Synthesis of highly-ordered TiO2 nanotubes for a hydrogen sensor

Highly-ordered, vertically oriented TiO₂ nanotubes are synthesized, and their hydrogen sensing properties are investigated. Self-organized TiO₂ nanotube arrays are grown by anodic oxidation of a titanium foil in an aqueous solution that contains 1 wt% hydrofluoric acid at 20 °C. We use a potential ramp at a rate of 100 mV s⁻¹, increasing from the initial open-circuit potential (OCP) to 20 V, and this final potential of 20 V is then held constant during the anodization process. The fabricated TiO₂ nanotubes are approximately 1 μm in length and 90 nm in diameter. For the sensor measurements, two platinum pads are used as electrodes on the TiO₂ nanotube arrays. The hydrogen sensing characteristics of the sensor are analyzed by measuring the sensor responses ((I − I₀)/I₀) in the temperature interval of 20–150 °C. We find that the sensitivity of the sensor is approximately 20 for 1000 ppm H₂ exposure at room temperature, and increases with increasing temperature. The sensing mechanism of the TiO₂ nanotube sensor could be explained with chemisorption of H₂ on the highly active nanotube surface.     

Hydrogen gas sensing based on polyaniline/anatase titania nanocomposite

Polyaniline (emeraldine)/anatase TiO₂ nanocomposite (PA-NC) was prepared by a chemical oxidative polymerization. The thin films of PA-NC for hydrogen gas sensing application were deposited on Cu-interdigited electrodes by spin coating technique. A study on characteristics of PA-NC thin films was demonstrated by a porous cylindrical morphology. The response and response/recovery time of sensors for hydrogen gas were evaluated by the change of TiO₂ wt% at environmental conditions. Resistance-sensing measurement was exhibited a high sensitivity about 1.63, a good Long-term response, low response time and recovery time about 83 s and 130 s, respectively, at 0.8 vol% hydrogen gas for PA-NC including 25% wt of anatase nanoparticles. The current–voltage characteristics of PA-NC gas sensors before and after hydrogen gas injection showed a nonlinear ohmic current. Moreover, we studied the formation of PA-NCs and their hydrogen gas sensing mechanism based on contact regions in PA-NC.     

Fast and highly-sensitive hydrogen sensing of Nb2O5 nanowires at room temperature

The single-crystalline Nb₂O₅ nanowires with tetragonal phase structures were synthesized through thermal oxidation process. The Nb₂O₅ nanowires were grown along [001] orientation and formed a layer of free-standing nanowire membrane. A pair of platinum electrodes was deposited on the surface of the nanowire layer to fabricate a Pt/Nb₂O₅ nanowire hydrogen sensor. The Pt/Nb₂O₅ nanowire hydrogen sensor exhibited fast, highly-sensitive and selective hydrogen response at room temperature, which may be attributed to the hydrogen induced interface and surface effects together with the high specific surface area of the Nb₂O₅ nanowires.     

Influence of Pt deposition on water-splitting hydrogen generation by highly-ordered TiO2 nanotube arrays

Highly-ordered TiO₂ nanotube arrays (TNTAs) were fabricated on Ti sheets by electrochemical anodization. Uniform Pt nanoparticles with an average diameter of 3 nm could be successfully located on the TiO₂ nanotubes on only one side (Pt/TNTAs) or both sides of the Ti sheet (Pt/TNTAs/Pt). Pt/TNTAs, the single-sided Pt deposited TNTAs, could be directly used to split water without a counter electrode. The hydrogen evolution rate can reach 120 μmol h⁻¹ cm⁻² in a mixed solution of 0.5 M Na₂SO₄ and 0.5 M ethylene glycol without any applied bias, which is six times of that by the pure TNTAs. In comparison to the traditional three electrode system, this single-sided Pt deposited TNTAs is a much more simple and efficient water splitting system. Meanwhile, the photoelectrical conversion mechanism has been investigated in detail.     

Cu(OH)2-modified TiO2 nanotube arrays for efficient photocatalytic hydrogen production

Cu(OH)₂/TNAs photocatalyst was prepared by loading Cu(OH)₂ nanoparticles on TiO₂ nanotube arrays (TNAs) using a chemical bath deposition method. The amount of Cu(OH)₂ loaded on the arrays was controlled by the repeated deposition times. The prepared catalyst was used to generate hydrogen under simulated solar light irradiation, and the results demonstrated that the hydrogen yield of Cu(OH)₂/TNAs was 20.3 times that of the pure TNAs. Furthermore, the photocatalytic efficiency for hydrogen production decreased only 5.8% after five cycles, indicating that Cu(OH)₂/TNAs photocatalyst showed excellent stability and reusability. This work presents an applicable and facile method to fabricate a highly active and stable photocatalyst for hydrogen production.     

Highly efficient CuO incorporated TiO2 nanotube photocatalyst for hydrogen production from water

Highly dispersed CuO was introduced into TiO₂ nanotube (TNT) made by hydrothermal method via adsorption-calcination process or wet impregnation process, to fabricate CuO incorporated TNT photocatalysts (CuO-TNT) for hydrogen production. It was found that CuO-TNT possessed excellent hydrogen generation activity, which was constantly vigorous throughout 5 h reaction. Depending on the preparation method, hydrogen evolution rates over CuO-TNT were founded in the range of 64.2-71.6 mmol h⁻¹ g⁻¹_catalyst, which was much higher than the benchmark P25 based photocatalysts, and even superior to some Pt/Ni incorporated TNT. This high photocatalytic activity of CuO-TNT was mainly attributed to the unique 1-D tubular structure, large BET surface area and high dispersion of copper component. Compared to wet impregnation, adsorption-calcination process was superior to produce active photocatalyst, since it was prone to produce photocatalyst with more highly dispersed CuO.     

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

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

Highly efficient TiO2 nanotube photocatalyst for simultaneous hydrogen production and copper removal from water

1-D mesoporous TiO₂ nanotube (TNT) with large BET surface area was successfully synthesized by a hydrothermal-calcination process, and employed for simultaneous photocatalytic H₂ production and Cu²⁺ removal from water. Cu²⁺, across a wide concentration range of 8-800 ppm, was removed rapidly from water under irradiation. The removed Cu²⁺ then combined with TNT to produce efficient Cu incorporated TNT (Cu-TNT) photocatalyst for H₂ production. Average H₂ generation rate recorded across a 4 h reaction was between 15.7 and 40.2 mmol h⁻¹ g⁻¹_catalyst, depending on initial Cu²⁺/Ti ratio in solution, which was optimized at 10 atom%. In addition, reduction process of Cu²⁺ was also a critical factor in governing H₂ evolution. In comparison with P25, its large surface area and 1-D tubular structure endowed TNT with higher photocatalytic activity in both Cu²⁺ removal and H₂ production.     

Design of TiO2 nanotube array-based water-splitting reactor for hydrogen generation

A water-splitting reactor yielding hydrogen and oxygen was designed with a titanium oxide (TiO₂) nanotube array photoelectrode vertically grown on a titanium substrate. The TiO₂ nanotube arrays were made by the method of anodization and annealed in an oxygen atmosphere. Hydrogen gas was collected from the reactor and the exact amount of hydrogen gas evolved from the photoanode was analyzed. The relationship between the amount of hydrogen evolution and three key factors, viz. the tube length, tube structure and crystal structure, was investigated.     

Effect of NaBF4 addition on the anodic synthesis of TiO2 nanotube arrays photocatalyst for production of hydrogen from glycerol–water solution

Addition of NaBF₄ during anodic synthesis of TiO₂ nanotube arrays (TNTAs) photocatalyst and its application for generating hydrogen from glycerol–water solution has been investigated. The TNTAs were synthesized by anodic oxidation of titanium metal in glycerol electrolyte solution containing NH₄F. During the process, the NaBF₄ with different concentrations were added to the solution. Annealing of the formatted TNTAs were performed at 500 °C for 3 h under 20% H₂ in argon atmosphere, to produce crystalline phase photocatalyst. FESEM analysis showed that self-organized and well ordered TNTAs have range of inner diameters, wall thicknesses and lengths approximately 62–130 nm, 27 nm and 1.53 μm, respectively. FTIR analysis indicated that carbon, nitrogen and boron were incorporated into the TNTAs lattice. Refer to UV–Vis DRS and XRD analysis, the TNTAs photocatalysts prepared have the band gap range of 2.70–3.10 eV, with mostly have anatase phase. The NaBF₄ addition during synthesis, resulted modified TNTAs that can reduce the recombination of photo-induced electrons-holes. Photocatalytic hydrogen production test, from glycerol–water solution, indicated that TNTAs with the addition of NaBF₄ during anodic synthesis process showed higher hydrogen production comparing to the one without NaBF₄ addition. Among them the TNTAs,b (with the addition 5 mM of NaBF₄) showed up to 32% improvement in the hydrogen production and can be considered as the optimum condition.     

The influence of the electrodeposition potential on the morphology of Cu2O/TiO2 nanotube arrays and their visible-light-driven photocatalytic activity for hydrogen evolution

The influence of the electrodeposition potential on the morphology of Cu₂O/TiO₂ nanotube arrays (Cu₂O/TNA) and their visible-light-driven photocatalytic activity for hydrogen evolution have been investigated for the first time in this work. The photocatalytic hydrogen evolution rate of the as-prepared Cu₂O/TNA at the deposition potential of −0.8 V was about 42.4 times that of the pure TNA under visible light irradiation. This work demonstrated a feasible and simple electrodeposition method to fabricate an effective and recyclable visible-light-driven photocatalyst for hydrogen evolution.     

Photoelectrochemical water splitting on highly smooth and ordered TiO2 nanotube arrays for hydrogen generation

To improve the photoelectrochemical (PEC) water splitting efficiency for hydrogen production, we reported the fabrication of lotus-root-shaped, highly smooth and ordered TiO₂ nanotube arrays (TiO₂ NTs) by a simple and effective two-step anodization method. The TiO₂ NTs prepared in the two-step anodization process (2-step TiO₂ NTs) showed better surface smoothness and tube orderliness than those of TiO₂ NTs prepared in one-step anodization process (1-step TiO₂ NTs). Under illumination of 100 mW/cm² (AM 1.5, simulated solar light) in 1 M KOH solution, water was oxidized on the 2-step TiO₂ NTs electrode with higher efficiency (incident-photon-to-current efficiency of 43.4% at 360 nm and photocurrent density of 0.90 mA/cm² at 1.23 V_RHE) than that on the 1-step TiO₂ NTs electrode. The effective photon-to-hydrogen conversion efficiency was found to be 0.18% and 0.49% for 1-step TiO₂ NTs and 2-step TiO₂ NTs, respectively. These results suggested that the structural smoothness and orderliness of TiO₂ NTs played an important role in improving the PEC water splitting application for hydrogen generation.     

Highly stable CdS-modified short TiO2 nanotube array electrode for efficient visible-light hydrogen generation

A highly stable photoelectrocatalytic electrode made of CdS-modified short, robust, and highly-ordered TiO₂ nanotube array for efficient visible-light hydrogen generation was prepared via sonoelectrochemical anodization and sonoelectrochemical deposition method. The short nanotube electrode possesses excellent charge separation and transfer properties, while the sonoelectrochemical deposition method improves the combination between CdS and TiO₂ nanotubes, as well as the dispersion of CdS nanoparticles. Different characterization techniques were used to study the nanocomposite electrode. UV-vis absorption and photoelectrochemical measurements proved that the CdS coating extends the visible spectrum absorption and the solar spectrum-induced photocurrent response. Comparing the photoactivity of the CdS/TiO₂ electrode obtained using sonoelectrochemical deposition method with others that synthesized using plain electrochemical deposition, the current density of the former electrode is ∼1.2 times higher that of the latter when biased at 0.5 V. A ∼7-fold enhancement in photocurrent response is obtained using the sonoelectrochemically fabricated CdS/TiO₂ electrode in comparison with the pure TiO₂ nanotube electrode. Under AM1.5 illumination the composite photoelectrode generate hydrogen at a rate of 30.3 μmol h⁻¹ cm⁻², nearly 13 times higher than that of pure titania nanotube electrode. Recycle experiments demonstrated the excellent stability and reliability of CdS/TiO₂ electrode prepared by sonoelectrochemical deposition. This composite electrode, with its strong mechanical stability and excellent combination of CdS and TiO₂ nanotubes, offers promising applications in visible-light-driven renewable energy generation.     

Photocatalytic hydrogen production from water/methanol solutions over highly ordered Ag-SrTiO3 nanotube arrays

The highly ordered Ag-SrTiO₃ nanotube arrays (NTAs) with uniform size were successfully synthesized by a combination of anodic oxidation, hydrothermal process and photocatalytic reduction method. X-ray photoelectron spectroscopy analysis reveals that Ag exists in the form of metallic silver, which is in good agreement with the X-ray diffraction characterization. Moreover, the UV-vis diffuse reflectance spectra indicate that Ag-SrTiO₃ NTAs have a strong absorption in the visible region which is attributed to the plasmon resonance of silver nanoparticles. After Ag loading, a further improvement of the photocatalytic activity for hydrogen production was obtained. Based on the above results, a possible electron-hole transfer mechanism was also assumed.     

Single crystalline TiO2 nanorods with enhanced visible light activity for solar hydrogen generation

Single crystalline TiO₂ nanorods and polycrystalline nanotubes were fabricated with same length to investigate the effects of their nanostructures on photocatalytic properties for splitting water. In order to enhance the visible light absorbance, TiO₂ nanorods and nanotubes were sensitized with semiconductor nanoparticles such as CdS, CdSe, and CdS/CdSe, and compared in viewpoint of solar hydrogen generation. It was observed that single-crystalline nanorods showed superior photocatalytic properties to polycrystalline nanotubes, and also the potential level of the nanorods with rutile phase was measured as lower than that of the nanotubes with mixture of anatase and rutile. Further improvement of photo-conversion efficiency was obtained by subsequent heat treatments of the sensitized photoelectrodes. It turns out that the improvement is attributed to the improved crystallinity and the increased size of the nanoparticles during the post-annealing treatments. It was demonstrated that TiO₂ nanorods with lower potential level and a single crystalline phase on FTO glass were advantageous for effective charge injection from the sensitized nanoparticles and transport without recombination lost at grain boundaries.     

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

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

High nuclearity Co polyoxometalate based artificial photosynthesis for solar hydrogen generation

The photocatalytic hydrogen (H₂) generation by the high nuclearity Co substituted polyoxometalates (POMs), K₁₀Na₁₂[{Co₃(B-β-SiW₉O₃₃(OH))(B-β-SiW₈O₂₉ (OH)₂)}₂]·49H₂O (abbreviated as CoPOM) was reported. Owing to the multielectron redox capabilities of the high nuclearity CoPOM, the POM showed excellent photocatalytic activities toward H₂ evolution in both molecule scale (homogeneous) and composite (heterogeneous) systems. The photocatalytic activities of CoPOM were much better than H₃PW₁₂O₄₀. UV–Vis–NIR absorption spectral, Raman spectral, cyclic voltammetric behavior and intermittent photoelectrochemical current response were used to characterize the structure of the TiO₂/CoPOM composite and interaction between TiO₂ and CoPOM. Analogous “Z-scheme” and “dye” sensitized mechanisms were proposed for the homogeneous and heterogeneous systems toward photocatalytic H₂ evolution under solar light irradiation, respectively.     

MOS hydrogen sensor with very fast response based on ultra-thin thermal SiO2 film

An MOS capacitor-type hydrogen gas sensor was fabricated with the structure of Ni/SiO₂/Si by using conventional silicon wafer technologies. Grown by dry oxidation at 900°C, the thickness of the SiO₂ film was only 24 Å. At 150°C, comparing to another MOS capacitor with 148 Å-thick oxide and otherwise identical configurations, this sensor showed much faster response speed (the time interval to reach half of the magnitude of the steady-state signal, or t_50%, was only 4 s in response to 1% H₂ without deduction of the delay from the gas delivery system), as well as enhanced signal magnitude (about two times of the former for 1% H₂). Based on the hydrogen-binding to the traps in the bulk SiO₂, a mechanism was proposed to explain the very short response time on the device with the ultra-thin SiO₂. The gate leakage in the device is also discussed. The presented sensor demonstrates a promising step in designing low-cost H₂ detectors with very fast responses.     

Photoelectrical and charge transfer properties of hydrogen-evolving TiO2 nanotube arrays electrodes annealed in different gases

TiO₂ nanotube arrays were fabricated by sonoelectrochemical anodic oxidation and calcined in nitrogen, air, or 5% hydrogen/nitrogen which was denoted as TNT-A, TNT-N, and TNT-H, respectively. All annealed TiO₂ nanotube arrays samples exhibited similar surface morphology. With UV illumination (365 ± 15 nm), the photocurrent density of the TNT-A, TNT-N and TNT-H was about 0.27 mA/cm², 0.45 mA/cm² and 0.60 mA/cm², respectively. The trapped electron at the Ti⁴⁺ center of TiO₂ nanotube arrays shows absorption at around 500-700 nm. From the XPS measurement, it was found that annealing in 5% hydrogen/nitrogen helped the sample obtain a greater defect density. Because of the reduction of Ti⁴⁺ and the formation of oxygen vacancies, the charge transfer resistance appeared in this order: TNT-A > TNT-N > TNT-H. Thus TNT-H harvested the greatest charge carrier density of 9.86 × 10²⁰ cm⁻³, TNT-N and TNT-A obtained a charge carrier density of 1.38 × 10²⁰ cm⁻³ and 1.06 × 10²⁰ cm⁻³, respectively. Accordingly, the hydrogen production rate by water splitting over TNT-A, TNT-N and TNT-H (320-780 nm irradiation, 3 h) was about 120 μL/h cm², 159 μL/h cm² and 231 μL/h cm², respectively.     

Facile fabrication of Bi2O3/TiO2-xNx nanocomposites for excellent visible light driven photocatalytic hydrogen evolution

Mesoporous Bi₂O₃/TiO_2−xN_x nanocomposites (BiNT) were synthesized by soft chemical template free homogeneous co-precipitation technique. XRD, XPS, TEM, UV-Vis DRS and photoluminescence studies were adapted to determine the structural, electronic and optical properties. The photocatalytic activities of the catalysts were evaluated for water splitting to generate clean hydrogen fuel under visible light irradiation (λ ≥ 400 nm). BiNT-400 catalyst showed highest results towards hydrogen production (198.4 μmol/h) with an apparent quantum efficiency of 4.3%. The pronounced activity of BiNT-400 sample towards hydrogen production was well consistent with high crystallinity, large surface area, proper excitation by N doping and Bi₂O₃ sensitization.