen vacancy doping for enhancement of field emission

The partially reduced TiO2 nanotube arrays (TNAs) are prepared via an uncomplicated and low-cost liquid phase reduction strategy using NaBH4 as the reducing agent. By controlling and adjusting the reduction temperatures from 30 to 90 °C, the reduction treatment can not only change their surface morphology but also introduce oxygen vacancies into them, resulting in an optimized morphology, elevated Fermi-level, reduced effective work function and improved conductivity of the TNAs. Meanwhile, the thermal and long-term stability of oxygen vacancy are also investigated, indicating that the oxygen vacancies retain long-term stability from room temperature up to 150 °C. More interesting, partially reduced TNAs show drastically enhanced field emission (FE) performances including substantially decreased turn-on field from 18.86 to 1.53 V μm−1, a high current density of 4.00 mA cm−2 at 4.52 V μm−1, and an excellent FE stability and repeatability. These very promising results are attributed to the combination of the optimized morphology and introduced oxygen vacancies, which can increase FE sites, reduce effective work function and increase conductivity.

Reduction Mechanism and Capacitive Properties of Highly Electrochemically Reduced TiO2 Nanotube Arrays

Highly reduced and ordered TiO2 nanotube arrays have been fabricated using two-step anodization and three-electrode reduction. A proton-electron coupled reduction mechanism has been proposed based on the combined paradigms of a conventional energy-band model and chemical evolution of basic building blocks of TiO2. Under optimized reduction conditions, about 22% of Ti4+ ions in tube surface regions are converted into Ti3+ ions while the morphology of the highly reduced TiO2 nanotube arrays keeps unchanged. The reduced nanotube arrays show superior electrochemical properties such as high areal capacitance, good rate capability, and high cycling stability. The areal capacitance of the reduced electrode is 24.07mFcm−2 at a scan rate of 10mVs−1, much higher than that of the pristine TiO2 nanotube arrays (0.02mFcm−2). This kind of highly reduced one-dimensional oxide nanostructures can find a large array of applications in supercapacitors, photocatalysis, electrochromic display, and Li ion batteries.

Enhancement of the field emission from the TiO₂ nanotube arrays by reducing in a NaBH₄ solution.

A mass of oxygen vacancies are successfully introduced into TiO2 nanotube arrays using low-cost NaBH4 as a reductant in a liquid-phase environment. By controlling and adjusting the reduction time over the range of 0-24 h, the doping concentration of the oxygen vacancy is controllable and eventually reaches saturation. Meanwhile, the thermal stability of oxygen vacancies is also investigated, indicating that part of the oxygen vacancies remain stable up to 250 °C. In addition, this liquid-phase reduction strategy significantly lowers the requirements of instruments and cost. More interesting, reduced TiO2 nanotube arrays show drastically enhanced field emission performances including substantially decreased turn-on field from 25.01 to 2.65 V/μm, a high current density of 3.5 mA/cm(2) at 7.2 V/μm, and an excellent field emission stability and repeatability. These results are attributed to the oxygen vacancies obtained by reducing in NaBH4 solution, resulting in a reduced effective work function and an increased conductivity.