Synthesis of hierarchical nanosheet-assembled V₂O₅ microflowers with high sensing properties towards amines

Abstract

Hierarchical three-dimensional nanosheet-assembled vanadium pentoxide (V₂O₅) microflowers are successfully synthesized by a hydrothermal method, followed by a high-temperature sintering treatment. Several advanced techniques are used to characterize the morphology and composition of the resulting nanostructures, such as TEM, HRTEM, SEM, XRD, and BET. The HRTEM image shows that the microflowers are assembled from the nanosheets with highly exposed {010} facets, as confirmed by selected area electron diffraction (SAED). According to N₂ sorption isothermal studies, the as-prepared V₂O₅ microflowers show high specific surface area of 61.5 m² g⁻¹. The formation of the microflowers with assistance of NaHCO₃, which may play a critical role in the self-assembly process, may be attributed to a “reproduction mechanism”. The gas sensing performances of both the V₂O₅ microflowers and the V₂O₅ nanosheets were evaluated towards several volatile organic compounds (VOCs), such as 1-butylamine, ethanol, acetone, and formaldehyde. The results show that the flower-like structure exhibits a superior sensing response and selectivity towards amines compared to that of the sheet-like structure at an optimum working temperature of ∼300 °C. The high selectivity towards 1-butylamine can be ascribed to the selective oxidation mechanism. This work will help explore vanadium oxides as gas sensors toward volatile organic compounds with high performance.

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Introduction

Three-dimensional (3D) hierarchical structures assembled with low dimensional nanostructures (1D nanorods and 2D nanosheets) have attracted increasing attention in the last two decades.^1–3 Various metal oxides with hierarchical architectures, such as TiO₂ (ref. 4), SnO₂ (ref. 2), Co₃O₄ (ref. 5), and MnO₂ (ref. 6), have been reported recently. Benefiting from the unique secondary units, the hierarchical structures inherit superior characteristics of the nanostructures, and make the hierarchical structures attractive for many important applications, such as in catalysts,⁴ sensors² and lithium-ion batteries.⁷

Vanadium oxides, especially vanadium pentoxide (V₂O₅, an n-type semiconductor), have been intensively studied as sensing materials recently due to their high chemical stability, low manufacturing cost, and high resistance to corrosion. These advantages make V₂O₅ high potentials for drunken driving tests, indoor gas detection, and disease diagnosis.^11–13 These materials are highly shape-dependent on sensing performance towards some volatile organic compounds (VOCs), such as acetone,⁸ organic ammine,⁹ and ethanol.¹⁰ V₂O₅ nanostructures with all kinds of morphologies, such as 1D (nanotubes,¹¹ wires,⁹ belts,¹² rods¹³), 2D (nanosheets),¹⁴ and 3D structures (urchin-like,⁸ flower-like,¹⁵ hollow spheres¹⁶) have been synthesized to improve their sensing and electrical performance. For example, Liu et al. demonstrated that V₂O₅ nanobelts prepared by a hydrothermal method, having width and length in the ranges of 20–30 nm and 0.2–1 μm, show short response/recovery times of 30–50 s and a low detection limit (5 ppm) towards ethanol at 200 °C.¹⁰ Recently, we have reported nanorod-assembled urchin-like microstructures with good sensitivity toward acetone.⁸ However, there still exists a great challenge to control nano/microstructures with desirable morphologies for excellent sensing performance towards volatile gases.

Organic amines (e.g., 1-butylamine) are widely applied in polymer industries, rubber production, and manufacturing of dyes, pharmaceuticals and textiles.¹⁷ However, the 1-butylamine is toxic and easily absorbed through skin. Direct exposure of 1-butylamine vapor can lead to eye, skin and upper respiratory tract irritation.¹⁸ Therefore, it is necessary to properly detect and alert for those exposed to such gases. Currently, atmospheric 1-butylamine has been quantified by several advanced techniques, such as liquid chromatogram, fluorescence and other optical spectrum, which require expensive apparatuses and are not suitable for portable carrier.¹⁹ Resis-chemical sensors are proposed as a low-cost and high efficient tool to detect organic amines. Thus, the sensing materials become vital to the devices.

In this area, some metal oxide nanoparticles (V₂O₅, In₂O₃, ZnO and MoO₃) have been investigated for detection of organic amines.^20–24 For instance, the black rice pigment sensitized TiO₂ thin film can be used as sensing layer to detect biogenic amines at room temperature.²⁵ It is reported that ZnO nanoparticles films present a good sensing performance towards some volatile amines at room temperature, and the detection limit could reach to 162 ppm.²⁶ Cao et al. reported that the three dimensional In₂O₃ nanosheets with a high surface area composed of ultrathin nanosheets show excellent amine sensing performance at room temperature. The 3D structure takes advantage of avoiding aggregation and facilitating the diffusion of the target gas.²⁷ V₂O₅ nanowires, prepared by a sol–gel method, exhibit a very low detection limit of 30 ppb toward 1-butylamine.⁹ Shah et al. reported that V₂O₅/In₂O₃ core–shell nanorods, prepared by a combination of solid solution and hydrothermal methods, can be used for sensing n-propylamine with high selectivity and sensitivity (S = R_a/R_g, S = ∼14), but suffering from long response (48 s) and recovery time (121 s).²⁸ Furthermore, our study shows that silver vanadate nanoparticles can be used for sensing organic amines and exhibit moderate response with sensitivity (S = 4.5 for Ag₂V₄O₁₁) towards 100 ppm of 1-butylamine at 260 °C.^19,29 Despite of the efforts above, the responses of these sensing materials toward 1-butylamine are not satisfactory for practical use, and still requires further enhancement.

Herein, we report a simple and effective hydrothermal method for the synthesis of nanosheet self-assembled V₂O₅ microflowers with large surface areas for sensing organic amines. The composition and morphology of such structures will be characterized with several analytical techniques, such as transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and Brunauer–Emmett–Teller (BET) surface area. The gas sensing performance (e.g., sensitivity, selectivity, and stability) of the as-prepared V₂O₅ microparticles toward several VOCs (including 1-butylamine) will be measured and evaluated. Finally, the possible mechanisms for achieving high sensing performance of such nanostructures will be discussed.

Experimental

Chemicals

Vanadium pentoxide (V₂O₅, 99%), ethylene glycol (EG, 99.9%), dicarbonate (NaHCO₃, 99.9%), ethanol (C₂H₆O, 99.8%), acetone (C₃H₆O, 99%), pyridine (C₅H₅N, 99%), methanol (CH₄O, 99.9%), formaldehyde (HCHO, 37 wt% in H₂O), 1-butylamine (C₄H₁₁N, 99%), and 1-butanol (C₄H₁₀O, 99.8%) were all purchased from Sinopharm Chemical Reagent Co., Ltd. and used as received without further purification. All the chemicals were of analytical grade. Ultra-pure water was used throughout the experiments.

Synthesis of nanosheet-assembled V₂O₅ microflowers

In a typical synthesis procedure, V₂O₅ (0.14 g) and various amounts of NaHCO₃ (1.25 M) were added in 16 ml of EG with vigorous stirring at 60 °C for several hours until the color of the suspension changed from brownish yellow to light yellow. Then, 20 ml of such obtained suspension was added into a 50 ml Teflon container. After the container was sealed in an autoclave, it was heated at 260 °C for 24 hours. The pertinent variables effecting on the particle formation and growth were also investigated, such as the concentration of vanadium, the molar ratio of vanadium to sodium bicarbonate, and temperatures. After cooled down to room temperature naturally, the resulting black precipitate was collected by centrifugation, washed thoroughly with deionized water and ethanol several times and finally dried at 70 °C overnight before further use. Finally, the precipitate was sintered at 400 and 600 °C in the atmosphere of nitrogen and air for 2 hours, respectively.

Characterization

The phase composition and purity of the synthesized particles were examined using Phillips X'pert Multipurpose X-ray Diffraction System (MPD) equipped with graphite mono-chromatized Cu-Kα radiation (λ = 1.54 Å) in the 2θ range of 10–80°. The particle morphologies were observed on a JEOL 1400 microscope (TEM), operated at an accelerated voltage of 100 kV. HRTEM images were recorded on a JEM 2100F field emission transmission electron microscope with an accelerating voltage of 200 kV. SEM images were recorded with a JEM-7001F field emission scanning electron microscope. The Brunauer–Emmett–Teller (BET) surface area and pore size distribution of the products were obtained from nitrogen physisorption isotherms (adsorption–desorption branches) at 77 K on a Micromeritics ASAP 2020 instrument. TGA was employed to determine phase transformation from precursors to oxides, operated by a thermal analyzer (DSC404F3) under air and nitrogen flow with heating rate of 10 °C min⁻¹ in the range of 25–600 °C.

Gas sensor construction and measurements

The gas sensors were fabricated according to the procedures reported in our previous studies.^8,19 Specifically, the as-prepared V₂O₅ particles were first dispersed in ethanol to form slurry, which was then coated on a ceramic tube with previously printed Au electrodes and Pt conducting wires. The ceramic tube was subsequently dried at 100 °C for 2 h to evaporate the solvent. To be stable, all the materials were treated and aged at 300 °C in air until the resistance became stable (∼24 h). The sensing properties of the V₂O₅ particles were evaluated using a computer-controlled WS-30A gas-sensing measurement system (Weisheng Electronics Co., Ltd., Henan, China).

Before the measurement, a Ni–Cr resistor was inserted into the ceramic tube as a heater, which is used to control the operating temperature by adjusting the heating voltage (V_heating). A reference resistor was placed in series with the sensor to form a complete measurement circuit. The change of the voltage at the two end of the reference resistor (R_reference) can be monitored by the computer. The target gas (e.g., 1-butylamine) was injected into the testing chamber using a microsyringe. The response (S) of n-type semiconductors can be defined as the ratio of the resistances measured in air (R_a) and in the tested gas atmosphere (R_g): S = R_a/R_g. The output voltage was set at 5 V and the gas-sensing measurements were conducted at a relative humidity of 55–65%.

Results and discussion

Composition of the as-prepared precursors and final products

The phase composition of vanadium oxide precursors and V₂O₅ microflowers was characterized by XRD, as shown in Fig. 1. The figure depicts the XRD patterns of the precursors (Fig. 1a) and V₂O₅ microflowers (Fig. 1b). It can be seen that V₂O₃, labeled as diamond, is the domain compound in the precursors. Other peaks can be indexed as vanadyl ethylene glycolate (VEG) (2θ = 13.6°, labeled as circle), VOOH (2θ = 27.2°, labeled as squire) and some unknown species (2θ = 47.8° and 76.9°, labeled as triangle). Fig. 1b shows the typical XRD patterns of the pure V₂O₅, corresponding to the sintered products of the precursors. All of the diffraction peaks can be well indexed to the diffraction peaks of orthogonal phase of V₂O₅ (JCPDS 01-089-0612). Due to the V₂O₅ product sintered from the precursors, it can be deduced that the unknown species in the precursor are still vanadium species.

Fig. 1 XRD patterns of (a) precursors and (b) V₂O₅ microflowers.

To further confirm the composition of the precursor and investigate the transformation from precursor to V₂O₅ particles, FT-IR was used to characterize the precursor and the sintered product. Fig. 2a shows the FT-IR spectra of the precursor and V₂O₅ microflowers. In the curve of precursor, the peaks concentrated at ∼3444 and 1637 cm⁻¹ corresponds to the O–H stretching bond, which may generates from VOOH species. The small peaks located at ∼2973 and 2925 cm⁻¹ represent CH₂O– group, probably originated from VEG. The peak centered at ∼991 cm⁻¹ can be assigned to V=O stretching bond, while those centered at 638 and 617 cm⁻¹ are attributed to the V–O–V bending mode and coupled bending vibration of V–O. From the FT-IR spectra, it can be confirmed that the VEG and VOOH species exist in the precursor, in good agreement with the deduction from the XRD pattern. The black curves in Fig. 2a shows a typical FT-IR spectrum of V₂O₅, in which the V=O stretching bond appears at ∼1014 cm⁻¹. While the peak at 522 cm⁻¹ is assigned to the V–O–V stretching bond. The V–O stretching bond and the coupled vibration between V=O and V–O–V bonds can be found at 664 and 848 cm⁻¹, respectively.^8,30

Fig. 2 (a) FT-IR patterns of the precursor and V₂O₅ microflowers. (b) TGA (solid line) and DSC (dot line) curves of the precursor sintered in air. EDS patterns of (c) the precursor and (d) the as-prepared V₂O₅.

To identify the phase transformation of the precursor, TGA/DSC technique was also used to monitor the transformation process of the precursor in air, as shown in Fig. 2b. Considering the melting point of V₂O₅ (690 °C), the testing temperature ranges from room temperature to 600 °C. From the TGA curve (black solid line), there is a weight gain of ∼10% at ∼300 °C, corresponding to the transformation from low valence vanadium compounds to V₂O₅. Due to the complicated composition of the precursor, the weight gain does not exactly follow the theoretical value (∼21% from pure V₂O₃ to V₂O₅). The gain stops before ∼600 °C, which means the oxidation has completed. The DSC curve (blue dot line) indicates the oxidizing reaction is a exothermic reaction, while the reaction temperature can be determined as precise as 328.2 °C. Fig. 2c and d show the elemental EDS spectra of the precursor and sintered V₂O₅ products, respectively. The EDS spectrum of precursor clearly indicates the existence of carbon, due to the presence of VEG; while the peak corresponding to carbon (C) vanishes in the EDS spectrum of V₂O₅. The element of aluminium (Al) is generated from sample holders, while Au is attributed to Au spin-coating for SEM observation.

Morphology of the as-prepared precursors and the final products

The microstructure of the as-prepared nanoparticles was inspected by microscopic techniques. Fig. 3a displays the SEM image of the nanosheet self-assembled precursor, suggesting that the microflowers with diameter of ∼5 μm are composed of numerous nanosheets with thickness of ∼10 nm. Fig. 3b shows the TEM image of the sheet-like precursor, and the selected diffraction pattern of the red circle (Fig. 3b) is exhibited in Fig. 3c. The circular pattern clearly indicates that the precursor is polycrystalline, which may be attributed to the complicated composition of the precursor. The SEM image of the V₂O₅ microflowers sintered from the precursor is shown in Fig. 3d, indicating that the microstructure is retained but accompanied with slight shrink to ∼4 μm. The TEM image of the V₂O₅ nanosheets is shown in Fig. 3e. Fig. 3f shows the high resolution TEM (HRTEM) image of the selected area (red circle in Fig. 3e). It is clearly observed that the distinct lattice fringes with interplanar spacings of 0.57 and 0.43 nm are perpendicular to each other, corresponding to the (200) and (001) V₂O₅ crystal planes. The inset displays the SAED pattern, in which two typical orthorhombic crystalline planes are indexed as (010) and (200), and the corresponding zone axis is [010].³¹

Fig. 3 (a) SEM image of the nanosheet self-assembled flower-like precursor; (b) TEM image of the sheet-like precursor; (c) the selected area electron diffraction (SAED) pattern of the precursor at the red circular area in (b); (d) SEM image of the nanosheet self-assembled flower-like V₂O₅ particles; (e) TEM image of the sheet-like V₂O₅ particles; (f) HRTEM image of the V₂O₅ nanosheet at the red circular area in (e); the inset is the corresponding SAED pattern.

A few of pertinent parameters (including reaction temperatures, concentration of vanadium, reaction time, pH and molar ratio of vanadium to bicarbonate) have been optimized to investigate their effects on the morphologies. It is found that pH heavily affects the morphologies (Fig. 4), when other conditions are fixed at 0.75 mM (vanadium concentration), 260 °C, and 48 h. The effect of other parameters (e.g., concentration of vanadium, temperature, and reaction time) on the morphology can be found in Fig. S1–3.† Fig. 4 shows the effect of pH on the morphology of the precursor. The pH adjustment was achieved by HNO₃ and NaHCO₃. It can be observed that the sheet-like structure can form with wide range of pH, from 1 to 8. The self-assembled microflowers appear at pH 10. Due to the buffer properties of bicarbonate, the continuing addition of the amount of bicarbonate cannot increase the pH of the system. Therefore, the effect of the molar ratio of vanadium to bicarbonate needs to be investigated. The SEM images are shown in Fig. S4.† It can be found that the morphology of the precursors is not heavily changed at the molar ratio from 1 : 1 to 1 : 2 (Fig. S4a and b†). Continuously increasing the molar ratio to 1 : 3 and 1 : 4, the microflowers trend to aggregate with each other and form larger microparticles (Fig. S4c and d†). Meanwhile, we also tried other bases (e.g., NaOH) to adjust high pH (≥10). However, no self-assembled microflowers are observed.

Fig. 4 The SEM images of the precursors obtained with various pH (a–e) 1, 4, 6, 8, and 10; (f) XRD patterns of the corresponding precursors. Other parameters are fixed at 0.75 mM (concentration of vanadium), 260 °C, and 48 h.

The mechanism for the formation of the nanosheet-assembled microflowers can be attributed to so-called “reproduction mechanism”. In the initial process of growth, the sheet-like particles might form at the low molar ratio of vanadium to bicarbonate. With the ratio increase, the concentration of H⁺ released from bicarbonate will ascent, leading to the incomplete nanosheets under the reducing conditions. The incomplete nanosheets agglomerate to form a new growth point. As a result, these new points further grow to form random nanosheet self-assembled microflowers.³² This mechanism can be confirmed by the aggregation of microflowers at high molar ratio of vanadium to bicarbonate.

Gas sensing performance

The gas sensing performance of the nanosheet-assembled V₂O₅ microflowers was investigated via measuring the selectivity to organic ammines, methanol, formaldehyde, acetone, 1-butanol, isopropanol, ethanol, and pyridine. In comparison, the V₂O₅ nanosheets and commercial V₂O₅ particles (Fig. S5†) were also used as sensing materials. As an n-type semiconductor, V₂O₅ is expected to adsorb O and OH groups which trap electrons from the conduction band of the V₂O₅ crystals and increase the resistance. The adsorbed oxygen and lattice oxygen of V₂O₅ can react with gas molecules which are chemical adsorbed at the active sites on the surface of V₂O₅ materials. In this process, electrons will transfer to the surface of V₂O₅ to lower the number of trapped electrons, which induces a decrease in the resistance.^8,33

Surface area is one key factor affecting the gas sensing performance.³⁴ The surface areas of the as-prepared precursors and the corresponding V₂O₅ microstructure were measured by the BET method via N₂ sorption isotherms. The isotherms of the sheet-like precursor and the corresponding V₂O₅ nanosheets are displayed in Fig. 5a and b. The BET surface area of the V₂O₅ nanosheets is ∼56.4 m² g⁻¹, much higher than that of the precursor (24.6 m² g⁻¹). While the isotherms of the nanosheet-assembled precursors and the corresponding V₂O₅ microflowers are shown in Fig. 6c and d. It can be seen that sintering can significantly increase the surface area of the particles (29.9 to 61.5 m² g⁻¹). It is also noted that the nanosheet-assembled microflowers show higher surface area than that of nanosheets. This may be the reason that the V₂O₅ microflower show higher sensing performance than that of V₂O₅ nanosheets. The microflowers may provide more sites for the adsorption of O₂ molecules, which play an important role in the sensing mechanism. The pore sizes distributions (Fig. 5c and d) derived from both adsorption and desorption branches of the isotherms using the BJH method indicate the difference in porosity for the two materials.

Fig. 5 N₂ sorption isotherms of (a) the sheet-like precursor, (b) the sheet-like V₂O₅ particles, (c) the flower-like precursor, and (d) the flower-like V₂O₅ particles. The insets in each figure are pore size distribution.

Fig. 6 (a) Response to 100 ppm 1-butylamine of the sensors based on the flower-like V₂O₅ particles, the sheet-like V₂O₅ particles, and commercial V₂O₅ particles at various working temperatures. (b) Sensor isothermal response of the flower-like, sheet-like and commercial V₂O₅ particles to various concentration of 1-butylamine at the optimum working temperature of 300 °C. (c) Response of the three V₂O₅ particles to different concentration of 1-butylamine testing at the optimum temperature. (d) Relative selectivity of the sensors based on the three V₂O₅ particles to carious gases at the optimum working temperature, while the gas concentration is fixed at 100 ppm.

The diffusion rates of gas molecules are dependent on temperatures, thus the working temperature significantly may affect both the response and the sensitivity. Fig. 6a displays the response of the sensors made from the nanosheet-assembled V₂O₅ microflowers, the V₂O₅ nanosheets and the commercial V₂O₅ particles toward 100 ppm of 1-butylamine with increasing working temperature from 240–340 °C. All V₂O₅ microflower-based sensors exhibit an optimum working temperature of 300 °C with S = 3.3 toward 100 ppm of 1-butylamine. In comparison, other two V₂O₅ based sensors (the V₂O₅ nanosheets and the commercial V₂O₅ particles) show the same optimum working temperatures, but the responses are lower than that of V₂O₅ microflowers.

The dynamic response–recovery transients of the V₂O₅-based sensors are shown in Fig. 6b. It is obvious that the output voltages of the sensors increase when the 1-butylamine is input, and decrease with the removal of the gas. Fig. 6c depicts the responses of three V₂O₅ based sensors toward different concentrations of 1-butylamine at 300 °C. It can be seen that the flower-like V₂O₅ particles show much higher response (∼3 times) towards 1-butylamine than the commercial V₂O₅ particles. The response of the sheet-like V₂O₅ particles is between those of the microflowers and the commercial V₂O₅ particles, however, the former exhibit ∼2 times higher response than the latter.

Selectivity is another critical characteristic in gas sensing, which needs to be evaluated to ensure an accurate detection of a single gas in a mixer with a higher response. The response of the sensors based on the flower-like, the sheet-like and the commercial V₂O₅ particles towards 100 ppm of different gases (e.g., 1-butylamine, acetone, pyridine, formaldehyde and alcohols) were measured at the working temperature of 300 °C, as displayed in Fig. 6d. It is noted that the flower-like V₂O₅ exhibit excellent selectivity to 1-butylamine, with much higher response than that of other gases. In comparison, the high selectivity towards 1-butylamine is observed on the sensors based on the sheet-like V₂O₅ particles, while the selectivity of the commercial V₂O₅ particles is not ideal. This indicates that the morphology of the V₂O₅ particles could highly affect the selectivity.

Recover time is also a key criterion to judge gas sensing performance of nanoparticles. Fig. 7a and b show the response/recovery time of the flower-like and sheet-like V₂O₅ particles, respectively. The response time is defined as the time required to 90% of the final equilibrium value, following exposure to 1-butylamine vapor. Inversely, the recovery time is defined as the time needed by the sensor to return to 10% of the original conductance in air, with removal of 1-butylamine vapor. It can be observed that the response time of the flower-like V₂O₅ particles (Fig. 7a) upon exposure to 5–100 ppm of 1-butylamine differs from 10–25 s, whereas the recovery time is 3–14 s. For the sheet-like V₂O₅ particles (Fig. 7b), the response time and recovery time vary from 10–17 s and 5–14 s, respectively. Furthermore, we also observed that the response/recovery time increases with gas concentration, probably due to the low vapor and diffusion rate of 1-butylamine in the gas chamber at high concentration. The above results show that the response speed of the flower-like V₂O₅ particles is slower than that of the sheet-like V₂O₅ particles. It can be concluded that the nanosheet assembly can enhance the sensitivity and selectivity to 1-butylamine of V₂O₅ particles, however, the response speed slightly decreased.

Fig. 7 Response and recovery time values of the flower-like V₂O₅ particles (a) and the sheet-like V₂O₅ particles (b) for various concentration of 1-butylamine at 300 °C. (c) Repeatability tests of the three samples towards 100 ppm of 1-butylamine for repeating 10 times at 300 °C.

Repeatability is one of important parameters of a sensing material, related to long-term reliability of the sensor. As shown in Fig. 7c, the repeatabilities of the samples (the flower-like, the sheet-like, and commercial V₂O₅ particles) were evaluated by testing the response towards 100 ppm of 1-butylamine at 300 °C by repeating 10 times. It can be found that the response of the sensing materials remains constant during 10 tests, suggesting the excellent potential of the flower-like V₂O₅ as a sensitive, selective and stable sensing material for detection of 1-butylamine. In the case of sensing 1-butylamine, the nanosheet-assembled V₂O₅ microflowers show superior sensitivity and selectivity to Ag₂V₄O₁₁ (ref. 29), Ag_0.35V₂O₅ (ref. 19), VO_x/Ag nanobelts,²⁹ and V₂O₅ nanofibers.⁹

Regarding the gas sensing mechanisms of V₂O₅ nanoparticles, the general surface absorbed redox mechanism is usually used for fundamental understanding. However, the sensing mechanism toward amines is slightly different. When exposed to amines, the sensing material (V₂O₅) interacts with the chemisorbed oxygen species on the surface to form CO₂, H₂O, and NO_x, and then release the trapped electrons to the oxide particles.¹⁹ The superior sensing response of the flower-like V₂O₅ particles to the nanosheets can be attributed to the higher surface area and the unique hierarchical structure. For the microflower structures, the radial and non-aggregated sheets may allow more gas molecules to contact. In contrast, the nanosheets may pack more densely than the microflowers. Hence the contact area with gases would be smaller than the microflowers. On the other hand, the highly selectivity of the V₂O₅ microflower sensor toward 1-butylamine is attributed to the selective oxidation of 1-butylamine. It is accepted that the selective oxidation of primary amines is favorable in the presence of vanadic acids.³⁵ The V₂O₅ particles may exhibit vanadic acids-like behavior under sensing condition because of reaction between the surface defect and water.^36,37 However, we need to point that the as-prepared V₂O₅ microflowers still show relatively low sensitivity compared with the Au doped ZnO nanoflakes.³⁸ The reason may be due to the difference of work functions between metal oxides (4.65 eV for ZnO,³⁸ 4.5 eV for SnO₂ (ref. 2) and 7 eV for V₂O₅ (ref. 28)). It is well-accepted that modifying by noble metal can improve the sensing performance, especially for the improvement of sensing performance by decoration of Au nanoparticles on the metal oxides (∼5.1 eV for Au).^2,38,39 Therefore, the future work will focus on the modification of Au nanoparticles on the surface of V₂O₅ microflowers for achieving high sensing performance.

Conclusions

We have demonstrated a facile synthesis strategy by combination of hydrothermal method and sintering treatment to prepare hierarchical nanosheet self-assembled V₂O₅ microflowers. A few unique findings can be summarized below:

(i) Unique microflower structures. The sheet-like V₂O₅ particles have been obtained under the reported conditions, especially for the self-assembled microflower structures with diameter of ∼5 μm, composed of numerous polycrystalline nanosheets, leading to a high surface area up to 61.5 m² g⁻¹. The formation and growth was dependent on high pH at high molar ratio of vanadium to bicarbonate.

(ii) Excellent gas sensing performance. The sheet-like and flower-like V₂O₅ particles have been used for sensing detection of ethanol, 1-butylamine, acetone, pyridine, methanol, isopropanol, formaldehyde and 1-butanol. The flower-like V₂O₅ particles show a low detection limit to 5 ppm and a significant high selectivity toward 1-butylamine among the tested particles, at the optimum working temperature of 300 °C. Furthermore, the flower-like and sheet-like particles show short response (10–25 s for microflowers, 10–17 s for nanosheets) and recovery time (3–14 s for microflowers, 5–14 s for nanosheets) at the concentration range from 5–100 ppm under the reported conditions.

The results in this work ascertain the potential use of vanadium oxide nanoparticles for industrial and environmental applications in the future.

Acknowledgements

We gratefully acknowledge the financial support of the National Natural Science Foundation of China (No. 51404066), the National Basic Research Program of China (N130102001, N150204011), as well as Australia Research Council (ARC) projects (DP160104456) and others.

Notes and references

1. A. Q. Pan, H. B. Wu, L. Zhang and X. W. Lou, Energy Environ. Sci., 2013, 6, 1476–1479.
2. Y. V. Kaneti, J. Yue, J. Moriceau, C. Chen, M. Liu, Y. Yuan, X. Jiang and A. Yu, Sens. Actuators, B, 2015, 219, 83–93.
3. A. Pan, H. B. Wu, L. Yu, T. Zhu and X. W. Lou, ACS Appl. Mater. Interfaces, 2012, 4, 3874–3879.
4. T. A. Arun, D. K. Chacko, A. A. Madhavan, T. G. Deepak, G. S. Anjusree, T. Sara, S. Ramakrishna, S. V. Nair and A. S. Nair, RSC Adv., 2014, 4, 1421–1424.
5. S. Lin, G. Su, M. Zheng, D. Ji, M. Jia and Y. Liu, Appl. Catal., B, 2012, 123–124, 440–447.
6. C. Yuan, L. Hou, L. Yang, D. Li, L. Shen, F. Zhang and X. Zhang, J. Mater. Chem., 2011, 21, 16035–16041.
7. H. Fei, Z. Shen, J. Wang, H. Zhou, D. Ding and T. Chen, J. Power Sources, 2009, 189, 1164–1166.
8. H. Fu, X. Jiang, X. Yang, A. Yu, D. Su and G. Wang, J. Nanopart. Res., 2012, 14, 1–14.
9. I. Raible, M. Burghard, U. Schlecht, A. Yasuda and T. Vossmeyer, Sens. Actuators, B, 2005, 106, 730–735.
10. J. F. Liu, X. Wang, Q. Peng and Y. D. Li, Adv. Mater., 2005, 17, 764–767.
11. J. Yue, X. Jiang and A. Yu, J. Phys. Chem. C, 2012, 116, 8145–8153.
12. C. L. Niu, J. B. Li, H. B. Jin, H. L. Shi, Y. Q. Zhu, W. Z. Wang and M. S. Cao, Electrochim. Acta, 2015, 182, 621–628.
13. J. Chu, Z. Z. Kong, D. Y. Lu, W. L. Zhang, X. S. Wang, Y. F. Yu, S. Li, X. Q. Wang, S. X. Xiong and J. Ma, Mater. Lett., 2016, 166, 179–182.
14. H. Q. Song, C. P. Zhang, Y. G. Liu, C. F. Liu, X. H. Nan and G. Z. Cao, J. Power Sources, 2015, 294, 1–7.
15. Y. Qin, G. Fan, K. Liu and M. Hu, Sens. Actuators, B, 2014, 190, 141–148.
16. J. Pan, L. Zhong, M. Li, Y. Y. Luo and G. H. Li, Chem.–Eur. J., 2016, 22, 1461–1466.
17. D. F. Paraguay, M. Miki-Yoshida, J. Morales, J. Solis and L. W. Estrada, Thin Solid Films, 2000, 373, 137–140.
18. K. Kim, J. W. Lee, D. H. Shin, J. Y. Choi and K. S. Shin, Analyst, 2012, 137, 1930–1936.
19. H. Fu, H. Xie, X. Yang, X. An, X. Jiang and A. Yu, Nanoscale Res. Lett., 2015, 10, 411.
20. V. Modafferi, G. Panzera, A. Donato, P. L. Antonucci, C. Cannilla, N. Donato, D. Spadaro and G. Neri, Sens. Actuators, B, 2012, 163, 61–68.
21. S.-J. Kim, I.-S. Hwang, J.-K. Choi, Y. C. Kang and J.-H. Lee, Sens. Actuators, B, 2011, 155, 512–518.
22. X. Wang, W. Liu, J. Liu, F. Wang, J. Kong, S. Qiu, C. He and L. Luan, ACS Appl. Mater. Interfaces, 2012, 4, 817–825.
23. M. B. Rahmani, S. H. Keshmiri, J. Yu, A. Z. Sadek, L. Al-Mashat, A. Moafi, K. Latham, Y. X. Li, W. Wlodarski and K. Kalantar-zadeh, Sens. Actuators, B, 2010, 145, 13–19.
24. P. Sundara Venkatesh, P. Dharmaraj, V. Purushothaman, V. Ramakrishnan and K. Jeganathan, Sens. Actuators, B, 2015, 212, 10–17.
25. L. Yanxiao, Z. Xiao-bo, H. Xiao-wei, S. Ji-yong, Z. Jie-wen, M. Holmes and L. Hao, Biosens. Bioelectron., 2015, 67, 35–41.
26. H. Xia, T. Liu, L. Gao, L. Yan and J. Wu, Appl. Surf. Sci., 2011, 258, 254–259.
27. Y. Cao, X. Huang, Y. Wu, Y.-C. Zou, J. Zhao, G.-D. Li and X. Zou, RSC Adv., 2015, 5, 60541–60548.
28. A. H. Shah, Y. L. Liu, V. T. Nguyen, G. S. Zakharova, I. Mehmood and W. Chen, RSC Adv., 2015, 5, 54412–54419.
29. H. Fu, X. Yang, X. Jiang and A. Yu, Sens. Actuators, B, 2014, 203, 705–711.
30. E. M. Guerra, G. R. Silva and M. Mulato, Solid State Sci., 2009, 11, 456–460.
31. H. Yu, X. Rui, H. Tan, J. Chen, X. Huang, C. Xu, W. Liu, D. Y. W. Yu, H. H. Hng, H. E. Hoster and Q. Yan, Nanoscale, 2013, 5, 4937–4943.
32. P. Kumar and L.-H. Hu, J. Alloys Compd., 2016, 655, 79–85.
33. B. Sun, J. Horvat, H. S. Kim, W.-S. Kim, J. Ahn and G. Wang, J. Phys. Chem. C, 2010, 114, 18753–18761.
34. X. Li, B. Wang, X. Wang, X. Zhou, Z. Chen, C. He, Z. Yu and Y. Wu, Nanoscale Res. Lett., 2015, 10, 373.
35. J. S. Reddy and A. Sayari, Appl. Catal., A, 1995, 128, 231–242.
36. M. Witko, K. Hermann and R. Tokarz, Catal. Today, 1999, 50, 553–565.
37. X. Yin, A. Fahmi, H. Han, A. Endou, S. S. C. Ammal, M. Kubo, K. Teraishi and A. Miyamoto, J. Phys. Chem. B, 1999, 103, 3218–3224.
38. Y. V. Kaneti, X. Zhang, M. Liu, D. Yu, Y. Yuan, L. Aldous and X. Jiang, Sens. Actuators, B, 2016, 230, 581–591.
39. J. Yue, X. Jiang and A. Yu, J. Phys. Chem. B, 2011, 115, 11693–11699.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra18848f

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