NiO TiO2 Research Articles

Ordered NiO-TiO2 nanotube arrays as an efficient catalyst support for methanol oxidation

Ordered NiO–TiO2 nanotube arrays (NiO–TiO2NTs) were fabricated on titanium–nickel (Ti–Ni) alloy substrate by anodic oxidation of Ti–Ni alloy foil and subsequently investigated as a catalyst support for methanol oxidation. The field emission scanning electron microscopy (FESEM) results demonstrate that NiO–TiO2NTs have the feature of regular tubular structure, which is advantageous to support catalysts for methanol oxidation reactions. The electrochemical results show that Pt nanoparticles supported on NiO–TiO2NTs have similar functionalities as PtNi alloy nanoparticles supported on TiO2 nanotube arrays for methanol oxidation. The NiO–TiO2NTs catalyst support developed here can be easily synthesized and thus show promising application prospect in direct methanol fuel cells (DMFC).

Enhanced Photocatalytic Activity in p-NiO Grafted n-TiO2 Porous Coatings

We synthesized NiO-loaded TiO2 porous p–n junctions by electrophoretic enhanced micro arc oxidation (EEMAO) technique which is a relatively simple and new method for producing composite coatings. Phase structure, chemical composition, and surface morphology of the NiO–TiO2 coatings were investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) techniques, respectively. Photocatalytic efficiency of the samples was studied through measuring the decomposition rate of 4-chlorophenol under UV irradiation. Results showed that the coatings mainly consisted of anatase, rutile, and nickel oxide phases. It was observed that a finer surface morphology, i.e. smaller pores, was obtained at lower voltages. Besides, the amount of nickel oxide loaded to the TiO2 host increased with the voltage. The NiO–TiO2 composite coatings showed a higher photoactivity than pure TiO2 coatings revealing the effective role of NiO in decreasing the recombination rate of photogenerated electrons and holes. The maximum photocatalytic reaction rate constants for TiO2 and NiO–TiO2 samples were obtained as 0.0064 and 0.0131 min−1, respectively. We also found that there was an optimum concentration of NiO where a maximum photocatalytic activity was achieved. A correlation between growth variables, structure, and photocatalytic efficiency was established.

NiO-decorated mesoporous TiO2 flowers for an improved photovoltaic dye sensitized solar cell.

Reducing light-induced e-h recombination is important for a dye sensitized solar cell (DSSC); the p-type NiO component in TiO2-NiO nanoparticles was reported to significantly decrease charge recombination, but its photovoltaic efficiency remains below 4% owing to a small surface area. In this work, we used a one-pot self-assembly process to fabricate flower-like mesoporous TiO2 decorated by NiO oxides, employing a pluronic polymer P123 as a structure directing and pore forming agent. The flower-like porous TiO2-NiO nanoparticles (F-TiO2-NiO NPs), possessing a high BET surface of 130 m(2) g(-1), are first used as a photoanode in DSSCs. These hybrid nanoparticles, decorated with NiO islands, are beneficial for improving photocurrent by increasing dye absorption and suppressing electron-hole recombination. The optimized F-TiO2-NiO NP anode (10 μm thick) achieved a power conversion efficiency of 8.20%, which is 26% and 47% higher than pristine flower-like TiO2 and commercially available P25 anodes, respectively. This efficiency is the highest among the reported TiO2-NiO hybrid anodes.

Single-site nickel-grafted anatase TiO2 for hydrogen production: Toward understanding the nature of visible-light photocatalysis

This work provides an engineering guide to constructing active sites on TiO2 with the surface organometallic chemistry of nickelocene for unveiling the molecular nature of so-called “p–n heterojunction”-induced visible-light photocatalysis. H2 evolution was used as a model reaction to evaluate the photocatalytic properties of Ni/TiO2 materials containing different Ni-oxo species, which are prepared by three methods. Ni nuclearity-dependent H2 evolution was shown by comparison of H2 production rates over such Ni/TiO2 photocatalysts. Detailed characterizations clearly revealed the triple role of atomically isolated Ni species as light-harvesting, electron-trapping, and hydrogen-evolving sites in the photocatalytic reaction. The TiONi molecular junctions formed at the NiO–TiO2 interface perform the function of p–n heterojunctions and create visible light absorption and photocatalysis. It was well established that visible light photocatalysis follows a physical mechanism of metal-to-metal charge transfer in TiONi linkages severing as visible-light chromophores. Hydrogen gas is reduced and evolved at oxygen vacancies of the TiO2 surface.

Effect of random p–n junctions on quasi-solid-state dye-sensitized solar cells with polymer electrolyte

In this paper, the effect of p–n junction additives via blending p-type semiconductors (NiO and CuI) with n-type TiO2 in polymer electrolyte on quasi-solid-state DSSCs is studied. The conduction mechanism of DSSCs modified with random p–n junctions has been investigated by FTIR, SEM, UV–Vis and electrochemical analysis. The results indicate that the CuI–TiO2 p–n blend modified polymer electrolyte reaches the maximum ionic conductivity of 6.44 mS cm−1, which is almost twice over than that of pure-TiO2 modified one. From photovoltaic study, it is found that introducing CuI–TiO2 and NiO–TiO2 p–n blends in polymer electrolyte can obviously improve electron recombination and light-to-electric conversion efficiency of DSSCs. The conversion efficiency of CuI–TiO2 p–n blend modified DSSC is 3.34%, which is much higher than that of pure TiO2 modified device (1.76%). It is also demonstrated that the photovoltaic properties of DSSCs directly depends on the ratio of p-type CuI and n-type TiO2 in the p–n blends and the optimal conversion efficiency of 4.27% is obtained at CuI:TiO2 ratio of 4:1.

Titanium dioxide–nickel oxide composite coatings: Preparation by mechanical coating/thermal oxidation and photocatalytic activity

Photocatalytic TiO2–NiO composite coatings were prepared by the mechanical coating technique and subsequent heat oxidation. The preparation process is examined in detail. The prepared coatings were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). The results showed that Ni particles were mainly located at the inner layer of these composite coatings if prepared by 1-step MCT, whereas the Ni particles adhered to the surfaces of the Ti–Ni composite coatings if prepared by 2-step MCT. At oxidation temperatures of 973 and 1073K, the Ti coatings were partly oxidized to rutile TiO2 and a Ti–TiO2–NiO composite microstructure was formed. When the oxidation temperature was further increased to 1173 and 1273K, the Ti coatings were oxidized to rutile TiO2 and a TiO2–NiO composite microstructure was obtained. The photocatalytic activity of these composite coatings was evaluated by the degradation of a methylene-blue (MB) solution under UV irradiation. The results showed that the sample prepared by 2-step MCT and subsequent heat oxidation at 1073K had the highest photocatalytic activity, i.e., more than 200nmolL−1h−1. The improvement in the photocatalytic activity could be attributed to the efficiency increase in charge separation, which may result from the formation of the TiO2–Ti composite microstructure and TiO2–NiO p–n heterojunctions.

Performance studies of electrolyte-supported solid oxide fuel cell with Ni–YSZ and Ni–TiO2–YSZ as anodes

Redox cycling of Ni-based anode induces cell degradation which limits the cell's lifetime during solid oxide fuel cell operation. In the present study, the redox testing of electrolyte-supported cells has been investigated with TiO2-added NiO–YSZ anode matrix. Button cells were fabricated by die-pressing YSZ powder as electrolyte, and onto which NiO–YSZ or NiO–TiO2–YSZ anode and LSM–YSZ composite cathode were painted. The electrochemical performance and stability have been evaluated by measuring current–voltage characteristics followed by impedance spectroscopy after each redox cycling. Anode matrices before and after cell operation have been characterized by X-ray diffraction (XRD), elemental dispersive X-ray (EDX), and scanning electron microscopy (SEM). During cell operation the peak power density decreases from 111 mW cm−2 (239 mA cm−2) to 84 mW cm−2 (188 mA cm−2) between 23 and 128 h with five redox cycles for cell having NiO–YSZ (40:60) anode. But for cell with NiO–TiO2–YSZ (30:10:60), the anode peak power density was constant and stable around 85 mW cm−2 (194 mA cm−2) throughout the cell run of 130 h and five redox cycles. No loss in the open circuit voltage was observed. SEM and XRD studies of NiO–TiO2–YSZ (30:10:60) anodes revealed formation of ZrTiO4, which may be responsible for inhibition of Ni coarsening leading to stable cell performance.

Changes in the mechanical properties of support materials for segmented-in-series type solid oxide fuel cells as a function of time

As the substrate materials for segmented-in-series type solid oxide fuel cells (SOFCs), the mechanical characteristics, such as Young's modulus, of Al2O3–NiO–YSZ, TiO2–NiO–YSZ and MgO–NiO–YSZ were examined under SOFC operating conditions. The Young's modulus for all of the samples decreased at a high temperature. For the Al2O3–NiO–YSZ samples, the behaviours of the Young's modulus changed at each process including as-sintered, 100 h reduction and 2000 h reduction. For the TiO2–NiO–YSZ samples, the Young's modulus behaviour varied between the as-sintered and reduced samples. However, further reduction did not change the behaviour of the Young's modulus. For the MgO–NiO–YSZ samples, the Young's modulus of the samples exhibited a nearly constant behaviour through the sequence of processes from as-sintered to 100 h reduction and 2000 h reduction. Based on the microstructural analysis, the compositions of the framework of the samples and the sliding of the grain boundary strongly affected the Young's modulus.

Electrochromic behavior of NiO–TiO2 films prepared with sodium dodecyl sulfonate added to the sol

The effects of the surfactant sodium dodecyl sulfonate (SDS) on NiO–TiO2 sols, xerogels and films have been studied. Xerogels and films with Ni concentration of 80mol% have been prepared using sols containing 3, 6, 9wt% of SDS added to nickel acetate tetrahydrate (Ni(CH3COO)2·4H2O) and tetra-n-butyl titanate (TNBT) as precursors in a mixture of ethanol and n-butanol. The thermal analysis of dried xerogels has been measured by thermogravimetry (TG) and differential thermal analysis (DTA) in air up to 500°C. The structural properties of xerogels sintered at 300°C have been determined by XRD and the xerogels consist predominantly of bunsenite nanoparticles embedded in an amorphous TiO2 phase. Single layers have been deposited by dip coating on FTO (fluorine doped tin oxide, SnO2:F) coated glasses and sintered at 300°C. Their thickness is found to increase with the SDS amount while the roughness of the surface is reduced. The electrochromic properties of films deposited on FTO coated glasses have been studied in 1M KOH aqueous electrolyte via cyclic voltammetry (CV), chronoamperometry (CA) and visible light transmission. The anodic and cathodic peaks in CV cycles and the charge intercalated during CA cycles increase with the SDS content. The transmission in the bleached and colored states as well as the change in optical density remain stable for the film prepared with 9wt% SDS up to almost 18,000 CA cycles, a great improvement compared to films prepared without surfactant. The coloration efficiency of the film prepared with 9wt% SDS is about 30cm2/C, a value comparable to that obtained for pure NiO film.

Corrosion behavior of TiO2–NiO nanocomposite thin films on AISI 316L stainless steel prepared by sol–gel method

TiO2–NiO nanocomposite thin films were deposited on the 316L stainless steel using sol–gel method by a dip coating technique. Different techniques such as differential thermal analysis, thermogravimetric analysis, X-ray diffraction, Fourier transform infrared spectrometry, scanning electron microscopy and scanning probe microscopy were carried out in order to characterize the structure of the coatings. The corrosion resistance of the coatings was evaluated by using Tafel polarization and electrochemical impedance spectroscopy tests of uncoated and coated specimens in a 3.5% NaCl solution at room temperature. It was found that to obtain desirable structure in coatings, the coatings should be calcined at 600°C for one and half hour. NiTiO3, anatase and rutile were the phases obtained in different calcination conditions in air atmosphere. The results of corrosion tests indicated that with increasing the dipping times from 2 to 4, the corrosion current density first decreases but when increasing the dipping times to 6, it increases. Also the corrosion current density decreased from 186.7nA.cm−2 (uncoated steel) to 34.21nA.cm−2 (80%TiO2–20%NiO) and corrosion potential increased from −150.2mV (uncoated steel) to −107.3mV (67%TiO2–33%NiO).

Surfactant-aided sol–gel synthesis of mesoporous-assembled TiO2–NiO mixed oxide nanocrystals and their photocatalytic azo dye degradation activity

In this work, the mesoporous-assembled TiO2–NiO mixed oxide nanocrystals were successfully synthesized by a sol–gel process with the aid of a structure-directing surfactant under mild conditions. The photocatalytic activity of the synthesized mixed oxide nanocrystals for degradation of Methyl Orange (MO) azo dye used as a model wastewater contaminant was investigated under UV light irradiation. The synthesized mixed oxide nanocrystals were systematically characterized by TG-DTA, N2 adsorption–desorption analysis, SEM-EDX, XRD, and UV–visible spectroscopy. The photocatalytic activity results showed that the incorporation of NiO phase with a suitable content into the TiO2–NiO mixed oxide increased the photocatalytic MO degradation efficiency. It was found that the mesoporous-assembled Ti75–Ni25 mixed oxide nanocrystal (with 75mol % TiO2 and 25mol % NiO) provided a maximum MO degradation efficiency as compared to the other investigated nanocrystals as well as the commercial P-25 TiO2 powder. The induced n–p junction at the interfacial contact between the TiO2 and NiO phases in the Ti75–Ni25 mixed oxide nanocrystal is supposed to efficiently prevent the electron–hole recombination upon the UV light excitation and consequently enhance its photocatalytic degradation activity.

Research of Bilayer Stacked Resistance Switching Memory Structure Based on Pt/NiO/TiO2/Pt

The large power consumption and the wide distribution of switching parameters for unipolar resistance switching are major hurdles to non-volatile memory application. In this letter, the stacked structure Pt/NiO/TiO2/Pt was investigated to overcome the mentioned problems. TiO2 thin film was formed on NiO by oxidation of metallic Ti thin film through rapid thermal process (RTP) in oxygen atmosphere. The sharp TiO2 (202) peak and NiO (200) peak observed by the X-ray diffraction measurement. This stacked bilayer structure is favorable for reducing reset current in comparison with single layer structure. This is attributed to a small amount of heat loss during rupturing process of conductive filaments. It is suggested that the conductive filament rupture and rejuvenation was happened at the interface between TiO2 and NiO by controlling switching sequence. Furthermore, the diffused Ti in NiO also can result in stabilization of switching parameters.

Catalytic hydrogenation of xylose to xylitol using ruthenium catalyst on NiO modified TiO2 support

The activity of Ru catalyst on a new class of NiO modified TiO2 support, Ru/(NiO–TiO2), was studied in the liquid phase catalytic hydrogenation of xylose to xylitol. The TiO2 support was modified by simple impregnation method using nickel chloride precursor and subsequent oxidation. Various catalysts with different targeted compositions of Ru (1.0 and 5.0wt%) and NiO (1.0, 5.0 and 10wt%) in NiO–TiO2 were prepared. These catalysts were characterized by using energy dispersive X-ray analysis (EDX/EDS), temperature-programmed reduction (TPR), inductively coupled plasma (ICP) mass spectrometry, transmission electron microscopy (TEM), X-ray powder diffraction (XRD) and CO chemisorption. The novel catalysts are evaluated for selective hydrogenation of xylose and the results compared with those obtained from conventional Raney Ni, Ru/C and Ru/TiO2 catalysts carried out under identical reaction conditions. The effect of NiO additive in the catalyst Ru/(NiO–TiO2), clearly found to enhance the conversion, yield and selectivity to xylitol. Furthermore, the order of catalytic activity may be given as Ru (1.0%)/NiO (5.0%)–TiO2>Ru (1.0%)/TiO2>Ru (1.0%)/C> Raney Ni. The effects of Ru and NiO loading, xylose concentration (2.5, 15 and 30wt%) and temperature (100, 120 and 140°C) were studied. Although at higher temp 140°C, the conversion of xylose was increased to optimum level, xylose to xylitol selectivity decreased due to formation of by-products.

Liquid phase hydrogenation of d-glucose to d-sorbitol over the catalyst (Ru/NiO–TiO2) of ruthenium on a NiO-modified TiO2 support

The catalyst of ruthenium (Ru) on a new class of NiO-modified TiO2 support material, that is, Ru/(NiO–TiO2) was employed especially for selective hydrogenation of d-glucose to d-sorbitol. The modification of the TiO2 support was carried out with a nickel precursor by a simple impregnation method. After calcinations, the stable NiO-modified TiO2 support was re-impregnated with ruthenium (III) chloride hydrate aqueous solution. An additive effect of NiO in its corresponding catalyst of ruthenium on a NiO-modified TiO2 support exhibits the enhancement in the selectivity to d-sorbitol by minimizing the d-fructose and d-mannitol. To develop our understanding for the activity of the catalysts, the hydrogenation of d-glucose was investigated with the effect of (1.0wt.% and 5.0wt.%) ruthenium loading and (2.0–10wt.%) NiO loading followed by the pretreatment of the catalysts. The reaction condition was optimized by varying the concentrations of d-glucose (from 10 to 30wt.%) and temperatures (from 90°C to 120°C) to reach up to the highest product selectivity to d-sorbitol. These catalysts were characterized by using energy dispersive X-ray (EDX) analysis, temperature-programmed reduction (TPR), inductively coupled plasma atomic emission spectrometry (ICP-AES), transmission electron microscopy (TEM), X-ray powder diffraction (XRD) and CO chemisorption.

Enhancement of photocatalytic properties of TiO2 nanobelts through surface-coarsening and surface nanoheterostructure construction

Abstract In this work, heterostructures were obtained by uniformly assembling NiO nanoparticles on the surface-coarsened TiO2 nanobelts through a precipitation process. The uniform assembling of NiO nanoparticles led to a large number of nano-p–n-junction heterostructures on the surface of the TiO2 nanobelts, where NiO and TiO2 are p- and n-type semiconductors, respectively. Compared with both pure NiO nanoparticles and TiO2 nanobelts, NiO nanoparticles/TiO2 surface-coarsened nanobelt heterostructured composite exhibited a greatly enhanced photocatalytic activity in the decomposition of a model dye compound methyl orange under ultraviolet light irradiation. It was argued that the nano-p–n-junctions effectively reduce the recombination of electrons and holes, thus resulting in enhanced photocatalytic property of the heterostructured composites. The better performance of the surface-coarsened nanobelts is due to the increased photo absorption and production of charge carriers, which renders the composites with further enhanced photocatalytic performance. The established approach allows the control of the nano-p–n junction heterostructure on the nanobelts, and hence, their photocatalytic effect.

P–N junction mechanism on improved NiO/TiO2 photocatalyst

The composite photocatalyst comprising p-type NiO and n-type TiO 2 showed improved photoactivity due to the inhibition of electron–hole recombination. A series of P–N junction photocatalysts, NiO/TiO 2 , were prepared by incipient wetness impregnation. The photocatalysts were characterized by UV–vis spectroscopy, XRD and XPS. The photoactivity of photocatalysts was tested by the degradation of methylene blue. The results showed that 0.5 wt.% NiO/TiO 2 had the highest activity as compared with TiO 2 . A P–N junction mechanism was proposed and verified that the photocurrent could be enhanced under forward bias in the NiO/TiO 2 film. The photoactivity enhancement is attributed to P–N junction and co-catalyst effects. The composite photocatalyst of p-type NiO and n-type TiO 2 improves photoactivity due to the inhibition of electron–hole recombination. A P–N junction mechanism is verified by increasing photocurrent under forward bias. The enhanced activity is attributed to P–N junction of NiO/TiO 2 . ► The P–N junction of photocatalyst was studied by the photocurrents using voltammetry. ► The electron–hole recombination is retarded by the P–N junction of NiO/TiO 2 . ► Combination of p-type NiO and n-type TiO 2 can improve photocatalytic activity.

NiO/TiO2 Nanoparticles for Photocatalytic Disinfection of Bacteria under Visible Light

NiO–TiO2 composite nanoparticles have been prepared by a modified ammonia-evaporation-induced synthetic method, sintered at 450°C and characterized by powder X-ray diffraction, energy-dispersive X-ray spectroscopy, scanning electron microscopy, UV-visible diffuse reflectance spectroscopy, photoluminescence spectroscopy, and electrochemical impedance spectroscopy. Doping shifts the optical absorption edge to the visible region but increases the charge-transfer resistance and decreases the capacitance. Under visible light, the composite nanoparticles effectively catalyze the Escherichia coli inactivation. The prepared oxide is selective in photocatalysis; with UV light, its photocatalytic activity to degrade sunset yellow, rhodamine B, and methylene blue dyes is less than that of the undoped one. However, it degrades phenol faster than TiO2 P25.

Nano-p–n junctions on surface-coarsened TiO2 nanobelts with enhanced photocatalytic activity

Nano-p–n junction heterostructure TiO2 nanobelts have been produced by assembling p-type semiconductor NiO nanoparticles on n-type TiO2 nanobelts for enhancement of the photocatalytic properties of TiO2 nanobelts. NiO/TiO2 nano-p–n junctions were synthesized on the surfaces of TiO2 nanobelts and surface-coarsened TiO2 nanobelts. The nanobelts were obtained using alkaline- and acid-assisted hydrothermal processes. The chemical-solution-deposition–decomposition process was used to form NiO nanoparticle/TiO2 heterostructure composite nanobelts (NiO-NP/TiO2 NBs), and NiO nanoparticle/surface-coarsened TiO2 heterostructure composite nanobelts (NiO-NP/TiO2 coarsened NBs). The uniform assembly of p-type NiO nanoparticles produces a large number of nano-p–n junction heterostructures on the surface of the TiO2 nanobelts, where NiO and TiO2 form p- and n-type semiconductors, respectively. Compared with both pure NiO nanoparticles and TiO2 nanobelts, NiO-NP/TiO2 NBs exhibit much enhanced photocatalytic activity. Interestingly, the optimized composite NiO-NP/TiO2 coarsened NBs exhibit an enhanced photocatalytic activity in the decomposition of a model dye compound, methyl orange (MO), under both ultraviolet and visible light irradiation. It is argued that the nano-p–n junctions effectively reduce the recombination of electrons and holes, thus leading to the enhancement of the photocatalytic properties of the heterostructure composites. The larger number of abundant photocatalytic-active surfaces in the surface-coarsened nanobelts increases photo-absorption and the production of charge carriers, which gives the composites an enhanced photocatalytic performance. The established approach allows for controlling the nano-p–n junction heterostructure of the nanobelts, and hence, their photocatalytic effect. The NiO/TiO2 nanobelt-based nano-p–n junction heterostructure TiO2 can provide a practical way to design and prepare nano-composites for applications as solar-cell electrodes, in solar photocatalysis, solar photolysis of water and other related fields.

A magnesium carbonate recyclable template to synthesize micro hollow structures at a large scale

A facile strategy by using magnesium carbonate as the recyclable template has been developed to synthesize a variety of uniform inorganic hollow structures whose composition, size and shape can be readily modulated to generate interesting structures such as TiO(2), Al(2)O(3), SiO(2), Gd(2)O(3) and NiO microtubes and TiO(2), SiO(2) hollow microboxes.

(Invited) Identity of the Conducting Nanofilaments in TiO2 and the Resistance Switching Mechanism of TiO2/NiO Stacked Layers

The resistance switching (RS) behaviors of TiO2 and TiO2/NiO thin films were examined. The presence of conductive nano-filaments was confirmed directly by high-resolution transmission electron microscopy (HRTEM) in a Pt/TiO2/Pt system. The HRTEM images identified Magnéli structured filaments (mostly Ti4O7 phase) of both set and reset states. The local electrical properties were confirmed by in-situ current-voltage measurements in HRTEM. The in-situ HRTEM observation confirmed the structural change of the filament during set and reset switching. The RS behaviour of a stacked material composed of n-type TiO2 and p-type NiO was also examined. The switching time could be decreased by ~ 1,000 times compared to the single layer material by stacking the two layers.