Characterization of surface-modified ceria oxide nanoparticles synthesized continuously in supercritical methanol

Surface-modified ceria oxide (CeO₂) nanoparticles were synthesized continuously in supercritical methanol at 400 °C, 30 MPa and a residence time of ∼40 s using a flow type reactor system. Oleic acid and decanoic acid were used as the surface modifiers. Transmission electron microscopy (TEM) showed that the surface modifiers changed drastically the shape and size of the nanoparticles. When 0.3 M of the surface modifiers were used, primary particles with diameter of 2–3 nm loosely aggregated and formed secondary particles with size of 30–50 nm. Wide angle X-ray diffraction (WAXD) analysis revealed that the surface-modified nanoparticles retained CeO₂ crystalline structure. The surface-modified CeO₂ nanoparticles had a very high surface area (140–193 m²/g) compared to the unmodified CeO₂ particles synthesized in supercritical water (8.5 m²/g). Fourier transform infrared (FT-IR) and thermogravimetric analysis (TGA) indicated that aliphatic, carboxylate and hydroxyl groups were chemically bounded on the surface of CeO₂ nanoparticles. Dispersability test using ultraviolet transmittance showed that most of the surface-modified CeO₂ nanoparticles were dispersed in ethylene glycol for 30 days while the unmodified CeO₂ particles synthesized in supercritical water or in supercritical methanol were precipitated after 7–15 days.     

Synthesis of Fe/Fe3C nanoparticles encapsulated in nitrogen-doped carbon with single-source molecular precursor for the oxygen reduction reaction

Ammonium ferric citrate (AFC) was used as a single-source molecular precursor to prepare Fe/Fe₃C nanoparticles encapsulated in nitrogen-doped carbon by pyrolysis in Ar atmosphere followed by acid-leaching. Comparative studies, using citric acid and ferric citrate as the precursors, indicated that the ammonia and ferric ion in AFC and the pyrolysis temperature affected the composition of iron species and the properties of carbon in AFC-derived materials. Above the pyrolysis temperature of 600 °C, the iron species were Fe/Fe₃C, and the carbon had a hollow graphitic nanoshell structure in AFC-derived materials. The specific surface area and content of nitrogen element decreased with increasing pyrolysis temperature. The AFC-derived material pyrolyzed at 600 °C had the optimal graphitization degree, specific surface area (489 m² g⁻¹) and content of nitrogen (1.8 wt.%), thus resulted in the greatest activity for oxygen reduction reaction among the AFC-derived materials pyrolyzed at different temperatures. The AFC-derived material pyrolyzed at 600 °C exhibited improved methanol-resistance ability compared with Pt/C catalyst.     

Microwave-assisted synthesis of ZnO doped CeO2 nanoparticles as potential scaffold for highly sensitive nitroaniline chemical sensor

Herein, we report the successful fabrication of highly sensitive, reproducible and reliable nitroaniline chemical sensor based on ZnO doped CeO₂ nanoparticles. The ZnO doped CeO₂ nanoparticles were synthesized through a simple, facile and rapid microwave-assisted method and characterized by several techniques. The detailed characterizations confirmed that the synthesized nanoparticles were monodisperse and grown in high density and possessing good crystallinity. Further, the synthesized ZnO doped CeO₂ nanoparticles were used as efficient electron mediators for the fabrication of high sensitive nitroaniline chemical sensors. The fabricated nitroaniline chemical sensor exhibited very high sensitivity of 550.42 μA mM⁻¹ cm⁻² and experimental detection limit of 0.25 mM. To the best of our knowledge, this is the first report in which CeO₂–ZnO nanoparticles were used as efficient electron mediators for the fabrication of nitroaniline chemical sensors. Thus, presented work demonstrates that ZnO doped CeO₂ nanoparticles are potential material to fabricate highly efficient and reliable chemical sensors.     

Effect of Pt or Pd doping on stability of TiO2 nanoparticle suspension in water

Stability of suspensions of TiO₂ nanoparticles synthesized by the flame aerosol reactor (FLAR) could be altered by doping TiO₂ nanoparticles with Pt, Pd, or Pt–Pd dopants. It was found that doping of TiO₂ with Pd or Pt could contribute to the control of the agglomeration of TiO₂ suspended in water. With the change of doping content, the isoelectric point (IEP) of stable TiO₂ suspension decreased gradually from 5 to 3.6 while the specific surface area was increased from 43.27 to 60.84 m²/g. With pH > 6.0, 2 wt% Pt–Pd/TiO₂ suspension exhibited the lowest agglomeration behavior. The plausible intrinsic structures of Pt, Pd, and Pt–Pd doped TiO₂ nanoparticles were proposed and discussed with respect to their IEP based on the DLVO theory.     

Formation and stabilization of submicron particles via rapid expansion processes

Submicron particles were produced by rapid expansion of supercritical solution into air (RESS) or an aqueous surfactant solution (RESSAS) to minimize particle growth and to prevent particle agglomeration. Thereby the effect of process conditions on the size of the particles precipitated was investigated. The obtained product was evaluated by measuring particle size by 3-wavelength extinction measurements, dynamic light scattering, specific surface areas by nitrogen gas adsorption, melting behaviour by differential scanning calorimetry, particle morphology by X-ray diffraction, scanning electron micrographs (SEM), and drug loading by high performance liquid chromatography.Prior to the particle formation experiments, the melting temperature of Salicylic acid under CO₂ pressure and the solubility of Salicylic acid in CO₂ were measured. The size of Salicylic acid particles produced via RESS decreased from 230 to 130 nm as the pre-expansion temperature decreased from 388 to 328 K and the specific surface area of the micronized particles was found to be up to 60 times higher than that of the unprocessed material. RESSAS experiments demonstrate that in 1 wt.% Tween 80 solutions Salicylic acid concentrations of 4.6 g/dm³ could be stabilized with particle diameters in the range of 180 nm. Additional experiments show that Ibuprofen nanoparticles with an average size of 80 nm and a drug concentration of 2.4 g/dm³ could be stabilized in 1 wt.% Tween^® 80 solutions. The use of a SDS solution instead of Tween^® 80 results in a stable aqueous suspension of phytosterol nanoparticles, where the average particle size is 50 nm at a drug concentration of 5.6 g/dm³.     

Using modified Pechini method to synthesize α-Al2O3 nanoparticles of high surface area

A modified Pechini method for the preparation of a high surface area α-alumina is proposed. The synthesis of these nanoparticles was carried out using a polymer as a chelating agent. The polymer was prepared from citric acid and acrylic acid by the melt blending method. The resulting α-alumina (98.16%) after calcination at 900 °C consisted of cylindrical nanoparticles of 100–200 nm in length and <25 nm in diameter with a relatively high surface area (18 m² g^?1).     

Superior carbon-based CO2 adsorbents prepared from poplar anthers

A series of renewable nitrogen-containing granular porous carbons with developed porosities and controlled surface chemical properties were prepared from poplar anthers. The preparation conditions such as pre-carbonization and activation temperatures and KOH amount significantly influence the structures and chemical compositions of the porous carbons, the CO₂ adsorption capacities of which are highly dependent on their pore structures, surface areas, nitrogen contents and adsorption conditions. The sample with developed microporosity, especially with the pores between 0.43 and 1 nm and high nitrogen content shows high CO₂ adsorption capacity at 1 bar and 25 °C. In contrast, when the adsorption pressure is higher than 5 bar, its CO₂ adsorption capacity is dominated by its surface area, and more accurately by its pore volume. Irrespective of this, if the pressure was decreased to 0.1 bar, its CO₂ capture ability is closely correlated to its nitrogen content but not to its porosity. By optimizing the preparation conditions, a porous carbon with a surface area of 3322 m² g⁻¹ and a CO₂ adsorption capacity as high as 51.3 mmol g⁻¹ at 50 bar and 25 °C was prepared.     

The Ba-hexaaluminate doped with CeO2 nanoparticles for catalytic combustion of methane

The Ba-hexaaluminate doped with CeO₂ nanoparticles with high surface area for catalytic combustion have been prepared by using the alumina sol as the (NH₄)₂CO₃ coprecipitation precursor and supercritical drying method. The catalysts are composed of the rod-like particles and granular ones. The CeO₂/BaAl₁₂O_19−α catalyst possesses the highest surface area (83.5 m²/g) and the smallest CeO₂ mean crystallite size (24.3 nm). Introduction of transition metal ion into the Al₂O₃ spinel leads to the increase of the catalytic activity. Nevertheless the hexaaluminate cannot be obtained when further increasing the introduction, the components of the main crystalline phases are Al_2.267O₄ and CeO₂. The CeO₂/BaFeMnAl₁₀O_19−α catalyst possesses the lowest complete conversion of methane temperature, probably due to the high surface area and the excellent performance of activating oxygen.     

Mesoporous carbons synthesized by direct carbonization of citrate salts for use as high-performance capacitors

A simple one-step synthesis methodology for the fabrication of mesoporous carbons with an excellent performance as supercapacitor electrodes is presented. The procedure is based on the carbonization of non-alkali organic salts such as citrate salts of iron, zinc or calcium. The carbonized products contain numerous inorganic nanoparticles (i.e. Fe, ZnO or CaO) embedded within a carbonaceous matrix. These nanoparticles act as endotemplate, which when removed, leaves a mesoporous network. The resulting carbon samples have a large specific surface area up to ∼1600 m² g⁻¹ and a porosity made up almost exclusively of mesopores. An appropriate heat-treatment of these materials with melamine allows the synthesis of N-doped carbons which have a high nitrogen content (∼8–9 wt.%), a large specific surface area and retain the mesoporous structure. The mesoporous carbon samples were employed as electrode materials in supercapacitors. They exhibit specific capacitances of 200–240 F g⁻¹ in 1 M H₂SO₄ and 100–130 F g⁻¹ in EMImTFSI/AN. More importantly, the carbon samples possess a good capacitance retention in both electrolytes (>50% in H₂SO₄ and >80% in EMImTFSI/AN at 100 A g⁻¹) owing to their mesoporous structure which facilitates the penetration and transportation of ions.     

Synthesis and characterization of mesoporous anatase TiO2 nanostructures via organic acid precursor process for dye-sensitized solar cells applications

Titania (TiO₂) nanoparticles have been synthesized using organic precursor technique. The titania nanoparticles were characterized. The results indicated that the prepared titanium oxalate and citrate precursors were transformed to anatase TiO₂ phase at temperature 400 °C for 2 h. Dye-sensitized solar cells were assembled using the prepared nanocrystalline TiO₂ with large surface area. The specific surface area S_BET was 80.9 and 78.6 m²/g using oxalic and citric acids, respectively. The power efficiency was 3.5 and 2.4%. A brief discussion on the possible reasons behind the low power conversion efficiency observed for these type of solar cells was reported.     

Nitrogen-doped porous carbons through KOH activation with superior performance in supercapacitors

A series of nitrogen-doped porous carbons are prepared through KOH activation of a nonporous nitrogen-enriched carbon which is synthesized by pyrolysis of the polymerized ethylenediamine and carbon tetrachloride. The porosity and nitrogen content of the nitrogen-doped porous carbons depend strongly on the weight ratio of KOH/carbon. As the weight ratio of KOH/carbon increases from 0.5 to 2, the specific surface area increases from 521 to 1913 m² g⁻¹, while the nitrogen content decreases from 10.8 to 1.1 wt.%. The nitrogen-doped porous carbon prepared with a moderate KOH/carbon weight ratio of 1, which possesses a balanced specific surface area (1463 m² g⁻¹) and nitrogen content (3.3 wt.%), exhibits the largest specific capacitance of 363 F g⁻¹ at a current density of 0.1 A g⁻¹ in 1 M H₂SO₄ aqueous electrolyte, attributed to the co-contribution of double-layer capacitance and pseudocapacitance. Moreover, it shows excellent rate capability (182 F g⁻¹ remained at 20 A g⁻¹) and good cycling stability (97% capacitance retention over 5000 cycles), making it a promising electrode material for supercapacitors.     

Strong adsorption of arsenic species by amorphous zirconium oxide nanoparticles

A novel oxide adsorbent of amorphous zirconium oxide (am-ZrO₂) nanoparticles was synthesized by a simple hydrothermal process for effective arsenic removal from aqueous environment. Due to their high specific surface area (327.1 m²/g), large mesopore volume (0.68 cm³/g), and the presence of high affinity surface hydroxyl groups, am-ZrO₂ nanoparticles demonstrated exceptional adsorption performance on both As(III) (arsenite) and As(V) (arsenate) without pre-treatment at near neutral condition. At pH ～ 7, the adsorption kinetic is fast and the adsorption capacity is high (over 83 mg/g for As(III) and over 32.4 mg/g for As(V), respectively). Under low equilibrium arsenic concentrations (C_e at 0.01 mg/L, the maximum contaminant level (MCL) for arsenic in drinking water), the amount of arsenic adsorbed by am-ZrO₂ nanoparticles is over 0.92 mg/g for As(III) and over 5.2 mg/g for As(V), respectively. The adsorption mechanism of arsenic species onto am-ZrO₂ nanoparticles was found to follow the inner-sphere complex mechanism. Testing with arsenic contaminated natural lake water confirmed the effectiveness of these am-ZrO₂ nanoparticles in removing arsenic from natural water. The immobilized am-ZrO₂ nanoparticles on glass fiber cloth demonstrated an even better arsenic removal performance than dispersed am-ZrO₂ nanoparticles in water, paving the way for their potential applications in water treatment facility to treat arsenic contaminated water body without pre-treatment.     

Simultaneous reduction,exfoliation, and nitrogen doping of graphene oxide via a hydrothermal reaction for energy storage electrode materials

Nitrogen (N)-doped graphene (NG) sheets were prepared using (NH₄)₂CO₃ and an aqueous dispersion of graphene oxide (GO) by an eco-friendly hydrothermal reaction. The in situ produced ammonia played an important role in the simultaneous nitrogen doping, the reduction and exfoliation of GO. The (NH₄)₂CO₃/GO mass ratio and reaction temperature were varied to investigate the effects on the N doping level. The elemental analysis determined from the X-ray photoelectron spectroscopy showed that the nitrogen content of the NG was about 10.1 at.% and the oxygen content decreased significantly due to the hydrothermal reduction of GO. The electrochemical performances of the NG sheets increased with increasing doped N content. The highest specific capacitance of 295 F g⁻¹ at a current density of 5 A g⁻¹ and the highest specific surface area of 412 m² g⁻¹ were observed with the sample processed at 130 °C. The retention of the specific capacitance was maintained at ∼89.8% after 5000 charge–discharge cycles. These results imply that NG sheets obtained by this simple eco-friendly approach are suitable for use in high performance energy storage electrode materials.     

Fabrication of nitrogen-doped hierarchically porous carbons through a hybrid dual-template route for CO2 capture and haemoperfusion

Hierarchically porous carbon materials have many important technological applications; however, most of them were fabricated using either expensive materials or complicated procedures. Based on a general chelate-assisted multi-component co-assembly strategy, nitrogen-doped hierarchically porous carbon materials were fabricated by using Al-based composite and commercial triblock copolymer Pluronic F127 as co-templates, and natural banana peel as precursor. This versatile strategy allowed to easily achieve tunable surface area (700–2100 m² g⁻¹), pore volume (0.38–1.65 cm³ g⁻¹) and a narrow average mesoporous size of ca. 2.72–4.03 nm by simply varying the dosages of Al³⁺ and F127, and to attain high N content (4.54 wt%) in a large-scale fabrication system (2 L). X-ray photoelectron spectroscope characterization of the as-prepared sample revealed nitrogen atoms are mainly in the form of pyridinic nitrogen, quaternary nitrogen and pyridine-N-oxide. Importantly, these as-obtained carbon materials showed excellent performance in CO₂ capture and bilirubin removal with high adsorption capacities and selectivities. The present fabrication strategy is also applicable to the design of porous carbons doped with other elements by choosing appropriate biomass precursors.     

A comparative study of the effect of Pd-doping on the structural,optical, and photocatalytic properties of sol–gel derived anatase TiO2 nanoparticles

Pd-doped anatase TiO₂ nanoparticles were synthesized by a modified sol–gel deposition technique. The synthetic strategy is applicable to other transition and post-transition metals to obtain phase-pure anatase titania nanoparticles. This is important in the sense that anatase titania forms the most hydroxyl radicals (compared to other polymorphs like rutile, brookite, etc.) for better photocatalytic performance. XRD and Raman data confirm the phase-pure anatase formation. Doping of Pd²⁺ into Ti⁴⁺ sites (for substitutional doping) or interstitial sites (for interstitial doping) creates strain within the nanoparticles and is reflected in the XRD peak broadening and Raman peak shifts. This is because of the ionic radii difference between Ti⁴⁺(∼68 pm) and Pd²⁺(∼86 pm). XPS data confirm the formation of high surface titanol groups at the nanoparticle surface and a large number of loosely bound Ti³⁺–O bonds, both of which considerably enhance the photocatalytic activity of the doped nanoparticles. A comparative study with other metal doping (Ga) shows that TiO₂: Pd nanoparticles have more Ti³⁺–O bonds, which enhance the charge transfer rate and hence improve the photocatalytic activity compared to other transition and post-transition metal-doped titania nanostructures.     

Structural and luminescence properties of undoped,Nd3+ and Er3+ doped TiO2 nanoparticles synthesized by flame spray pyrolysis method

Bulk and thin film forms of titanium dioxide (TiO₂) have been studied many times due to its very promising optical properties. In this study, low-cost flame spray pyrolysis (FSP) synthesis of Nd³⁺/Er³⁺doped TiO₂ nanoparticles has been reported for the first time. The produced particles were post-annealed after FSP process at 550 °C in order to obtain crystalline structure. The phase and elemental analysis of the produced materials were performed by X-Ray Diffraction (XRD) and X-Ray Photoelectron Spectroscopy (XPS), respectively. The surface morphology, accurate size and specific surface area of the primary particles were identified using scanning electron microscopy (SEM) and particle size analyser. Luminescent properties of the produced nanoparticles were investigated by steady state and time resolved fluorescence spectra. Doping of TiO₂ nanoparticles with the rare earths of Nd³⁺and Er³⁺resulted in visible and near-infrared light emission when excited at 364 nm. The utilized nanoparticles yielded bi-and tri-exponential decay curves. Additionally, they exhibited typical upconversion luminescence when radiated by 810 nm.     

Improvement of field-emission-lamp characteristics using nitrogen-doped carbon nanocoils

Carbon nanocoils (CNCs) synthesized using thermal pyrolysis chemical vapor deposition on 304 stainless steel wire substrates were used as the cathodes of field emission lamps (FELs). The effects of the growth temperature on the FE performances were studied, and we observed that uniform and dense CNCs that are suitable for use as FE cathodes can be synthesized at 600 °C. We also found that a nitrogen doping post-treatment can significantly improve the FE efficiency of the CNCs. When doped at 200 °C with a nitrogen flow rate of 500 sccm for 30 min, the nitrogen content of the CNC surface could reach 4.9 wt.%. ESCA analysis indicates that the doped nitrogen atoms formed CN_x bonding and increased the sp² clusters in the CNCs. The turn-on voltage was reduced from 2.1 V/μm to 1.4 V/μm, and the β value increased considerably from 2465 to 3241 after N-doping post-treatment. The bulb-type FELs using our N-doped CNC cathodes showed a good luminous efficiency as high as 75 lm/W at 8 kV.     

Nitrogen/sulfur dual-doped mesoporous carbon with controllable morphology as a catalyst support for the methanol oxidation reaction

Ordered mesoporous carbon (OMC) was synthesized by nano-casting method using novel fluidic precursor – acrylonitrile telomer (ANT). By the penetration of mesoporous silica template with pure ANT, followed by the stabilization, carbonization and removal of the template, we obtained highly ordered mesoporous carbon rods (specific area 408 m² g⁻¹). When an acetone solution of ANT (66 and 33 wt.%) was used instead of pure ANT, carbon materials with mesopore ranging from 2 to 7 nm were obtained (specific area 843 and 1012 m² g⁻¹ respectively). Both nitrogen and sulfur atoms were doped into mesoporous carbon with 4 and 0.6 at.% using nitrogen containing monomer and sulfur containing chain transfer agent, without involving complicated synthetic technique and poisonous gaseous compounds. This method was proved to be a facile way to synthesize nitrogen and sulfur containing OMC with partially controllable pore distribution and morphology. More importantly, due to unique mesopore structure and heteroatom doping, Pt nano-particles deposited on the OMCs showed electrocatalytic activity as high as 508 mA mg⁻¹ Pt in methanol oxidation which is 1.7-fold of activity of Pt deposited on commercial Vulcan carbon black.     

Molecular hydrogen and spiltover hydrogen storage on high surface area carbon sorbents

A series of templated carbons with various high surface areas (2033–3798 m²/g) have been prepared using various microporous zeolites as hard templates. Molecular hydrogen storage and spiltover hydrogen storage on these templated carbons were investigated and compared with superactivated carbon AX-21 and other reported porous carbon sorbents at 298 K and 100 atm. Two relationships between the surface areas of these carbons and their hydrogen capacities were obtained. The relationship between molecular hydrogen capacity and surface area showed a 0.23 wt.% H₂/1000 m²/g of carbon sorbent at 298 K and 100 atm, indicating that merely increasing surface areas of the carbon sorbents cannot achieve a significant molecular hydrogen capacity at ambient temperature. Spiltover hydrogen storage was achieved by doping Pt nanoparticles (as dissociative hydrogen source) on these carbons (spiltover hydrogen receptor). Our first result on the relationship between the spiltover hydrogen capacity and surface area showed 0.4 wt.% H₂/1000 m²/g of carbon sorbent at 298 K and 100 atm, which indicated that storage via spillover can lead to an average of 70% enhancement compared to molecular hydrogen storage.     

Synthesis and electrochemical performance of reduced graphene oxide/maghemite composite anode for lithium ion batteries

Reduced graphene oxide (rGO) tethered with maghemite (γ-Fe₂O₃) was synthesized using a novel modified sol–gel process, where sodium dodecylbenzenesulfonate was introduced into the suspension to prevent the undesirable formation of an iron oxide 3D network. Thus, nearly monodispersed and homogeneously distributed γ-Fe₂O₃ magnetic nanoparticles could be obtained on surface of graphene sheets. The utilized thermal treatment process did not require a reducing agent for reduction of graphene oxide. The morphology and structure of the composites were investigated using various characterization techniques. As-prepared rGO/Fe₂O₃ composites were utilized as anodes for half lithium ion cells. The 40 wt.%-rGO/Fe₂O₃ composite exhibited high reversible capacity of 690 mA h g⁻¹ at current density of 500 mA g⁻¹ and good stability for over 100 cycles, in contrast with that of the pure-Fe₂O₃ nanoparticles which demonstrated rapid degradation to 224 mA h g⁻¹ after 50 cycles. Furthermore, the composite showed good rate capability of 280 mA h g⁻¹ at 10C (∼10,000 mA g⁻¹). These characteristics could be mainly attributed to both the use of an effective binder, poly(acrylic acid) (PAA), and the specific hybrid structures that prevent agglomeration of nanoparticles and provide buffering spaces needed for volume changes of nanoparticles during insertion/extraction of Li ions.