Self-doped TiO2 Nanotube Arrays Research Articles

Nano-Polycrystalline Cu Layer Interlaced with Ti3+-Self-Doped TiO2 Nanotube Arrays as an Electrocatalyst for Reduction of Nitrate to Ammonia

The electrochemical nitrate reduction reaction (NO3RR) is considered as a promising strategy to degrade nitrate-containing wastewater and synthesize recyclable ammonia at atmospheric pressure and room temperature. In this work, the copper oxides-derived nano-polycrystalline Cu (NPC Cu) was integrated with Ti3+-self-doped TiO2 nanotube arrays (NTA) to fabricate the NPC Cu/H-TiO2 NTA. Ti3+-self-doped TiO2 NTAs and the NPC Cu facilitate electron transfer and mass transportation and create abundant active sites. The unique nanostructure in which Cu nano-polycrystals interlace with the TiO2 nanotube accelerates the electron transfer from the substrate to surface NPC Cu. The density functional theory calculations confirm that the built-in electric field between Cu and TiO2 improves the adsorption characteristic of the NPC Cu/H-TiO2 NTA, thereby converting the endothermic NO3- adsorption step into an exothermic process. Therefore, the high NO3- conversion of 98.97%, the Faradic efficiency of 95.59%, and the ammonia production yield of 0.81 mg cm-2 h-1 are achieved at -0.45 V vs reversible hydrogen electrode in 10 mM NaNO3 (140 mg L-1)-0.1 M Na2SO4. This well-designed NPC Cu/H-TiO2 NTA as an effective electrocatalyst for the 8e- NO3RR possesses promising potential in the applications of ammonia production.

Synergistic Effect of Self-Doped TiO2 Nanotube Arrays and Ultraviolet (UV) on Enhanced Disinfection of Rainwater

Synergistic Effect of Self-Doped TiO2 Nanotube Arrays and Ultraviolet (UV) on Enhanced Disinfection of Rainwater

Ti3+ self-doped TiO2 nanotube arrays revisited as Janus photoelectrodes for persulfate activation and water treatment

This study investigates the binary role of Ti3+ self-doped TiO2 nanotubes (bl-TNA) as an anode as well as a cathode in the photoelectrochemical (PEC) oxidative treatment of organic contaminants. Compared to the Ti cathode-based asymmetric system, the symmetric PEC system with Janus bl-TNA photoelectrodes demonstrated higher interfacial charge transfer, photoresponsive activity, and kinetic constant for bisphenol-A (BPA) degradation. The bl-TNA photocathode enhanced the formation of radicals by providing additional reaction sites for photocatalytic reactions and cathodic peroxydisulfate (PDS) activation. Additionally, the rate constant ratio for BPA degradation between the anode and cathode in the symmetric system changed from 7.5:1–1.3:1 under PEC and PEC/PDS conditions, indicating that PDS addition enhanced the photoactivity of the electrodes, with a higher rate at the cathode. Finally, only 2.4% performance decline was observed after 64 h with the periodic polarity reversal operation, demonstrating the long-term stability of the proposed PEC system.

Ti3+ Self-Doped Tio2 Nanotube Arrays Revisited as Janus Photoelectrodes for Persulfate Activation and Water Treatment

Ti3+ Self-Doped Tio2 Nanotube Arrays Revisited as Janus Photoelectrodes for Persulfate Activation and Water Treatment

High-efficiency electrochemical decomposition of artificial sweetener aspartame using a Ti3+ self-doped TiO2 nanotube arrays anode

Artificial sweeteners have attracted growing concern considering their large usage, wide detection and ecological toxicity in the last decade. In this work, high-efficiency decomposition of artificial sweetener aspartame was investigated by using a Ti3+ self-doped TiO2 nanotube arrays electrode. Effects of current density, initial concentration and inorganic anions on the degradation kinetics were studied. Results showed that the electrochemical degradation of aspartame followed pseudo-first-order kinetics, and the rate constant was 0.039 min−1 at a current density of 2 mA cm−2. The electrochemical degradation was not susceptible to nitrate, and the degradation was significantly enhanced by chloridion. Twenty-four transformation products were identified by UPLC-Orbitrap-MS/MS, and possible electrochemical degradation pathway was proposed. The degradation process of aspartame mainly included hydroxylated, demethylation, dehydrogenation, dehydration condensation and side-chain cleavage. Quantitative structure-activity relationship model revealed that the potential risks in electrolysis process should not be ignored before complete mineralization of aspartame. Energy consumption was greatly reduced in the presence of Cl-, and the lowest value for 90% aspartame degradation was 0.07 Wh L−1. The findings presented here demonstrate that electrocatalysis exhibits high efficiency in degrading aspartame and is potentially suitable for eliminating artificial sweeteners from wastewaters.

Treatment of organic wastewater by a synergic electrocatalysis process with Ti3+ self-doped TiO2 nanotube arrays electrode as both cathode and anode

Electrochemical anodic oxidation (AO) is a promising technology for wastewater treatment due to its strong oxidation property and environmental compatibility. However, it suffers from high energy consumption for pollutants removal due to the side-reactions of hydrogen evolution reaction on cathode and oxygen evolution reaction on anode. Combining electro-Fenton (EF) with AO not only generated •OH for pollutants degradation but also increased current efficiency. This work investigated a synergic electrocatalysis process between EF and AO with Ti3+ self-doped TiO2 nanotube arrays (Ti3+/TNTAs) electrode as both cathode and anode for wastewater treatment. The pseudo-first-order kinetic rate constant of phenol degradation by EF+AO (0.107 min−1) was 9.7 or 6.3 times as much as that of only EF (0.011 min−1) or AO (0.017 min−1) process, respectively. Enhanced pollutants removal of EF+AO could be attributed to the coexistence of •OH oxidation and direct oxidation on Ti3+/TNTAs surface. The COD of secondary effluent of coking wastewater decreased from 159.3 mg L−1 to 47.0 mg L−1 by EF+AO within 120 min with low specific energy consumption (9.5 kWh kg−1 COD−1). This work provided a new insight into design of the energy-efficient synergic electrocatalysis process for refractory pollutants degradation.

Treatment of Ni-EDTA containing wastewater by electrochemical degradation using Ti3+ self-doped TiO2 nanotube arrays anode

Ethylene diamine tetraacetic acid (EDTA) could form stable complexes with nickel due to its strong chelation. Ni-EDTA has significant impacts on human health because of its acute toxicity and low biodegradability, thus some appropriate approaches are required for its removal. In this research, a Ti3+ self-doped TiO2 nanotube arrays electrode (ECR-TiO2 NTA) was prepared and employed in electrochemical degradation of Ni-EDTA. The oxygen evolution potential of ECR-TiO2 NTA was 2.6 V vs. SCE. More than 96% Ni-EDTA and 88% TOC was removed after reaction for 120 min at current density 2 mA cm−2 at pH 4.34. The degradation of Ni-EDTA was mainly through the cleavage of amine group within Ni-EDTA and furthermore decomposed it into small molecular acids and inorganic ions including NH4+and NO3−. The electro-deposition of nickel ions at cathode was confirmed by XPS and was greatly affected by the pH of solution. The effects of current density, initial Ni-EDTA concentration, initial pH of solution and HCO3− concentration on Ni-EDTA degradation were investigated. The results exhibited that the ECR-TiO2 NTA had excellent efficiencies in electrochemical degradation of Ni-EDTA. The LSV analysis suggested that Ni-EDTA oxidation on ECR-TiO2 NTA anode and the production of hydroxyl radical (·OH) on the anode played an important role in the removal of Ni-EDTA.

Persulfate enhanced photoelectrochemical oxidation of organic pollutants using self-doped TiO2nanotube arrays: Effect of operating parameters and water matrix

This study investigated the influence of adding peroxydisulfate (PDS) to a photoelectrocatalysis (PEC) system using self-doped TiO2 nanotube arrays (bl-TNAs) for organic pollutant degradation. The addition of 1.0 mM PDS increased the bisphenol-A (BPA) removal efficiency of PEC (PEC/PDS) from 65.0% to 85.9% within 1 h. The enhancement could be attributed to the high formation yield of hydroxyl radicals (·OH), increased charge separation, and assistance of the sulfate radicals (SO4·−). The PDS concentration and applied potential bias were influential operating parameters for the PEC/PDS system. In addition, the system exhibited a highly stable performance over a wide range of pH values and background inorganic and organic constituents, such as chloride ions, bicarbonate, and humic acid. Further, the degradation performance of the organic pollutant mixture, including BPA, 4-chlorophenol (4-CP), sulfamethoxazole (SMX), and carbamazepine (CBZ), was evaluated in 0.1 M (NH4)2SO4 solution and real surface water. The degradation efficiency increased in the order of CBZ < SMX < 4-CP < BPA in the PEC and PEC/PDS systems with both water matrices. Compared with the PEC system, the PEC/PDS (1.0 mM) system showed a threefold higher pseudo first-order reaction rate constant for BPA among pollutant mixtures in surface water. This was attributed to enhanced ·OH production and the selective nature of SO4·−. The pseudo first-order reaction rate constants of other pollutants, i.e., 4-CP, SMX, and CBZ increased ca. twofold in the PEC/PDS system. The results of this study showed that the PEC/PDS system with bl-TNAs is a viable technology for oxidative treatment.

Preparation of ZnO/two-layer self-doped black TiO2 nanotube arrays and their enhanced photochemical properties.

Highly efficient TiO2 photoanodes can be achieved by enhancing electrical conductivity and improving charge separation and transfer. In this paper, Ti foils were used to fabricate TiO2 nanotubes by anodic oxidation and ZnO/two-layer self-doped black TiO2 nanotubes were prepared by electrochemical reduction and a hydrothermal method. The formed black TiO2 nanotubes have a better photoconversion efficiency and the maximum photoconversion efficiency increased by 59% compared with the pure nanotubes. The deposition of ZnO further improves the maximum photoconversion efficiency to 456% based on black TiO2. The photocurrent responses also increase by about 5 times in our results. This work is instructive for the development of highly robust and efficient photoanode materials in fields including photoelectrochemistry and photocatalysis.

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.

Oxygen vacancy self-doped black TiO2 nanotube arrays by aluminothermic reduction for photocatalytic CO2 reduction under visible light illumination

In this work, black titania nanotube arrays (B-TiO2NTAs) were prepared by aluminothermic reduction of anodized TiO2-NTAs. It was found that the oxygen partial pressure atmicro-region of TiO2-NTAs surfacewas critical for the creation of black TiO2 NTAs. The oxygen vacancies in the prepared B-TiO2 NTAs induced new defect energy levels in the band gap of TiO2, which reduced the band gap and broadened their visible light absorption. Furthermore, the oxygen vacancies could also act as the catalytic sites and accelerate surface reactions for the photocatalytic reduction of CO2 to CO, which also was proved by isotope experiment. Especially, B-TiO2 NTAs annealed at 600℃ showed an excellent photocatalytic CO2 reduction to CO performance with the yield of 185.39 μmol g−1 h-1 under visible light illumination because the oxygen vacancy self-doping largely enhanced three key factors in this process, including photoinduced charge generation, charge separation and transportation and interfacial reaction. This facile and versatile method could be potentially used for large scale production of colored TiO2 with a high photocatalytic CO2 reduction capability in the visible light illumination.

Enhanced performance of lithium ion batteries from self-doped TiO2 nanotube anodes via an adjustable electrochemical process

In this work, we report on fabricating of self-doped TiO2 nanotube arrays via a facile electrochemical method by modify the electrochemical anodized amorphous TiO2 nanotube arrays with cathodic pulse doping. The self-doped TiO2 nanotube arrays is black appearance with abundant oxygen vacancy and hydroxyl groups in comparison with that of as-anodized ones. The initial discharge specific capacity of self-doped TiO2 nanotube anode is 1355 μA h/cm2 which is more than four times higher than 338 μA h/cm2 of the as-anodized TiO2 nanotube anode in lithium ion batteries. After 100 cycles, the specific capacity of the self-doped TiO2 nanotube electrodes still much higher than that of the as-anodized samples, which is attributed to good tubular morphology retention and secondary growth of the self-doped TiO2 nanotube arrays through the process of insertion/extraction Li+. The controllable regulation of the capacity of self-doped TiO2 anode is also achieved by adjusting electrochemical cathode pulse parameters. It is noteworthy that the cycled anode materials can be used as a catalyst for photocatalytic degradation of methylene blue which may shed light on the resource recovery and the environmental protection.

Photoelectrochemical degradation of 2,4-dichlorophenoxyacetic acid using electrochemically self-doped Blue TiO2 nanotube arrays with formic acid as electrolyte

Blue TiO2 nanotube arrays (Blue-TNTs) were fabricated via an electrochemical reduction method with formic acid as the electrolyte. The optimum reduction conditions were obtained as bias potential of −1.3 V, reduction time of 5 min and formic acid of 3 M. Blue-TNTs were remarkably corroded compared with the intact TNTs. Similar crystal structures of the two catalysts were observed using X-ray diffraction, while red-shift was observed for Blue-TNTs using Raman spectra. X-ray photoelectron spectroscopy indicated of the presence of Ti3+ in Blue-TNTs that resulted from the reduction of Ti4+ and reduced the resistance of the catalyst. Blue-TNTs exhibited much stronger light-absorption than intact TNTs over the entire ultraviolet-visible region, especially in the visible region. The catalyst was used toward the photoelectrochemical oxidation of 2,4-dichlorophenoxyacetic acid (2,4-D) for the first time where the influencing factors were studied. Photoelectrocatalysis with Blue-TNTs presented a 2,4-D degradation rate constant (0.0295 min−1) more than twice the sum of that of electrocatalysis (0.0055 min−1) and photocatalysis (0.0089 min−1). Blue-TNTs fabricated in formic acid showed a better photoelectrocatalytic performance for 2,4-D removal compared with that prepared in ethylene glycol, Na2SO4 and NaNO3 solution. Blue-TNTs is considered to be a promising photoelectric anode for contaminant degradation.

Facile synthesis of Ti3+ self-doped and sulfur-doped TiO2 nanotube arrays with enhanced visible-light photoelectrochemical performance

Titanium dioxide nanotube arrays (TiO2 NTAs) has shown its promising properties for use as a photoelectrode material. However, the efficiency in this regard is restricted by the wide band gap of TiO2. In this study, a facile method was employed to prepare Ti3+ self-doped and sulfur-doped (S-doped) TiO2 NTAs (H-S-TiO2 NTAs). The Ti3+ and S co-doping greatly improved the photoelectrochemical performances of TiO2 NTAs. The optimized H-S-TiO2 NTAs showed obviously red-shifted compared with the optimized TiO2 NTAs. The saturation photocurrent density and the photoelectric conversion efficiency of the optimized H-S-TiO2 NTAs reached 1.97 mA/cm2 and 1.18%, which were 3.72 and 4.92 times higher than that of the optimized TiO2 NTAs, respectively. Furthermore, the optimized H-S-TiO2 NTAs exhibited the highest transient photocurrent density (1.08 mA/cm2) with excellent photoelectric response characteristics, indicating that the transfer efficiently of photogenerated carriers was increased in the optimized H-S-TiO2 NTAs. On the basis of research results, such enhanced performances could prove that the Ti3+ self-doping and S-doping had synergistic effects. The established S-doping as well as introduced Ti3+ self-doping of TiO2 NTAs resulted in enhanced visible-light absorption and rapid electron transfer rate.

Electrocatalytic reduction of nitrobenzene using TiO2 nanotube electrodes with different morphologies: Kinetics, mechanism, and degradation pathways

Here we report on one of the first studies utilizing self-doped TiO2 nanotube arrays (NTAs) for electrocatalytic reduction of water pollutants with nitrobenzene (NB) as the model compound. In particular, we focused on the effects of morphological and crystallographic characteristics on the electrochemical reactivity of TiO2 NTAs towards the target pollutant. The TiO2 NTAs with different morphologies and exposed facets were synthesized by tuning a suite of anodization parameters and applying a post-anodization treatment. A series of electrochemical reduction tests using TiO2 NTAs as the cathode were carried out to investigate the effects of nanotube morphologies and operating conditions. Results indicate that the {0 0 1}-exposed facet, longer nanotube length, and larger nanotube diameter are the structural and morphological features that can lead to enhanced performance of TiO2 NTA electrodes. With the applied cathode potential of −1.20 V/SCE and an initial concentration of 300 mg/L, >95% NB can be efficiently degraded at the energy consumption of 2.07 kWh/m3 (EE/O), 7.67 kWh/kg (NB), and 30 kWh/kg (C), all of which are much lower than those of electrochemical oxidation processes. The good stability of the TiO2 NTA cathode was also demonstrated in 10 cyclic runs. Furthermore, CV measurements and scavenger tests were employed to study the reaction mechanism. It was found that both the direct reduction and surface adsorbed H* contributed to NB degradation. The NB transformation pathway and degradation kinetics of NB and the intermediate products were also investigated. Results obtained from this study suggest that the self-doped TiO2 NTA is a promising cost-effective electrode material for electrocatalytic reduction of water pollutants with high efficiency and stability.

Self-doped TiO2 nanotube arrays for electrochemical mineralization of phenols

Self-doped TiO2 nanotube arrays (DNTA) were prepared for the electrooxidation of resistant organics. The anatase TiO2 NTAs had an improved carrier density and conductivity from Ti3+ doping, and the oxygen-evolution potential remained at a high value of 2.48 V versus the standard hydrogen electrode, and thus, achieved a highly enhanced removal efficiency of phenol. The second anodization could stabilize Ti3+ and improve the performance by removing surface TiO2 particles. Improper preparation parameters (i.e., a short anodization time, a high calcination temperature and cathodization current density) harmed the electrooxidation activity. Although boron-doped diamond (BDD) anodes performed best in removing phenol, DNTA exhibited a higher mineralization of phenol than Pt/Ti and BDD at 120 min because intermediates were oxidized once they are produced with DNTA. Mechanism investigations using reagents such as tert-butanol, oxalic acid, terephthalic acid, and coumarin showed that the DNTA mineralization resulted mainly from surface-bound OH, and the DNTA produced more than twice the amount of OH compared with BDD. The free OH on the BDD electrode was more conducive to initial substrate oxidation, whereas the adsorbed OH on the DNTA electrode mineralized the organics in situ. The preferential removal of p-substituted phenols on DNTA was attributed mainly to their electromigration and the aromatic intermediates that are hydrophobic were beneficial to mineralization.

Fabrication of IrO2 decorated vertical aligned self-doped TiO2 nanotube arrays for oxygen evolution in water electrolysis

To fabricate effective and stable OER electrode in water electrolysis, self-doped TiO2 nanotube arrays which has a higher electrical conductivity than pristine TiO2 nanotube arrays was used as the support for loading IrO2. The self-doped TNTA was fabricated by a simple electrochemical reduction of TNTA in neutral electrolyte solution, and then IrO2 nano particles were deposited by pulse electro-deposition method. The cyclic voltammetric behavior, electrical conductivity and micro structure of self-doped TNTA prepared at different reduction potential were characterized to obtain an optimal performance, and self-doped TNTA prepared under −1.9 V (vs. Ag/AgCl) shows the best electrochemical performance. After depositing IrO2, the OER activity and stability of new electrodes were also determined. Due to the enhanced electrical conductivity of support, the mass activity of IrO2/self-doped TNTA are 50 times higher than IrO2/TNTA. The OER stability of new electrode was evaluated under constant current of 5 mA/cm2, IrO2/TNTA and IrO2/Ti were also tested for comparison. A higher stability of IrO2/self-doped TNTA electrode is observed than the other two electrodes, and XPS studies indicate a lower oxidation state of Ir in IrO2/self-doped TNTA, this shows a possible interactions between IrO2 and the new support.

IrO2 Decorated Self-Doped TiO2 Nanotube Arrays: A Binder-Free and More Stable Electrode for Oxygen Evolution Reaction in Acid Condition

The effective, stable and low cost anodic catalyst or electrode for OER is still a big challenge for water electrolysis in acid condition. Many efforts were devoted to develop suitable supports for depositing Ir and Ru metal or oxides to obtain an enhanced activity or stability for OER. In this work, self-doped TiO2 nanotube arrays (sTNTA) which has a higher electrical conductivity than TNTA was chosen as the support, it was fabricated by a simple electrochemical reduction of TNTA in neutral electrolyte. Then IrO2 nano particles were deposited on it by electrochemical pulse deposition method to form an OER electrode. The cyclic voltammetric behavior, electrical conductivity and micro structure of sTNTA prepared under different reduction potential were characterized to obtain an optimal performance for depositing IrO2, the OER activity and stability of new electrode were also determined. The obtained sTNTA shows an increased current in CV test than TNTA, which is due to the increased carrier density. As the reduction potential move to more negative level (from -1.5V to -1.9V vs. Ag/AgCl), the carrier density was found increased for 3 order of magnitudes, and XPS characterization confirm the existence of Ti3+ which formed in electrochemical reduction process, and sTNTA prepared under -1.9V shows the best electrochemical performance. After depositing IrO2, IrO2/sTNTA exhibited higher OER activity than IrO2/TNTA, according to the CV and EIS test, this can be ascribed to the enhanced electrical conductivity of support, which results in more active sites and lower charge transfer resistance of the electrode. The OER stability of IrO2/sTNTA was tested under constant current of 5mA/cm2, IrO2/TNTA and IrO2/Ti which prepared by electrochemical deposition on Ti foil were also tested for comparison. A higher stability of IrO2/sTNTA electrode was observed than the other two electrodes, and this may be due to the interaction between the IrO2 and sTNTA. XPS studies show a clear peak shift of Ir4f in IrO2/sTNTA, indicating a possible lower valence of Iridium, and such lower oxidation state of Iridium may be response for the enhanced OER stability. In summary, our studies have demonstrated the possibility of self-doped TNTA as a potential anodic support for OER. Comparing with the TNTA, the high electrical conductivity of sTNTA facilitate the charge transfer rate and thus improve the OER activity. Additionally, a higher OER stability was observed for IrO2/sTNTA electrode, and this may be due to possible catalyst-support interactions between IrO2 and self-doped TNTA. Figure 1

Electrochemically induced Ti3+ self-doping of TiO2 nanotube arrays for improved photoelectrochemical water splitting

We report a facile electrochemical reduction method to synthesize Ti3+-self-doped TiO2 nanotube arrays (TNTs), where the effects of reduction duration and potential on the photoelectrochemical performance were systematically investigated. The X-ray photoelectron spectroscopy and electron paramagnetic resonance spectra confirmed the presence of Ti3+ in the TNTs. Under the optimum reduction condition, the Ti3+-self-doped TNTs exhibited remarkably enhanced photocurrent density and photoconversion efficiency, which were nearly 3.1 and 1.75 times that of pristine TNTs, respectively. The enhancement of PEC performance is due to the improved electrical conductivity, accelerated charge transfer rate at the TNTs/electrolyte interface, as well as the improved visible light response, which is elucidated by electrochemical impedance spectra, Mott–Schottky, and UV–Vis diffuse reflection spectra.

All‐Solid‐State Cable‐Type Supercapacitors with Ultrahigh Rate Capability

All‐Solid‐State Cable‐Type Supercapacitors with Ultrahigh Rate Capability