NiFe2O4@ nitrogen-doped carbon hollow spheres with highly efficient and recyclable adsorption of tetracycline

Antibiotics can affect ecosystems and threaten human health; therefore, methods for removing antibiotics have become a popular subject in environmental management and for the protection of human health. Adsorption is considered an effective approach for the removal of antibiotics from water. In this study, NiFe₂O₄@nitrogen-doped carbon hollow spheres (NiFe₂O₄/NCHS) were synthesized via a facile hydrothermal method followed by calcination using NCHS as a hard template. The nanocomposite exhibited high adsorption activity and good recyclability. The nanocomposite was characterized by X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and nitrogen adsorption–desorption to study its micromorphology, structure, and chemical composition/states. In addition, the factors affecting the adsorption process were systematically investigated, including tetracycline (TC) concentration, solution pH, ionic strength, and temperature. The maximum adsorption capacity for TC was calculated to be 271.739 mg g⁻¹ based on the Langmuir adsorption model, which was higher than various other materials. This study provides an effective method for constructing the NiFe₂O₄/NHCS core–shell structure, which can be applied for the removal of TC from water.

Antibiotics can affect ecosystems and threaten human health; therefore, methods for removing antibiotics have become a popular subject in environmental management and for the protection of human health.     

Synthesis of Pd-loaded mesoporous SnO2 hollow spheres for highly sensitive and stable methane gas sensors

High performance methane gas sensors have become more and more essential in different fields such as coal mining, kitchens and industrial production, which necessitates the design and synthesis of highly sensitive materials. Herein, mesoporous SnO₂ hollow spheres with high surface area (>90 m² g⁻¹) are prepared by a progressive inward crystallization routine, showing a high response of 1.31 to 250 ppm CH₄ at a working temperature of 400 °C. Furthermore, loading noble metal Pd onto the surface of SnO₂ hollow spheres by an adsorption–calcination process improves the response to 4.88 (250 ppm CH₄) at the optimal dosage of 1 wt% Pd. Meanwhile, the working temperature decreases to 300 °C, showing the prominent spillover effect of catalytic Pd and PdO–SnO₂ heterostructure sensitization as evidenced by the binding energy shift in the X-ray photoelectron spectroscopy (XPS) analysis. The response/recovery time is as short as 3/7 s and the sensitivity is stable for a test period as long as 15 weeks. All these performances show the promise of the highly porous Pd-loaded SnO₂ hollow spheres for high performance methane sensors.

This work reports a simple, rapid, effective and reliable CH₄ sensor based on Pd-loaded SnO₂ hollow spheres with high surface area and porosity, which is of great importance to gas sensing performance.     

Absorption induced ordered ring and inner network structures on a nanoporous substrate

Interaction of colloidal droplets with a porous medium is the key issue for many industrial applications, such as direct-ink-write printing on flexible wearable clothing. In this work, we find a novel pattern of an ordered ring with inner network from a colloidal droplet resting on the nanoporous substrate. Experimental results show that the outward flow caused by the lateral absorption is responsible for the ring structures. The mutual competition between the inward dewetting and the outward flow determines the formation of the inner network pattern. The capillary immersion forces dominate the self-assembly of particles and promote the ordered arrays of the structures.

Liquid absorption induced the formation of a novel pattern of an ordered ring with inner networks on the nanoporous substrate.     

Nitrogen-doped hollow carbon spheres with tunable shell thickness for high-performance supercapacitors

Nitrogen-doped hollow carbon spheres (NHCSs) are well prepared by using Cu₂O microspheres as a hard template and 3-aminophenol formaldehyde resin polymer as carbon and nitrogen precursors. The thickness of the carbon shell can be easily controlled in the range of 15–84 nm by simply adjusting the weight ratios of the precursors to Cu₂O microspheres, and the Cu₂O templates can also be further reused. Physicochemical characterization demonstrates that the obtained NHCSs possess a well-developed hollow spherical structure, thin carbon shell and high nitrogen doping content. Due to these characteristics, when further utilized as electrodes for supercapacitors, the NHCSs with the carbon shell thickness of 15 nm show a high capacitance of 263.6 F g⁻¹ at 0.5 A g⁻¹, an outstanding rate performance of 122 F g⁻¹ at 20 A g⁻¹ and an excellent long-term cycling stability with only 9.8% loss after 1000 cycles at 5 A g⁻¹ in 6 M KOH aqueous electrolyte. This finding may push forward the development of carbon materials, exhibiting huge potential for electrochemical energy storage applications.

Nitrogen-doped hollow carbon spheres (NHCSs) are well prepared by using Cu₂O microspheres as a hard template and 3-aminophenol formaldehyde resin polymer as carbon and nitrogen precursors.     

Preparation and electrochemical performance of VO2(A) hollow spheres as a cathode for aqueous zinc ion batteries

Rechargeable aqueous zinc ion batteries (ZIBs), owing to their low-cost zinc metal, high safety and nontoxic aqueous electrolyte, have the potential to accelerate the development of large-scale energy storage applications. However, the desired development is significantly restricted by cathode materials, which are hampered by the intense charge repulsion of bivalent Zn²⁺. Herein, the as-prepared VO₂(A) hollow spheres via a feasible hydrothermal reaction exhibit superior zinc ion storage performance, large reversible capacity of 357 mA h g⁻¹ at 0.1 A g⁻¹, high rate capability of 165 mA h g⁻¹ at 10 A g⁻¹ and good cycling stability with a capacity retention of 76% over 500 cycles at 5 A g⁻¹. Our study not only provides the possibility of the practical application of ZIBs, but also brings a new prospect of designing high-performance cathode materials.

VO₂(A) hollow spheres exhibit superior zinc ion storage performance, large reversible capacity of 357 mA h g⁻¹ at 0.1 A g⁻¹, and good cycling stability with a capacity retention of 76% over 500 cycles at 5 A g⁻¹     

Facile fabrication of porous BiVO4 hollow spheres with improved visible-light photocatalytic properties

Bismuth vanadate (BiVO₄) hollow spheres with porous structure have been successfully fabricated by a one-step wet solution method with no surfactant and template. The structure, morphologies, and composition of the as-prepared products were studied with X-ray powder diffraction (XRD), transmission electron morphology (TEM), Brunauer–Emmett–Teller (BET), X-ray photoelectron spectroscopy (XPS) and UV-vis spectroscopy. Based upon the time-dependent experimental results, BiVO₄ nanospheres with hollow and solid structures can be controlled effectively through the reaction time, and a reasonable formation process was suggested in this work. Moreover, the experiment of degrading methyl orange (MO) under visible-light illumination was conducted to evaluate the photocatalytic performance of the obtained BiVO₄ samples. The porous BiVO₄ hollow spheres exhibit superior visible-light photocatalytic properties for MO degradation than other photocatalysts under irradiation, and could be reused for up to five times without significant reduction in the photocatalytic activity. In addition, based on active group trapping experiments, ˙OH radicals as the main active species from H₂O₂ molecules play a vital role in the photocatalytic degradation of MO, and a photocatalytic mechanism for the BiVO₄ system was proposed. High photocatalytic activity, universality and stability suggest that the porous BiVO₄ hollow spheres may have potential applications in wastewater treatment.

Bismuth vanadate (BiVO₄) hollow spheres with porous structure have been successfully fabricated by a one-step wet solution method with no surfactant and template.     

Preparation of a nanoporous Cu–Ag solid solution with enhanced sono-Fenton-like catalytic activity

Uniform 3D bi-continuous nanoporous Cu–Ag solid solution (NPCS) and nanoporous copper (NPC) were successfully synthesized by dealloying Cu₇₀Y₂₈Ag₂ and Cu₇₂Y₂₈ metallic glasses, respectively, which was confirmed by using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SEM and TEM images show that the ligament size of NPCS (d_SEM = 65 nm, d_TEM = 45 nm) is much smaller than that of NPC (d_SEM = 402 nm, d_TEM = 370 nm), which reveals that the ligaments of NPC can be significantly refined by the substitution of 2 at% Ag for Cu in the amorphous precursor. The obtained NPCS exhibits much larger specific surface area and higher total pore volume (S_BET = 8.34 m² g⁻¹, V_p = 0.093 cm³ g⁻¹) compared to NPC (S_BET = 1.77 m² g⁻¹, V_p = 0.050 cm³ g⁻¹). Furthermore, the catalytic activities of the samples were evaluated by decomposing methyl orange (MO) dye under the irradiation of ultrasound. The results show that NPCS with an extreme fine microstructure displayed superior sono-Fenton-like catalytic activity compared to NPC and commercial copper foil.

NPCS with an extreme fine microstructure displayed superior sono-Fenton-like catalytic activity compared to NPC.     

Simplified waste-free process for synthesis of nanoporous compact alumina under technologically advantageous conditions

Precipitated ammonium aluminium carbonate hydroxide (NH₄Al(OH)₂CO₃) is a promising precursor for preparation of nanostructured Al₂O₃. However, the experimental conditions, such as the low concentration of Al³⁺ salt solution, high temperature and/or pressure, long reaction time, and excessive amount of the (NH₄)₂CO₃ precipitating agent, make this process expensive for large-scale production. Here, we report a simpler and cheaper route to prepare nanostructured alumina by partial neutralisation of a nearly saturated aqueous solution of Al(NO₃)₃ with (NH₄)₂CO₃ as a base at pH < 4. Synthesis in the acidic region led to formation of a polynuclear aluminium cluster (Al₁₃), which is an important “green” solution precursor for large-area preparation of Al₂O₃ thin films and nanoparticles. Control of the textural properties of the final alumina product during calcination of the prepared aluminium (oxy)hydroxide gel was accomplished by adding low-solubility aluminium acetate hydroxide (Al(OH)(CH₃COO)₂) as a seed to the Al(NO₃)₃ solution before neutralisation. The large Brunauer–Emmett–Teller specific surface area (376 m² g⁻¹) and narrow pore size distribution (2–20 nm) of the prepared compact alumina suggest that the chelating effect of the acetate ions affects the structures of the forming transition aluminas, and the evolved gases produced by decomposition of Al(OH)(CH₃COO)₂ and NH₄NO₃ as a by-product of the reaction during calcination prevent particle agglomeration. Other advantages of the proposed process are its versatility and the ability to obtain high purity materials without producing large amounts of by-products without the need for washing and energy saving by using a low processing temperature, and the possibility of recycling the generated CO₂ and NH₃ gases as the (NH₄)₂CO₃ reagent.

Flow chart of the proposed process for production of nanoporous alumina monoliths.     

Improving the photocatalytic reduction of CO2 to CO for TiO2 hollow spheres through hybridization with a cobalt complex

A chemical system with enhanced efficiency for electron generation and transfer was constructed by the integration of TiO₂ hollow spheres with [Co(bipy)₃]²⁺. The introduction of [Co(bipy)₃]²⁺ remarkably enhances the photocatalytic activity of pristine semiconductor photocatalysts for heterogeneous CO₂ conversion, which is attributable to the acceleration of charge separation. Of particular interest is that the excellent photocatalytic activity of the heterogeneous catalysts can be utilised for a universal photocatalytic CO₂ reduction system. Yields of 16.8 μmol CO and 6.6 μmol H₂ can be obtained after 2 h of the photoredox reaction, and the apparent overall quantum yield was estimated to be 0.66% under irradiation at λ = 365 nm. The present findings clearly demonstrate that the integration of electron mediators with semiconductors is a feasible process for the design and development of efficient photochemical systems for CO₂ conversion.

A chemical system with enhanced efficiency for electron generation and transfer was constructed by the integration of TiO₂ hollow spheres with [Co(bipy)₃]²⁺.     

Preparation of superhydrophobic surfaces with micro/nano alumina molds

Polymer micro/nano hierarchical structures were successfully formed by photo-nanoimprinting using anodic porous alumina molds. Anodic porous alumina molds with hierarchical structures were prepared by the anodization of an Al substrate with a micro-concave array. The obtained surfaces with hierarchical structures exhibited superhydrophobicity. The hydrophobic properties of the obtained samples were dependent on the surface structures and could be optimized by changing the micro- and nanopatterns in the hierarchical structure.

Superhydrophobic surface with hierarchical structures prepared by nanoimprinting using anodic porous alumina molds.     

Facile synthesis of ordered mesoporous zinc alumina catalysts and their dehydrogenation behavior

Ordered mesoporous Zn/Al₂O₃ materials with varying Zn content were simply prepared via an evaporation-induced self-assembly (EISA) method. Dehydrogenation of isobutane to isobutene was carried out on these materials; an isobutane conversion of 45.0% and isobutene yield of 39.0% were obtained over the 10%Zn/Al₂O₃ catalyst at 580 °C with 300 h⁻¹ GHSV. The obtained materials with Zn content up to 10% possess large specific surface area and big pore volume and zinc species can be highly dispersed on the surface or incorporated into the framework. The acidity of these catalysts was changed by the introduction of Zn, the decrease of strong acid sites is conducive to the promotion of isobutene selectivity and the weak and medium acidic sites played an important role in isobutane conversion. In addition, these catalysts exhibited good catalytic stability, due to the effective inhibition of coke formation by the ordered mesoporous structure.

Facile synthesis of ordered mesoporous zinc alumina catalysts that exhibited great catalytic activity (45.0% isobutane conversion, 86.7% isobutene selectivity) and stability.     

Synthesis of nitrogen-doped graphene wrapped SnO2 hollow spheres as high-performance microwave absorbers

Nitrogen-doped graphene (NG)/SnO₂ hollow sphere hybrids were synthesized in this work. The chemical composition, crystal structure and morphology have been characterized by FT-IR spectra, XRD, Raman spectra, XPS, SEM and TEM in detail. Reflection loss (RL) values of NG/SnO₂ hollow sphere hybrids less than −10 dB and −20 dB are found in the wide frequency range of 4.5–18 GHz and 5–16.2 GHz within 1.3–3.5 mm, and a minimum RL of −50.3 dB is achieved at 8.6 GHz with the matching thickness of only 2.3 mm. The results indicate that the NG/SnO₂ hollow sphere hybrids with high-performance microwave absorption properties have a promising future in decreasing electromagnetic wave irradiation and interference.

Nitrogen-doped graphene/SnO₂ hollow sphere hybrids were synthesized and show high-performance microwave absorption properties.     

Efficient fabrication of ordered alumina through-hole membranes using a TiO2 protective layer prepared by atomic layer deposition

Ordered alumina through-hole membranes were obtained by a combination of the anodization of Al, formation of a TiO₂ protective layer, and subsequent etching. Two-layered anodic porous alumina materials composed of TiO₂-coated and noncoated alumina were prepared by the combination of the anodization of Al and the formation of a TiO₂ protective layer by atomic layer deposition (ALD). The obtained two layers of anodic porous alumina have different solubilities because the TiO₂ thin layer formed by ALD acts as a protective layer that prevents the dissolution of the alumina layer during wet etching of the sample in an etchant. After the selective dissolution of the bottom layer of porous alumina without the TiO₂ layer, an ordered alumina through-hole membrane could be detached from the Al substrate. This process allows the repeated preparation of ordered alumina through-hole membranes from a single Al substrate. By this process, ordered alumina through-hole membranes with large interhole distances could also be obtained. The obtained alumina through-hole membrane can be used in various applications.

Ordered alumina through-hole membrane with an interhole distance of 1 μm.     

Chemo-photothermal effects of doxorubicin/silica–carbon hollow spheres on liver cancer

Chemo-photothermal therapy, which exhibits synergistic effects, is more effective than either of the treatments administered alone because of its superior ability to target and destroy cancer cells. An anti-cancer compound (doxorubicin, DOX) was embedded in silica–carbon hollow spheres (SCHSs) using heat and vacuum to integrate multi-therapeutic effects onto one platform and subsequently improve the anti-cancer efficacy. SCHSs were synthesized via a surface activation method and its highly porous surface enhanced the loading content of the desired drug. SCHSs are an infrared photothermal material that can destroy targeted cells by heating under near-infrared (NIR) laser illumination at 808 nm. NIR laser illumination also enhances DOX release from SCHSs to increase the anti-cancer efficiency of DOX–loaded SCHSs (DOX–SCHSs) in both two-dimensional and three-dimensional multicellular tumor spheroid cultures. SCHSs exhibited high heat-generating ability and pH-responsive drug delivery. In conclusion, this study demonstrated that DOX–SCHSs represent a potential tool for chemo-photothermal therapy due to its photothermal effects. Thus, our findings imply that the high cancer cell killing efficiency of DOX–SCHSs induced by NIR illumination can be used for the treatment of tumors.

SCHSs were applied as vectors for drug delivery and thermal production under NIR laser irradiation. DOX-loaded SCHSs conjugated with ConA were found to kill liver cancer cells efficiently.     

Template-free preparation of iron oxide loaded hollow silica spheres and their anticancer proliferation capabilities

Hollow silica spheres (HSS) exhibited high-specific surface area, low toxicity, low density, and good biocompatibility. The effectivity of HSS material can be improved further by loading nanoparticles for smart biological applications. In this work, magnetic nanoparticle (iron oxide; Fe₃O₄)-loaded pure HSS (c-HSS-Fe) were synthesized successfully using a template-free chemical route and investigated for their anticancer cell proliferation capabilities against cancerous cell lines: human colorectal carcinoma cells (HCT-116). The structure, morphology, chemical bonding, and thermal stability of the prepared HSS derivatives were studied using spectroscopic and microscopic techniques. Our analyses confirmed the successful preparation of Fe₃O₄ loaded HSS material (sphere diameter ∼515 nm). The elemental analysis revealed the existence of Fe along with Si and O in the Fe₃O₄ loaded HSS material, thus reaffirming the production of the c-HSS-Fe product. The effects of silica spheres on HCT-116 cells were examined microscopically and by MTT assays. It was observed that the c-HSS-Fe demonstrated dose-dependent behavior and significantly reduced the cancer cell proliferation at higher doses. Our results showed that c-HSS-Fe was more effective and profound in reducing the cancer cells'' activities as compared to unloaded HSS material where the cancer cells have undergone nuclear disintegration and fragmentation. It is concluded that c-HSS-Fe is a powerful bio-active material against cancerous cells.

Hollow silica spheres were loaded with Fe₃O₄ NPs (u-HSS-Fe) and calcined further to remove the non-degradable phenyl groups (c-HSS-Fe) for anticancer applications.     

One-pot preparation of hierarchical Cu2O hollow spheres for improved visible-light photocatalytic properties

As visible light photocatalysts, narrow bandgap semiconductors can effectively convert solar energy to chemical energy, exhibiting potential applications in alleviating energy shortage and environmental pollution. Cu₂O hollow spheres with a narrow band gap and uniform hierarchical structures have been fabricated in a controlled way. The one-pot solvothermal method without any template is simple and facile. The morphologies, crystal structures, composition, specific surface areas, and optical and photoelectric properties of the products were analyzed by various techniques. The hollow and solid Cu₂O spheres could be fabricated by controlling the reaction time, and a possible growth process of the Cu₂O hollow spheres was revealed. The degradation of methyl orange (MO) was used to investigate the visible-light catalytic properties of the Cu₂O samples. More than 90% of MO is degraded under visible light illumination of 20 min, exhibiting a quick catalytic reaction. The rate constant of the Cu₂O hollow spheres was 2.54 times and 46.6 times larger than those of the Cu₂O solid spheres and commercial Cu₂O powder, respectively. The possible photocatalytic mechanism of MO was revealed over Cu₂O hollow spheres through the detection of active species. The as-prepared Cu₂O hollow spheres display improved visible-light catalytic activity and stability, indicating their potential application in wastewater treatment.

As visible light photocatalysts, narrow bandgap semiconductors can effectively convert solar energy to chemical energy, exhibiting potential applications in alleviating energy shortage and environmental pollution.     

Synthesis of an ordered nanoporous Cu/Ni/Au film for sensitive non-enzymatic glucose sensing

Ordered nanoporous Cu/Ni/Au film was prepared by electrochemical deposition and magnetron sputtering using an anodic aluminium oxide template. The fabricated porous film has a uniform hexagonal pore size structure, a long-range ordered arrangement, and a pore diameter of approximately 40 nm. Following the dissolution of the template, the independent Cu/Ni/Au film is devolved to an ITO substrate as an effective non-enzyme glucose detection sensor. The sensor has good electrocatalytic performance with two specific linear ranges of 0.5 μM to 3.0 mM and 3.0–7.0 mM and high sensitivities of 4135 and 2972 μA mM⁻¹ cm⁻², respectively. The lower detection limit was 0.1 μM with a signal-to-noise ratio of 3. Additionally, the sensor features excellent selectivity and stability. These satisfactory results indicate that Cu/Ni/Au film is a promising platform for the development of non-enzymatic glucose sensors.

Ordered nanoporous Cu/Ni/Au film prepared by template method could be transferred and used as an effective glucose sensor.     

Formation of highly ordered micro fillers in polymeric matrix by electro-field-assisted aligning

Nanocomposites composed by polymeric matrix with micro/nano fillers have drawn lots of attention since their dramatic properties beyond pristine polymers. The spatial distribution of the micro/nano fillers in the polymeric matrix determines the final desired properties of the nanocomposites, thus deserves to investigate. Here, we proposed an effective method of assembling the micro/nano fillers to pre-designed patterns within the polymeric matrix by AC-electro-field-assisted aligning. By pre-designed AC electric fields which could be dynamically controllable, the distribution of microparticles (acting as fillers) in the matrix was tuned to various patterns related to the electric fields, such as linear alignment and circular alignment. The field-oriented particles chains could act as endoskeletal structures, showing unique properties (i.e., mechanical, optical, and anisotropic properties) beyond those of the conventional composites with randomly distributed particles.

Nanocomposites composed by polymeric matrix with micro/nano fillers have drawn lots of attention since their dramatic properties beyond pristine polymers.     

Mesoporous hollow black TiO2 with controlled lattice disorder degrees for highly efficient visible-light-driven photocatalysis

Black TiO₂ has received tremendous attention because of its lattice disorder-induced reduction in the TiO₂ bandgap, which yields excellent light absorption and photocatalytic ability. In this report, a highly efficient visible-light-driven black TiO₂ photocatalyst was synthesized with a mesoporous hollow shell structure. It provided a higher specific surface area, more reaction sites and enhanced visible light absorption capability, which significantly promoted the photocatalytic reaction. Subsequently, the mesoporous hollow black TiO₂ with different lattice disorder-engineering degrees were designed. The structure disorder in the black TiO₂ obviously increased with reduction temperature, leading to improved visible light absorption. However, their visible-light-driven photocatalytic efficiency increased first and then decreased. The highest value can be observed for the sample reduced at 350 °C, which was 2-, 1.4- and 5-fold that of the samples reduced at 320 °C, 380 °C and 400 °C, respectively. This contradiction can be ascribed to the varied functions of the surface defects with different concentrations in the black TiO₂ during the catalytic process. In particular, the defects at low concentrations boost photocatalysis but reverse photocatalysis at high concentrations when they act as charge recombination centers. This study provides significant insight for the fabrication of high-efficiency visible-light-driven catalytic black TiO₂ and the understanding of its catalysis mechanism.

Our work provides significant insights into the design of hollow black TiO₂ spheres and the mechanism accounting for their high-efficient visible-light-driven catalysis.