Nb-doped TiO2 Nanotubes Research Articles

Flow-through Gas Phase Photocatalysis Using TiO2 Nanotubes on Wirelessly Anodized 3D-Printed TiNb Meshes.

In this work, for the first time 3D Ti-Nb meshes of different composition, i.e., Ti, Ti-1Nb, Ti-5Nb, and Ti-10 Nb, were produced by direct ink writing. This additive manufacturing method allows tuning of the mesh composition by simple blending of pure Ti and Nb powders. The 3D meshes are extremely robust with a high compressive strength, giving potential use in photocatalytic flow-through systems. After successful wireless anodization of the 3D meshes toward Nb-doped TiO2 nanotube (TNT) layers using bipolar electrochemistry, they were employed for the first time for photocatalytic degradation of acetaldehyde in a flow-through reactor built based on ISO standards. Nb-doped TNT layers with low concentrations of Nb show superior photocatalytic performance compared with nondoped TNT layers due to the lower amount of recombination surface centers. High concentrations of Nb lead to an increased number of recombination centers within the TNT layers and reduce the photocatalytic degradation rates.

Highly Selective, Defect-Induced Photocatalytic CO2 Reduction to Acetaldehyde by the Nb-Doped TiO2 Nanotube Array under Simulated Solar Illumination

The adsorption and activation of CO2 molecules on the surface of photocatalysts are critical steps to realize efficient solar energy-induced CO2 conversion to valuable chemicals. In this work, a defect engineering approach of a high-valence cation Nb-doping into TiO2 was developed, which effectively enhanced the adsorption and activation of CO2 molecules on the Nb-doped TiO2 surface. A highly ordered Nb-doped TiO2 nanotube array was prepared by anodization of the Ti-Nb alloy foil and subsequent annealing at 550 °C in air for 2 h for its crystallization. Our sample showed a superior photocatalytic CO2 reduction performance under simulated solar illumination. The main CO2 reduction product was a higher-energy compound of acetaldehyde, which could be easily transported and stored and used to produce various key chemicals as intermediates. The acetaldehyde production rate was over ∼500 μmol·g-1·h-1 with good stability for repeated long-time uses, and it also demonstrated a superior product selectivity to acetaldehyde of over 99%. Our work reveals that the Nb-doped TiO2 nanotube array could be a promising candidate with high efficiency and good product selectivity for the photocatalytic CO2 reduction with solar energy.

Platinum-catalyzed Nb–doped TiO2 and Nb-doped TiO2 nanotubes for hydrogen generation in proton exchange membrane water electrolyzers

This paper studies the catalytic activity of Pt deposited onto Nb-doped titania supports toward the hydrogen evolution reaction (HER). New catalysts based on Nb-doped TiO2 nanoparticles (nNb-TiO2) and Nb-doped TiO2 nanotubes (nNb-TNTs), with n in the range 3–10 at % (Nb + Ti), were synthesized. The specific surface areas of nNb-TNTs were 250–300 m2g-1, about three times higher than those of nNb-TiO2. X-ray diffraction showed the Nb incorporation into the TiO2 lattice with its consequent lattice expansion. The X-ray photoelectron spectra of Pt deposited onto Nb-doped titania revealed a negative charge accumulation on Pt, thus denoting strong metal-support interaction. The electrochemical characterization in acidic media showed that Pt supported on nNb-TiO2 and nNb-TNTs presented better activity toward the HER than that of Pt deposited onto the un-doped supports and commercial Pt on carbon. The best performance was obtained with a small Nb doping of 3 at %.

Effective combination of CuFeO2 with high temperature resistant Nb-doped TiO2 nanotube arrays for CO2 photoelectric reduction

Herein, we report a simple approach to synthesize CuFeO2/TNNTs photocathodes composed of high-temperature resistance n-type Nb-doped TiO2 nanotube arrays (TNNTs) and p-type CuFeO2 for CO2 reduction. TNNTs were prepared by anodic oxidation on TiNb alloy sheets and CuFeO2/TNNTs were then prepared by coating precursor liquid onto TNNTs followed by heat treatment in argon atmosphere. The microstructures of CuFeO2/TNNTs and TNNTs before and after heat treatment were investigated by SEM and TEM. The phase compositions of CuFeO2/TNNTs were studied by XRD and XPS, and the light absorption performance were tested by UV–vis diffuse reflectance spectrum. Results show that TNNTs exhibit a regular nanotube arrays structure and this structure is well remained after the calcination at 650 °C. In addition, TNNTs show similar semiconductor properties to n-type TiO2, which enables them to be integrated with p-type CuFeO2 to obtain composite photocathodes with a p-n junction. The synthesized CuFeO2/TNNTs photocathode is well crystallized because no other crystalline iron or copper compounds are included in the prepared photocathode. Furthermore, the photocathode shows high light absorption and fast carrier transport due to the appropriate band gap and p-n junction. As a result, high photoelectrocatalytic CO2 reduction performance with high selectivity to ethanol is obtained on this photocathode.

Supporting IrO2 and IrRuO nanoparticles on TiO2 and Nb-doped TiO2 nanotubes as electrocatalysts for the oxygen evolution reaction

Abstract IrO2 and IrRuOx (Ir:Ru 60:40 at%), supported by 50 wt% onto titania nanotubes (TNTs) and (3 at% Nb) Nb-doped titania nanotubes (Nb-TNTs), as electrocatalysts for the oxygen evolution reaction (OER), were synthesized and characterized by means of structural, surface analytical and electrochemical techniques. Nb doping of titania significantly increased the surface area of the support from 145 (TNTs) to 260 m2 g−1 (Nb-TNTs), which was significantly higher than those of the Nb-doped titania supports previously reported in the literature. The surface analytical techniques showed good dispersion of the catalysts onto the supports. The X-ray photoelectron spectroscopy analyses showed that Nb was mainly in the form of Nb(IV) species, the suitable form to behave as a donor introducing free electrons to the conduction band of titania. The redox transitions of the cyclic voltammograms, in agreement with the XPS results, were found to be reversible. Despite the supported materials presented bigger crystallite sizes than the unsupported ones, the total number of active sites of the former was also higher due to their better catalyst dispersion. Considering the outer and the total charges of the cyclic voltammograms in the range 0.1–1.4 V, stability and electrode potentials at given current densities, the preferred catalyst was IrO2 supported on the Nb-TNTs. The electrode potentials corresponding to given current densities were between the smallest ones given in the literature despite the small oxide loading used in this work and its Nb doping, thus making the Nb-TNTs-supported IrO2 catalyst a promising candidate for the OER. The good dispersion of IrO2, high specific surface area of the Nb-doped supports, accessibility of the electroactive centers, increased stability due to Nb doping and electron donor properties of the Nb(IV) oxide species were considered the main reasons for its good performance.

Pure and Highly Nb-Doped Titanium Dioxide Nanotubular Arrays: Characterization of Local Surface Properties.

This paper presents the results of studies of the local surface properties of pure and highly Nb-doped (12 wt %) TiO2 nanotubes (TNT) using the X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) methods, respectively. XPS analysis showed that the pure TNT exhibit an evident over-stoichiometry combined with high level of undesired C contaminations, which was confirmed by the relative concentration of specific elements O, Ti and C (with respect to all the surface atoms) equal to 0.46, 018 and 0.36, respectively. In turn, for the highly Nb-doped (12 wt %) TNT, a slightly different surface chemistry was observed because the relative concentration of specific elements O and Ti and, with respect to all the surface atoms, is slightly lower, that is, 0.42 and 0.12, respectively; this is directly related to the fact that Nb atoms appeared having the relative concentration at the level of 0.09, whereas the undesired C contaminations reached the same level (0.36), as is the case of pure TNT. In addition, SEM analysis confirms that there are evident free spaces between the specific slops containing several TNT, what was additionally confirmed by the contribution of specific surface bonding coming from the SiO2/Si substrate. The obtained information allowed us a new insight on the potential origin of aging effect at the surface of TNT in atmosphere being the undesired limitation for their potential application as the chemical resistive type sensors or in any other fields of their application related to their surface activity.

Thermal Decomposition of Some Sol-Gel Precursors for Mesoporous TiO2-Based Thin Films for Chemorezistive Environmental Sensors

This paper presents a comparative study on the thermal decomposition in air and FTIR analyses of complex sol-gel precursors used for the deposition of three titania based mesoporous thin films: TiO2, Nb-doped TiO2 and carbon nanotubes (MWCNT)-doped TiO2. The effect of doping with Nb or adding MWCNT on the thermal decomposition of titanium isopropoxide-based precursor and crystallization of TiO2 was investigated through thermogravimetric analysis (TGA), differential thermal analysis (DTA), and Fourier Transform Infrared (FTIR) data. TG-DTA results showed two main exothermic peaks were observed in all samples for the temperature range 280-300 oC and 429-444 oC, corresponding to isopropoxide oxidative decomposition and anatase crystallization. In case of Nb doped TiO2 an oxidative decomposition of Nb ethoxide appears at 198 �C. The acidified MWCNT are thermally stable up to 500�C. Doping TiO2 causes peaks shift to higher temperatures. Also, the effect of heating rate on thermal decomposition for precursors was studied. FTIR analyses were conducted on precursor systems thermally treated at different temperature to track the chemical transformations during the films formation.

A promising electrode material modified by Nb-doped TiO2 nanotubes for electrochemical degradation of AR 73

A distinctive SnO2Sb electrode with highly ordered Nb doped TiO2 nanotubes sheet as a new substrate, obtained by NbTi alloy anodization, is prepared by pulse electrochemical deposition for the first time as electrocatalytic oxidation anode for wastewater treatment. The novel electrode has a larger surface area and smaller crystallite particles than conventional SnO2Sb electrodes as obtained from the analysis of scanning electron microscopy and X-ray diffraction. Compared with Ti/SnO2Sb and Ti/TiO2-NTs/SnO2Sb prepared by pulse electrochemical deposition, the electrode modified by NbTiO2-NTs has the higher oxygen evolution potential of 2.29 V (vs. Ag/AgCl), and the lower charge transfer resistance, which decreased by 65% and 79%. The service lifetime of NbTi/NbTiO2-NTs/SnO2Sb is 4.9 times longer than that of Ti/SnO2Sb and 1.9 times longer than that of Ti/TiO2-NTs/SnO2Sb. The new electrode is proved to have an excellent electrochemical oxidation and degradation ability using Acid Red 73 as a target organic pollutant. The AR 73 removal, chemical oxygen demand removal and kinetic rate constant are increased obviously due to the introduction of NbTiO2-NTs. Besides, the energy consumption reduces 37.2% and 31.4% in contrast with Ti/SnO2Sb and Ti/TiO2-NTs/SnO2Sb. Hence, the Ti/SnO2Sb modified by NbTiO2-NTs is a very promising anode material for the electrochemical treatment of dye wastewater.

(Invited) Doped Titania Oxide Nanotubes for Photoelectochemical Water Splitting

Extensive studies have been carried out on clean and renewable energy due to the pollution and depletion of fossil fuel. Ions doping is the efficient ways to enhance photocatalytic activity of titanium oxide (TiO2). Here we report photocatalytic activity of the Ni and Nb-doped TiO2 nanotube arrays photoanodes, respectively. The Ni-doped TiO2 nanotube arrays were fabricated through anodization of Ti50Ni alloys in electrolyte containing 5% ethylene glycol/glycerol containing (NH4)2SO4 and NH4F. Nb-doped TiO2 nanotube arrays were fabricated through anodization of Ti35Nb alloys in electrolytes of 1M NaH2PO4 containing 0.5 wt% HF. X-ray diffraction (XRD), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to characterize the doped TiO2 nanotubes photoanodes. The experimental results indicated that Nb and Ni doping could enhance the photocatalytic properties of the titanium oxide. The photocatalytic activity of Nb-doped TiO2 and Ni-doped TiO2 nanotube arrays photoanodes were 1.5 and 2 times as high as that of the undoped TiO2 nanotubes photoanodes, respectively. Density functional theory (DFT) calculations indicated that Ni-doping could narrow the band gap of undoped TiO2 through formation of impurity states and Nb doping could result in the valence band shift of the undoped TiO2. The doped TiO2 nanotube photoanode are promising for photoelectochemical water splitting application.

Optimization of Photoelectrochemical Performance of Long TiO2 Nanotubes By Li-Doping Induced Defect Passivation

Photoelectrochemical conversion of sunlight into storable fuels has great potential to overcome the challenge posed by sunlight’s transient nature. TiO2 nanotubes are an attractive system for this application due to their high surface area, 1D charge transport, and stability in solution and under irradiation. The photoelectrochemical performance of TiO2 nanotubes need to be optimized by balancing increased light absorption in longer nanotubes against the limitations imposed by the presence of trap states which would increase recombination of photogenerated charges. Although nanotubes from anodizing in low water content electrolytes can access the widest range of nanotube lengths, their high density of trap states facilitates recombination processes.[1] It has been demonstrated that 7 mm long nanotubes maximize efficiency in Nb doped TiO2 nanotubes.[2] Recently, we have shown that trap state passivation can be readily achieved by electrochemically induced Li doping accompanied by the formation of Ti3+ states in TiO2 nanotubes. This treatment suppresses recombination by pre-filling the traps with electrons and yields up to a 2x enhancement in the photoelectrochemical performance for 1 mm long nanotubes (Figure 1 (a)), providing a potential route to extend the length-vs-photocurrent maximum to longer tubes.[3] With this Li doping process we are able to demonstrate a 15 micron long nanotube array yields the highest photocurrent between 1 and 21 mm, generating 1.5 mA/cm2 of photocurrent under simulated sunlight (Figure 1 (b)). Studies by electrochemical impedance have confirmed the trap state passivation mechanism by direct inspection of the density of trap-state levels and electron paramagnetic resonance spectroscopy measurements have identified Ti3+ states that accompany this doping process. Figure Caption Figure 1. (a) Lithium modified TiO2 nanotubes show up to 2x enhancement in photoelectrochemical performance at 1.0 VSCE. (b) Photoelectrochemical performance of TiO2 nanotubes up to 21 microns in length shows an optimum at 15 microns after Li doping. References References [1] L. Tsui, G. Zangari, Electrochim. Acta 121 (2014) 203. [2] C. Das, P. Roy, M. Yang, H. Jha, P. Schmuki, Nanoscale 3 (2011) 3094. [3] L. Tsui, M. Saito, T. Homma, G. Zangari, J. Mater. Chem. A (2014). In-Press. 10.1039/C4TA05620E Figure 1

Impact of the Anodizing Potential on the Electron Transport Properties of Nb-doped TiO2 Nanotubes

The effect of the anodizing potential on the photoelectrochemical properties of (Nb-doped)-TiO2 Nanotubes (NTs) has received little attention to date within the research community. Our study to understand the photochemistry of the NTs reveals a clear impact of the anodizing potential (15 to 60V) on the microstructure and the electron transport properties of the NTs. At 15V, the NTs film develops very slowly and is either too thin to absorb light effectively (after 20minutes anodization) or too damaged (after 60minutes), due to excessive dissolution by the electrolyte causing collapsing and bundling of the NTs. Consequently the 1D electron transport (associated with well-defined, undamaged NTs) is lost and the photoresponse is poor (~ 0.6mA/cm2 at 1V vs RHE). An anodizing potential of 30V appears instead to be optimal as it ensures a good balance between the growth and dissolution of the oxide; NTs obtained under these conditions provide a remarkable photocurrent density up to ~ 1.2mA/cm2. When anodizing at 45-60V instead, the oxidation proceeds too fast, inducing the formation of cracks within the anodic film, making the anodizing process more difficult to control, although in general a good photoresponse (0.9÷1.0mA/cm2) is still observed. Other than a small contribution (<5%) above 400nm due to the presence of shallow donors in the anodic film, the Incident Photon to Current Efficiency (IPCE) of NTs grown at 15-30V is characterized by a well-defined single slope, associated with the fundamental absorption of TiO2. In contrast, NTs grown at 45-60V exhibit a more complex electron transport; the presence of additional slopes in their IPCE response is tentatively explained in terms of new localized energy states induced in the anodic film above a certain potential threshold (30V).

Fabrication and investigation of gas sensing properties of Nb-doped TiO2 nanotubular arrays

Synthesis of Nb-containing titania nanotubular arrays at room temperature by electrochemical anodization is reported. Crystallization of pure and Nb-doped TiO2 nanotubes was carried out by post-growth annealing at 400 °C. The morphology of the tubes obtained was characterized by scanning electron microscopy (SEM). Crystal structure and composition of tubes were investigated by glancing incidence x-ray diffraction (GIXRD) and total reflection x-ray fluorescence (TXRF). For the first time gas sensing characteristics of Nb-doped TiO2 nanotubes were investigated and compared to those of undoped nanotubes. The functional properties of nanotubular arrays towards CO, H2, NO2, ethanol and acetone were tested in a wide range of operating temperature. The introduction of Nb largely improves conductivity and enhances gas sensing performances of TiO2 nanotubes.

Nb-doping of TiO2/SrTiO3 nanotubular heterostructures for enhanced photocatalytic water splitting

Ti alloys with low Nb contents (0.1at.% and 0.5at.%) were used to grow self-organized Nb-doped TiO2 nanotube arrays. These layers were partially converted to SrTiO3 using a hydrothermal treatment. Various factors, such as hydrothermal temperature, hydrothermal duration, and annealing conditions, were investigated in view of the photocatalytic water splitting performance of the formed Nb-doped TiO2/SrTiO3 heterostructure nanotubes. Photoelectrochemical characterization shows that Nb-doping efficiently enhances the photocatalytic water splitting performance of TiO2/SrTiO3 heterostructure nanotubes.

Fabrication of pure and Nb–TiO2 nanotubes and their functional properties

TiO2 and Nb-doped TiO2 nanotubes were obtained on flexible polymeric substrates (Kapton HN) and alumina with high roughness (alumina substrate with granular surface). Nanotubes were prepared by electrochemical anodization of a Ti thick film. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to investigate the morphology of the structures and the roughness of the substrates. The functional properties of tubular arrays obtained on flexible and rough substrates were investigated towards two applications, namely, chemoresistive gas sensors and flexible dye sensitized solar cells.

Wide-range hydrogen sensing with Nb-doped TiO2 nanotubes

Anatase-type titania nanotubes doped with Nb element were fabricated through an anodization of Ti35Nb alloy substrate and further annealing at 450 °C. Hydrogen sensitivity of the Nb-doped TiO2 nanotubes at room temperature was investigated through exposure of the nanotube samples to different hydrogen atmospheres. At room temperature, the Nb-doped nanotubes demonstrated a good sensitivity for wide-range detection of both dilute and high-concentration hydrogen atmospheres ranging from 50 ppm to 2% H2. The Nb-doped nanotubes also presented remarkable reversibility and repeatability as well as a quick response to the hydrogen atmosphere. The Nb-doped titania nanotubes have great advantages as robust and wide-range hydrogen sensors operating at room temperature.

Increased photocurrent response in Nb-doped TiO2 nanotubes

Nb-doped TiO2 nanotube (NT) layers were grown by electrochemical anodization of Ti–Nb alloys. To successfully activate Nb doping the oxide tubes need to be annealed at sufficiently high temperatures (∼650 °C). For tubes doped with 0.1 wt% Nb not only a considerable increase in the electrical conductivity but also a significantly enhanced photocurrent (superior to any pure TiO2 nanotube layer (anatase or rutile)) was obtained.

Nb doping of TiO2nanotubes for an enhanced efficiency of dye-sensitized solar cells

Nb-doped TiO(2) nanotube (with C(Nb) < 1 wt%) layers were successfully fabricated by self-ordered electrochemical anodization of Ti-Nb alloys. When used in dye-sensitized solar cells the efficiency enhanced by up to 30% compared to non-doped TiO(2) nanotubes. IMVS measurements indicate the beneficial effect to be due to lower recombination losses.

Nb doped TiO2 nanotubes for enhanced photoelectrochemical water-splitting

Nanostructured titanium dioxide is one of the classic materials for photoelectrochemical water splitting. In the present work we dope TiO(2) nanotube anodes. For this, various low concentration bulk-Nb-doped TiO(2) nanotube layers were grown by self-organizing anodization of Ti-Nb alloys. At Nb-contents around 0.1 at%, and after an adequate heat-treatment, a strongly increased and stable photoelectrochemical water-splitting rate is obtained.