Composite Hollow Spheres Research Articles

Mechanical properties of cementitious metallic hollow sphere composite under compression

This study presents a novel cementitious metallic hollow sphere composite by integrating metallic hollow spheres (MHS) into a cement matrix. Experimental results show that while MHS reduce the stiffness and strength of cement matrix, they enhance deformability and energy absorption. Adding 0.5% polypropylene fibers significantly improves mechanical properties and changes failure mode from brittle to ductile. This work offers insights into MHS materials in civil engineering.

Urchin-Like Mesoporous TiN Hollow Sphere Enabling Promoted Electrochemical Kinetics of Bromine-Based Flow Batteries.

Bromine-based flow batteries (BFB) have always suffered from poor kinetics due to the sluggish Br3 -/Br- redox, hindering their practical applications. Developing cathode materials with high catalytic activity is critical to address this challenge. Herein, the in-depth investigation for the free energy of the bromine redox electrode is conducted initially through DFT calculations, establishing the posterior desorption during oxidation as the rate-determining step. An urchin-like titanium nitride hollow sphere (TNHS) composite is designed and synthesized as the catalyst for bromine redox. The large difference in Br- and Br3 - adsorption capability of TNHS promotes rapid desorption of generated Br3 - during the oxidation process, liberating active sites timely to enable smooth ongoing reactions. Besides, the urchin-like microporous/mesoporous structure of TNHS provides abundant active surface for bromine redox reactions, and ample cavities for the bromine accommodation. The inherently high conductivity of TNHS enables facile electron transfer through multiple channels. Consequently, zinc-bromide flow batteries with TNHS catalyst exhibit significantly enhanced kinetics, stably operating at 80mAcm-2 with 82.78% energy efficiency. Overall, this study offers a solving strategy and catalyst design approach to the sluggish kinetics that has plagued bromine-based flow batteries.

Highly sensitive electrochemical sensing of norfloxacin by molecularly imprinted composite hollow spheres

Poly (3,4-ethylenedioxythiophene) (PEDOT)-based molecularly imprinted electrochemical sensors have attracted widespread attention for monitoring contaminants in food and the environment. However, there are still problems such as poor hydrophilicity, easy agglomeration, and low selectivity in its preparation. In this work, a novel molecularly imprinted composite hollow sphere was prepared by a molecular imprinting technique using nitrogen-doped hollow carbon spheres as matrix material, and PEDOT and poly(methacrylic acid) as monomers. The selective binding capabilities and mechanism of the material to norfloxacin (NOR) were systematically investigated. Then the material-based sensor was constructed, and its electrochemical detection performance toward NOR was thoroughly studied. The sensor exhibited a wide linear range (0.0005–31 μM), a low detection limit (0.061 nM), satisfactory immunity to interference and stability. Besides, the sensor displayed better sensitivity and reliability (spiked recoveries of 98.0–105.2%, relative standard deviation of 3.45–5.69%) for detecting NOR in lake water, honey, and milk than high-performance liquid chromatography. This work provides a new strategy for developing poly(3,4-ethylenedioxythiophene)-based molecularly imprinted electrochemical sensors.

Returnable MoS2@carbon nitride nanotube composite hollow spheres drive photo-self-Fenton-PMS system for synergistic catalytic and photocatalytic tetracycline degradation

In this work, we synthesized hollow sphere composites of MoS2@carbon nitride nanotube (MoS2@TCN-S) for in-situ formation of H2O2 under visible light. The photo-self-Fenton system was further coupled with peroxymonosulfate (P-SF-PMS) for promoted reactive oxygen species generation and tetracycline degradation, so that the P-SF-PMS(MoS2@TCN-S) system can remove 94.3 % TC within 120 min, and the removal efficiencies after 15 cycles decrease by less than 5 %. The P-SF-PMS(MoS2@TCN-S) system also shows excellent degradation performance for actual wastewater, with COD removal rates of domestic sewage and secondary sedimentation tank effluent reaching 64.6 % and 58.7 % within 120 min. Quenching experiment and ESR results reveal the synergy of PMS and P-SF system, which was the key to improving the degradability and pH (3–9) adaptability of the P-SF-PMS system. Under alkaline conditions, PMS and H2O2 generated on MoS2@TCN-S activate each other and form 1O2 and OH, which solves the problem of poor degradation of P-SF system under alkaline conditions. Under acidic and neutral conditions, O2– becomes the main ROS, the activation of PMS by O2– and the synergistic effect of PMS and H2O2 make the P-SF-PMS system get rid of the dependence on H+. In addition, direct Z-scheme heterojunction enables MoS2@TCN-S to exhibit faster electron transfer ability, faster H2O2 generation ability, stronger adsorption capacity for PMS, and lower activation barrier for *HSO5, which also enhances the synergistic effect between PMS and P-SF system. In this study, a new type of multi-functional water purification material is proposed, and the synergism of PMS and P-SF system is revealed.

Confined dissipation cage in dual-shell structured Ti3C2Tx@CNTs/Ni hollow spheres for lightweight and broadband electromagnetic wave absorption

Ti3C2Tx can dissipate energy well due to its high conductivity, but at the expense of severe electromagnetic (EM) wave reflection, which is exacerbated by self-stacking. Despite various attempts to tackle this problem, the excessive isolation of Ti3C2Tx and massive high-density inorganic components render these designs unfavorable for lightweight and broadband absorption. Meanwhile, improvements in absorption performance are related to active surface and polarization interface expansion, as well as conductive path reconstruction, but the main contributing factor remains unknown. In this work, the dual-shell structured Ti3C2Tx@CNTs/Ni-HS (hollow sphere) composites are fabricated. This structure creates a confined dissipation cage, where the CNTs/Ni shell confines the electron transfer among spheres and prevents self-restacking and the tightly contacted Ti3C2Tx shell forms an uninterrupted conductive path for effective dissipation. Therefore, Ti3C2Tx@CNTs/Ni-HS-1 exhibits a broad effective absorption band of 6.1 GHz and a minimum reflection loss of −64.6 dB at a mass ratio of only 15 wt%. Importantly, when hollow structure cracks, although large active surface and polarizable interface remain, the absorption capacity of fragmented Ti3C2Tx@CNTs/Ni-HS-1 deteriorates, indicating rational conductive path in confined dissipation cage is the main factor in achieving excellent performance. This work provides new insight for the rational design of high-performance absorbers.

MoS2 grown on hollow carbon nanospheres as photoanode for improved photoelectrocatalytic degradation of Bisphenol A

Photoelectrocatalytic (PEC) technology capable of fully mineralizing hazardous substances is a promising strategy to address growing environmental problems. Herein, MoS2 nanosheets grown on nitrogen doped hollow carbon spheres (MoS2–NHCS) composites was prepared, and then bonded on a foamed nickel matrix by electrodeposition as photoanode for PEC degradation of Bisphenol A (BPA). Due to the integrated synergistic effect of unique composites structure and photocatalysis/electrocatalysis, the separation efficiency and survival life of photogenerated carriers were improved, and then accelerated the degradation of pollutants. The maximum reaction rate constant was 7.30 × 10−3 min−1. Photoelectrochemical characterization and key influencing parameters were discussed to clarify the relations between structure and function. The activity loss of MoS2–NHCS photoanode after four cycles was less than 10%, showing the long-term stability of BPA removal. The radicals quenching results revealed that this composite system was dominated by •OH together with moiety contributed by h+ and O2•-. Based on the detection results of intermediates, a possible decomposition pathway of BPA was proposed. The mixed pollution experiments suggested that the synergistic effect between organic matter and heavy metal greatly increased the treatment load of this composite electrode, providing a new idea for the study of practical wastewater treatment by the PEC system.

Construction of a bioinspired Fe3O4/N-HCS nanozyme for highly sensitive detection of GSH

The catalytic synergy of different nanomaterials is a potential technique used to develop a highly active and stable nanozyme. Here, magnetite on nitrogen-doped hollow carbon sphere (Fe3O4/N-HCS) composites were prepared through solvent thermal method and exhibited excellent oxidase and peroxidase-like catalytic activity under different pH levels. The dual enzyme-like catalyzed reaction of Fe3O4/N-HCS conformed to the typical Michaelis–Menten kinetics, and Fe3O4/N-HCS had a strong affinity for substrate 3,3′,5,5′-tetramethylbenzidine (TMB) with H2O2. The remarkable performance of the composites was achieved by the synergistic effect of Fe3O4 and N-doped activated graphite carbon, which accelerated the formation of O2•- and •OH during the dual enzyme-like catalytic reaction. Furthermore, an effective colorimetric glutathione (GSH) sensor with a wide linear range from 0.02 µM to 250 µM and a detection limit as low as 9.3 nM was constructed based on the superior peroxidase-like activity of Fe3O4/N-HCS. The convenient colorimetric sensor monitored the content of GSH in the serum of mice, and the detection results were satisfactory, with a recovery of 96–103% and an RSD between 0.80% and 3.30%. This study discussed the internal mechanism of Fe3O4/N-HCS nanozyme and developed an efficient method for GSH detection to expand nanozyme application in bioanalysis.

A novel bio-inspired metal hollow sphere filled TC4/Al sandwich composite: Fabrication and compressive properties

A novel bio-inspired metal hollow sphere filled TC4/Al sandwich composite was proposed based on the cognition of biological materials with TC4/Al metal laminated composites (MLCs) as outer layers and metal hollow sphere composite (MHSC) made from 316 L stainless steel (316 L SS) metal hollow spheres (MHSs) and Al powder matrix as inner core. The sandwich composite was sintered by the hot pressing method in a one-step process. Scanning electron microscopy (SEM) observation, energy dispersive spectrometer analysis (EDS), and X-ray diffraction (XRD) analysis were carried for characterization on multiple interfaces. Al3Ti intermetallic compound layers were formed between laminations and 316 L/Al interfaces with Fe-Al intermetallic compounds were formed between 316 L SS MHSs and Al matrix in MHSC inner core. Compressive stress-strain curves of the sandwich composite exhibited the combination characteristic of the comparison MHSC and MLC curves. The deformation behavior of the sandwich composite was first dominated by MHSC inner core and then by MLCs outer layers. Failure modes and interface effects were also studied.

Effect of Preparation Process on Microstructure and Mechanical Properties of Metal Hollow Sphere Composites (Mhscs)

Effect of Preparation Process on Microstructure and Mechanical Properties of Metal Hollow Sphere Composites (Mhscs)

Investigation on the acoustic properties of structural gradient 316L stainless steel hollow spheres composites

Abstract In this study, a kind of structural gradient metal hollow spheres composites (SG-MHSCs) were fabricated using two kinds of 316L stainless steel hollow spheres with different diameters and A356 aluminum through the casting method. Then the density of the SG-MHSCs was measured by the direct measurement; the microstructure of the SG-MHSCs was characterized by the Scanning Electron Microscope. Meanwhile, the acoustic performance of MHSCs was tested by the impedance tube, and the sound absorption and insulation mechanism SG-MHSCs were discussed and analyzed.

Sol-gel Synthesis of Boron Doped TiO2/hollow Glass Bubbles Composite Powders for Photocatalytic Degradation of Azophloxine

Background: B-TiO2 was supported on the surface of iM16K glass bubbles to achieve a suitable density for the B-TiO2/iM16K composite hollow spheres. Aeration or stirring in the wastewater can lead to thorough mixing of photocatalyst and wastewater. Solid-water separation is quite easy because the materials can float on the water surface while stopping aeration or stirring. Methods: The iM16K glass bubbles were used to prepare boron-doped B-TiO2/iM16K composite hollow spheres through a sol-gel route. The materials were characterized by X-ray diffraction, scanning electron microscope, Fourier transform infrared spectroscopy, UV-Visible diffuse reflectance spectrometry, and N2 adsorption-desorption techniques. Photocatalytic degradation of azophloxine on the composites was determined. Results: The bandgap energy of the B-TiO2/iM16K composite was slightly less than 3.0 eV when the calcination temperature was below 500oC. The sample calcined at 350oC had a BET surface area of 88.6 m2 /g, while the value of the sample calcined at 800oC was 1.2 m2 /g. The maximum photocatalytic degradation efficiency was obtained for the sample calcined at 450oC, and nearly all of the original azophloxine molecules were decomposed after 120 min of irradiation. Photocatalytic degradation efficiency after 30 min of irradiation was enhanced from 18.8% to 47.9% when B-TiO2 dosage increased from 100 to 800 mg/L. Conclusion: Crystallization of anatase TiO2 was temperature dependent, and the properties of B-TiO2/iM16K composite hollow spheres were affected by phase composition of the boron-doped TiO2 layer. The change in calcination temperature can have significant effect on photocatalytic degradation of azophloxine. The production of hydroxyl radical depended on photocatalytic activity of the B-TiO2/iM16K composite hollow spheres.

Synergistic effect of Co3Fe7 alloy and N-doped hollow carbon spheres with high activity and stability for high-performance lithium-sulfur batteries

Lithium-sulfur (Li-S) batteries have attracted extensive attention as a promising next-generation electrochemical energy storage technology, owing to their high energy density and low material cost. However, issues such as severe polarization and poor cycle stability caused by shuttle effect and slow sulfur redox kinetics limit their practical applications. Here, Co 3 Fe 7 alloy embedded into nitrogen-doped hollow carbon sphere composite (CoFe/NHCS) was synthesized as an electrocatalyst for Li-S batteries. The Co 3 Fe 7 alloy demonstrates a strong chemisorption and superior electrocatalytic conversion towards polysulfides, while the nitrogen-doped carbon hollow spheres promote Li + /electron transfer and physically suppress polysulfides shuttling. Their synergistic effect therefore could both accelerate the polysulfides redox conversion and inhibit the polysulfides loss. As a consequence, the Li-S batteries assembled with CoFe/NHCS-modified separators exhibit a superior rate capacity (1029 mAh/g at 2 C) and excellent cycling stability (644 mAh/g at 1 C after 500 cycles). Furthermore, even at a high sulfur loading of 6.7 mg/cm 2 , a high areal capacity of 5.58 mAh/g is achieved, which is retained at 4.45 mAh/cm 2 after 100 cycles. In addition, the CoFe/NHCS possesses an excellent stability in both physical structures and chemical properties over extended cycles, demonstrating its great potential for high-performance and long-cycle-life Li-S batteries. The Co 3 Fe 7 alloy embedded into nitrogen-doped hollow carbon sphere composite exhibits an excellent cycling performance with a high sulfur loading for lithium-sulfur batteries. • Co 3 Fe 7 alloy embedded into nitrogen-doped hollow carbon sphere (CoFe/NHCS) was synthesized for Li-S batteries. • CoFe/NHCS demonstrates an improved adsorptive and electrolytic effect towards polysulfides. • CoFe/NHCS enables Li-S batteries with an excellent rate performance and superior cycling stability. • With a high sulfur loading of 6.7 mg/cm 2 , a high areal capacity of 4.45 mAh/cm 2 is retained after 100 cycles. • CoFe/NHCS possesses an excellent stability in both physical structures and chemical properties over extended cycles.

Enhanced photocatalytic H2 production performance of CdS hollow spheres using C and Pt as bi-cocatalysts

Photocatalytic H2 production from water splitting is an effective method to solve energy crisis and environmental pollution simultaneously. Herein, carbon@CdS composite hollow spheres (C@CdS-HS) are fabricated via a facile hydrothermal method using porous carbon hollow spheres (C-HS) as the template. The C@CdS-HS shows an excellent photocatalytic H2-generation rate of 20.9 mmol h−1 g−1 (apparent quantum efficiency of 15.3% at 420 nm), with 1.0 wt% Pt as a cocatalyst under simulated sunlight irradiation; this rate is 69.7, 13.9, and 3.9 times higher than that obtained with pure CdS hollow spheres (CdS-HS), C@CdS-HS, and CdS-HS/Pt, respectively. The enhanced photocatalytic H2-evolution activity of C@CdS-HS/Pt is due to the synergistic effect of C and Pt as the bi-cocatalyst. The C-HS serves not only as an active site provider but also as an electron transporter and reservoir. Moreover, C-HS has a strong photothermal effect that is induced by near infrared light, which kinetically accelerates the H2-production reaction. Additionally, the underlying charge transfer pathway and process from CdS to C−HS is revealed. This work highlights the potential application of C-HS-based nanocomposites in solar-to-chemical energy conversion.

Facile synthesis of multi-shelled MnO2–Co3O4 hollow spheres with superior catalytic activity for CO oxidation

Nano-oxide hollow structures have a wide range of applications in catalysis, but the preparation of multi-shelled composite oxide hollow structures still presents a major challenge. Herein, well-dispersed multi-shelled MnO2–Co3O4 hollow spheres have been successfully prepared via a facile “Kirkendall effect” method employing carbon spheres as sacrifice templates. The shell of the hollow spheres was constructed by interconnected Co3O4 nanosheets and catkin-like MnO2 nanorods. The interdiffusion of Carbon chain and MnO4− during synthesis could be the possible formation mechanism for the special morphology. Benefiting from the hierarchical porous structure, high surface area and strong synergistic effect between Co3O4 and MnO2 was obtained, the as-prepared Co–Mn composite hollow spheres showed reliably high activity and recycle stability for CO oxidation, achieving the complete CO conversion at 137 °C without significant decrease in activity after five cycles and 10h constant-temperature reaction. We believe that this work will provide a new method for the preparation of multi-shell composite hollow spheres.

One-pot synthesis of hematite-alumina hollow sphere composite by ultrasonic spray pyrolysis technique with high adsorption capacity toward PAHs

A hybrid nanocomposite of alumina and hematite was synthesized by ultrasonic spray pyrolysis technique. The study of microscopic images, mapping analysis, and XRD patterns revealed that the Al2O3 – Fe2O3 nanocomposite was composed of separated spherical particles with a thin layer ball-shaped structure that metal oxides are uniformly distributed in the wall of hollow sphere particles, led to a coherent and monotonous construction. A series of coefficients of equilibrium sorption of polycyclic aromatic hydrocarbons (PAHs) as hazardous materials were measured on the prepared composite material in a batch technique. The free or pure Al2O3 or Fe2O3 showed negligible removal efficiency for the mentioned analytes. The various significant variables, such as initial analyte concentration, solution pH, adsorbent dose, and contact time to remove analyte, were studied in the aqueous solutions. Adsorption data were modeled to Langmuir, Freundlich, and Temkin isotherms, and a good correlation found in the case of Langmuir isotherm and adsorption capacity for anthracene, phenanthrene, and naphthalene were 370, 333, and 322 mg g−1, respectively. Investigation of the kinetic models proved a pseudo-second-order, and the prepared adsorbent can be reused more than 7 times without a significant decrease of adsorption performance.

One-pot self-assembly preparation of thiol-functionalized poly(3,4-ethylenedioxythiophene) hollow nanosphere/Au composites, and their electrocatalytic properties.

In this work, we developed a thiol-functionalized poly(3,4-ethylenedioxythiophene) hollow sphere (poly(EDOT-MeSH)/Au) polymer through a simple one-pot self-assembly method using polyvinylpyrrolidone (PVP) as a soft template. The monomer was used as both a reductant and a stabilizer to decorate gold nanoparticles (Au NPs). FTIR, XRD, EDX, SEM and TEM analyses were used to characterize the composite hollow spheres. The chemical bond between S and Au was confirmed by XPS. The electrochemical performance of the composite hollow spheres was determined by cyclic voltammetry (CV) and an ampere response timing current test. The results revealed that the poly(EDOT-MeSH)/Au hollow-sphere-based electrochemical sensor possesses excellent conductivity and high redox reversibility with detection limits (S/N = 3) of 0.2, 0.02, 0.08 and 0.05 μM in the linear ranges of 0.1–650 μM, 0.05–100 μM and 0.1–600 μM for the determination of ascorbic acid (AA), dopamine (DA), uric acid (UA) and nitrate ions (NO2−), respectively. The preparation method for these composites will further the development of this type of conducting polymer/gold nano-composite material modified electrochemical sensor for biological species.

Magnetically-controllable hollow multi-shell magnetite spheres as stable and reusable SERS substrates

This study successfully synthesized magnetically-manipulatable and reusable SERS active buoyant substrates, i.e. Ag@SiO2@Fe3O4 composite hollow spheres. The composites comprised mesoscopic hollow spheres with a Fe3O4 inner shell of 15–30 nm in thickness and strong ferromagnetism, which were synthesized using spray pyrolysis and the subsequent reduction, and a SiO2 outer shell to maintain the stability of Fe3O4, as well as Ag nanoparticles on the sphere surface providing localized surface plasmon resonance. The coercive field (Hc) of bare Fe3O4 hollow sphere reached 200 Oe, and the saturation magnetization (Ms) was 130 emu/g at 300 K, which was at least 30% higher than the reported values no matter in bulk or nanoscales. Compared to composite spheres with Fe3O4 solid cores, the SERS signal intensity of hollow structures was 2.6 times greater, which could be attributed to the buoyancy of hollow structure which contribute to more interaction with the target analytes. Under external magnetic field, these ferromagnetic hollow composites can be concentrated and separated easily, resulting in a more augmented SERS effect (about 1.5 times higher than those collected without applying magnetic field). This study also demonstrated the possibility to reuse those composites for SERS measurements.

Acoustic Properties of 316L Stainless Steel Hollow Sphere Composites Fabricated by Pressure Casting

In this study, we prepared metal hollow sphere composites (MHSCs) using metal hollow spheres (MHSs) by pressure casting under vacuum conditions, and investigated the acoustic properties. The density of the MHSCs was measured using the mass to volume ratio, the microstructure of the MHSCs was observed using a scanning electron microscope, and the acoustic properties of the MHSCs were tested using an impedance tube. The measured MHSCs showed that the densities of the MHSCs with the random distribution of MHSs with diameter ~3.28 mm (1.74 g/cm3 to 1.77 g/cm3) (MHSC-3.28) were nearly equal to that of the MHSCs with the random distribution of MHSs with diameter ~5.76 mm (1.74 g/cm3 to 1.76 g/cm3) (MHSC-5.76), and lower than that of the MHSCs with the layered structure of MHSs with diameter ~3.28 mm (1.93 g/cm3 to 1.97 g/cm3) (MHSC-LS). Microstructural observations confirmed that the interface region between the MHSs and matrix demonstrated a simple physical combination pattern with pores. The acoustic properties of the MHSCs showed that the sound absorption coefficient of MHSC-LS was lower than that of MHSC-3.28 and higher than that of MHSC-5.76 at off-resonance. The sound absorption coefficient peak value of MHSC-3.28 was higher than that of MHSC-LS, and lower than that of MHSC-5.76 at resonance. The sound transmission loss of MHSC-3.28 was lower than that of MHSC-5.76, which shows the rules are independent from the resonance. The sound transmission loss of MHSC-LS was higher than that of MHSC-5.76 at resonance, but lower than that of MHSC-3.28 at off-resonance. In addition, we discuss the propagation mechanism of the sound waves in the MHSC, which is mainly determined by the distribution of the MHSs in the MHSC.

Acoustic insulation and absorption mechanism of metallic hollow spheres composites with different polymer matrix

In this study, the polyurethane/316L stainless steel hollow spheres (PU/316L HS) and silicone rubber/316L stainless steel hollow spheres (SR/316L HS) composites were prepared by casting method. The sound transmission loss of polyurethane composites and silicone rubber composites increased after adding metallic hollow spheres. The sound insulation performance of hollow sphere composite is closely related to the acoustic reflection and diffraction at the interface. In addition, silicone rubber with better molecular flexibility is easier to reflect sound waves than polyurethane, thus reducing the passing sound energy, so it has greater sound transmission loss. For the sound absorption performance of materials, the PU has a maximum absorption coefficient of 0.87 at 3550 Hz, but the absorption coefficient of the PU/316L HS composite moved to the lower frequency. The reason for this phenomenon is that the hollow spheres change the elastic modulus of polyurethane, which causes the resonance frequency of material and air to move to lower frequency, so the range of sound absorption frequency changes. Moreover, the different proportion of free volume in polyurethane and silicone rubber matrix is the main reason for the great difference of their sound absorption properties.

Synthesis and Low-Concentration Acetone Sensing Properties of ZnFe2O4/Rgo Hollow Spheres Composites

Introduction As a kind of synthetic raw material in industrial production, acetone is chemically active and extremely flammable. It also behaves toxic, a long-term exposure to acetone, if its concentration exceeds 173 ppm, will make a serious threat to human health [1-2]. Furthermore, acetone is also a fat metabolite in human’s body. According to the related literature, the concentration of acetone in the exhaled gas of healthy people is less than 0.8 ppm, while that in exhaled gas of diabetic patients is higher than 1.8 ppm [3-4]. In this view, it is possible to realize diagnosing diabetes with nondestructive testing. It is necessary to develop high performance gas sensor for practical application.With a unique spine structure and a narrow band gap width (~1.94 eV), zinc ferrite (ZnFe2O4) possesses several remarkable properties and shows a good potential in gas sensing filed. And our previous works had studied pure ZnFe2O4 hollow spheres and Ag-ZnFe2O4 gas sensing nanomaterials. Aiming at the high-performance acetone gas sensor, we prepared ZnFe2O4 hollow spheres/rGO composites via a facile solvothermal synthesis method followed with a high-temperature heat treatment process in an inert atmosphere. ZnFe2O4/rGO gas sensor The obtained pure ZnFe2O4 hollow spheres or ZnFe2O4/rGO powders were firstly dissolved into deionized water to form a homogeneous paste, which were then coated uniformly on the surface of Al2O3 ceramic plate with heating electrodes (Pt) and gold electrodes (Au) to fabricate a thin sensing films. Subsequently, the sensors were dried at 120℃ for 12 h, and after another aging for 24 h at 180℃, a series of gas sensors based ZnFe2O4/rGO (different mass contents of rGO: 0, 0.1, 0.25, 0.5 and 1 wt%) were obtained. The temperature of sensors was adjusted by aligning the applied voltage. Method Pure ZnFe2O4 hollow spheres and ZnFe2O4/rGO composites were prepared via a simple solvothermal method followed with a high-temperature heat treatment process in an inert atmosphere. Firstly, GO aqueous dispersion solution (0.5 mg/mL) was further ultrasonic treatment for 2 h, then various amounts of GO solution (0 mL, 0.242 mL, 0.602 mL, 1.205 mL, 2.410 mL) were added into the homogeneous solution of isopropanol (30 mL) and glycerol (8 mL) under slow stirring. Secondly, 0.1098g Zn(CH3COO)2·2H2O and 0.4042g Fe(NO3)3·9H2O were dissolved in the obtained homogeneous solution under magnetically stirring for 1 h. Subsequently, the mixed solutions were transferred into Teflon-lined stainless-steel autoclave (50 mL), and then maintained at 180oC for 12 h. After naturally cooling down to room temperature, the obtained suspensions were centrifuged using deionized water and anhydrous ethanol for 4 times and dried at 75 oC for 12 h. Ultimately, these as-prepared precursors were placed in an argon atmosphere, annealed at 400℃ for 2 h with a heating rate of 5oC/min to reduce GO to rGO, and five samples were obtained: pure ZnFe2O4 hollow spheres, 0.1 wt% ZnFe2O4/rGO, 0.25 wt% ZnFe2O4/rGO, 0.5 wt% ZnFe2O4/rGO and 1 wt% ZnFe2O4/rGO. Results and Conclusions In this work, the sensing properties of ZnFe2O4 sensors to low-ppm-level acetone were improved by compounding with reduced graphene oxide. As shown in Figure 1, as rGO was introduced, the sphere size and hollow structure were not affected while the surface morphology was modified and more regular. In Figure 2, 0.5 wt% ZnFe2O4/rGO sensor exhibited higher response, good selectivity and linear response to low concentration acetone at 200 oC. The response of 0.5 wt% ZnFe2O4/rGO to 10 ppm acetone was 8.18, which was about 2.5 times that of pure ZnFe2O4 gas sensor. Therefore, this sensor can be a promising material to detect low concentration (ppm) acetone, and it is expected to be a potential candidate to diagnose diabetes via nondestructive testing the exhaled acetone vapor if the sensitivity and response speed to ppb-level acetone are further improved.