Synthesis and Triethylamine Sensing Performance of Nanowires Assembled Leaf-like MoO3Nanostructure

TIAN Wen-DiLI Xiao-ZeCAO Jian-LiangWANG Yan

(1College of Chemistry and Chemical Engineering,Henan Polytechnic University,Jiaozuo,Henan 454000,China)

(2College of Safety Science and Engineering,State Collaborative Innovation Center of Coal Work Safety and Clean-Efficiency Utilization,Henan Polytechnic University,Jiaozuo,Henan 454000,China)

Abstract:A multichip leaf-like MoO3nanostructure assembled from one-dimensional nanowires has been synthesized by a simple solvothermal method using bis(acetylacetonato)dioxomolybdenum（Ⅵ）in a component solvent of acetic acid and deionized water at relatively lower temperature of 150℃.The field emission scanning electron microscopic(FESEM)and transmission electron microscopy(TEM)observations indicate that the multichip MoO3 nanostructures are composed of a plurality of fine nanowires with rough surface.The X-ray powder diffraction(XRD),high-resolution transmission electron microscope(HRTEM)results signified that the MoO3nanoleaves with the orthorhombic system,a distorted octahedral structure.And,the dominant exposed crystal plane of the leaf-like MoO3was(021).In addition,the leaf-like MoO3possessed distinctive selectivity,ultrahigh sensitivity and response time(～5 s)towards 2.25 g·m-3triethylamine(TEA)at the operating temperature of 300 ℃,even attained 12.4 at the low detection level(4.5 mg·m-3).Based on the analysis results,the growth and formation of crystalline phase was deemed to follow the oriented attachment mechanism.Accordingly,the enhanced sensing ability could be mainly ascribed to its unique nanostructure,linear combination of large density,and high specific surface area,which promote the substantial free diffusion of TEA gases into the blade hole,showing excellent response recovery.

Keywords:leaf-like MoO3;nanostructure;gas sensing material;triethylamine;fast detection

With the improvement of the safety awareness of people in recent years,more and more attention has been paid to the devices in the gas sensing field for detecting volatile organic compound(VOCs)vapors.Therefore,a large amount of functional nanomaterials are widely used in this field because of their transferability,easy manufacture,low cost,miniaturization,and so forth.As a layered n-type metal oxide semiconductor(MOS),nanometric molybdenum oxide(MoO3)possesses an indirect band gap of approximately 2.39～2.9 eV[1].It demonstrates wide promising application in fields including gas sensors[2],photocatalysis[3],gaschromic[4],energy storage[5],and photochromic devices[6]due to its high reactivity characteristics,surface effect and intractable quantum confinement effect.Well-known,there are three different MoO3crystal forms[7]:the thermodynamically stable α-MoO3,metastable hexagonal h-MoO3,and monoclinic β-MoO3.Thereinto,orthorhombic MoO3(α-MoO3)hasgrabbed more attention because of its unique layered crystal structure that facilitates the diffusion of gas molecules.For example,Mohamed et al.[8]reported the framework of 1% GO.MoO3-xirregular circular shape via an environmentally solvothermal and next annealing route,which exhibited efficient visible light photocatalysis and gas sensing applications.The gas sensor based on 3D α-MoO3hierarchical nanoflowers prepared by Sui et al.[9]showed more splendidly sensitive and selective towards 45 mg·m-3triethylamine(TEA)detection at the operating temperature of 170 ℃.Li et al.[10]also used novel 1D α-MoO3/ZnO compounds to greatly enhance the sensing properties from 6.5 to 19 for 450 mg·m-3ethanol.These studies all show that α-MoO3is an excellent sensing material.Nevertheless,the advance in highperformance gas detection technology remains a great challenge in the morphology design of α-MoO3nanomaterials.

TEA,as a representative VOC,is a pungent organic amine released from corrupted fish and shells,which has been mainly used in organic solvents and efficient catalysts in industrial production at present.But it′s prone to generate risks to human beings in virtue of its inflammable,toxic and explosive nature,and causes a significant damage to the respiratory system,resulting in pulmonary edema and even death[11-12].In line with Occupational Safety and Health Authority(OSHA)prescribed levels of permissible exposure,the actual threshold limit value(TLV)of TEA concentration in the air is 45 mg·m-3,and the TLV is 4.5 mg·m-3recognized by American Conference of Governmental Industrial Hygienists(ACGIH)[13].Hence,setting strict criterion to quickly detect and real-time monitoring mg·m-3(or even lower)levels of TEA is considerably significant for assessing fish freshness and identifying deleterious environments.

At present,nanosized MoO3with different morphologies and structures have been prepared to qualitatively detect various VOCs,such as nanoparticles[14],nanoplates[15],nanowires[16],nanobelts[17],nanorods[18],nanofibers[19],nanotubes[20],films[21],hollow spheres[22],and other hierarchical structures[23].Some studies also have proven that the gas sensitivity of nanomaterials can be dramatically affected by structure characteristics.Li et al.[24]obtained different kinds of SnO2nanoflowers and proved that the surface area with sufficient gas diffusion is beneficial to enhance the sensing performance of SnO2nanoflower sensor.Zeng et al.[25]constructed hierarchical WO3flower-like architectures and measured the thickness as well as pore size of thin petal structures,expounding the mechanism of excellent ethanol gas sensing properties.Li reported the morphology controllable synthesis of MoO3lamellar flowers and preparation of reticulated porous microspheres,and reflected the ideal ethanol gas response[26].Nevertheless,most of the above measures are usually complicated or doped surfactants or need templates,and the characteristics of these MoO3gas sensing technology with leaf-like nanostructures have not been announced.

Herein,we reported a facile strategy to design and fabricate multichip MoO3leaf-like architecture by the oxidization conversion of synthesized low-crystallization MoO3precursors by solvothermal method.In actual testing,it manifests preeminent TEA gas response and topgallant recovery performance at lower(mg·m-3)fast detection level due to its large specific surface area and porous texture.The mechanism is discussed in detail in this article,therebyprovidingpotential research orientation and technical prospect for the preparation of higher performance sensors on single semiconductor metal oxide.

1 Experimental

1.1 Materials

All organic chemical reagents used for the synthesis of MoO3nanoleaves were in the analytic grade purity and applied to starting materials without any deeper purification process.Bis(acetylacetonato)dioxomolybdenum（Ⅵ）was purchased from Macklin Biochemical Co.,Ltd(Shanghai,China).Acetic acid was purchased from Luoyang Shangyanggong Chemical Reagent Technologies Co.,Ltd(Luoyang,China).

1.2 Synthesis of MoO3nanoleaves

Originally,0.35 mmol bis(acetylacetonato)dioxomolybdenum（Ⅵ）(C10H14MoO6)was dissolved in acetic acid(33 mL)and deionized water(2 mL)mixed solution under the magnet stirring to form a light blue precursor emulsion.After agitating for 30 min,the obtained mixture solution was poured into a 50 mL Teflon-lined stainless steel autoclave and kept for 8 h at 150℃.Naturally cooled,the light blue precipitate was collected by centrifugation at 8 000 r·min-1and washed with deionized water plus ethanol for three times,and then dried overnight in air at 60℃.Finally,the precursor was annealed at 350℃for 2 h in a muffle furnace with a heating rate of 2℃·min-1,the dark blue powders were received after cooling to room temperature.

1.3 Characterization

The individual crystal structure and purity of asfabricated pure MoO3specimens were characterized by X-ray powder diffraction(XRD,Bruker AXS D8,40 kV,150 mA)with Cu Kα radiation(λ=0.154 1 nm)in a recorded range of 10°～70°(2θ)at a rate of 10(°)·min-1.The microstructure was identified by field emission scanning electron microscopy(FESEM,FEI Quanta 250 FEG)equipped with an energy dispersive X-ray spectroscopic(EDS).High-resolution transmission electron microscopy(HRTEM,JEOL JEM-2100,200 kV)was used to investigate its lattice conformation and surface structure.Simultaneously,the ultraviolet-visible spectroscopy(UV-Vis)was judged by a UV spectrophotometer(Cary5000,Agilent).Thermogravimetry differential scanning calorimetry(TG-DSC)analysis was completed on a Simultaneous Thermal Analyzer(STD Q600)in an air flow of 20 mL·min-1at a heating rate of 10 ℃ ·min-1.In this process the starting temperature was room temperature and the terminating temperature was 1 000℃.The N2nitrogen adsorption-desorption isotherms were researched at liquid nitrogen temperature using a Quantachrome Autosorb-iQ2 absorption analyzer,and the specimen was degassed at 180℃for more than 8 h.

1.4 Gas sensor fabrication and sensing performance measurement

The process of fabricating the sensor substrates was the same as our previous published articles[27-29].Electrical properties of the devices were real-time detected by Beijing Elite Tech.CGS-4 intelligent test meter.The response of the sensor(Ra/Rg)was calculated as the ratio of the sensor resistance balance in air(Ra)to that in test gas(Rg).What′s more,the response and recovery time(Tresand Trec)is defined by the time for 90% of the initial equilibrium resistance change during gas injection and release,successively.During this test,the relative humidity(RH)was kept at 35% and floats 2%.

2 Results and discussion

2.1 Morphology and structure characterization

The XRD patterns of multichip leaf-like MoO3nanopore structures were characterized and the results are shown in Fig.1.Thereinto,curve(a)has many weak impurity peaks,which indicates the poor crystallinity of the unannealed MoO3precursors,and curve(b)is the phase after annealed at 350℃in air for 2 h.Apparently,all intensity diffraction peaks shown in the patterns can be assigned to the pristine orthorhombic α-MoO3phase information and the space group of Pbnm(62)according to PDF No.35-0609 with parameters:a=0.396 3 nm,b=1.385 6 nm,c=0.369 7 nm.Furthermore,based on the Bragg position at the bottom,it shows no any detectable impurities,indicating the com-plete crystallization transformation of MoO3.The intensity of diffraction peaks of(110),(040)and(021)are extremely stronger than other peaks,revealing a highly anisotropic growth of the leaf-like MoO3pore structures.Similarly,the two layers twist the connected[MoO6]6-octahedron to form a layered crystal structure[30],thereby exhibiting a texture effect corresponding to the orientation of its shape.According to Debye-Scherrer principle[31]as shown in equation:Dsize=kλ/(βcos θ),where k and λ represent Scherrer constant and the wavelength of the X-ray source,taking fixed values 0.89 for k and 0.154 nm for λ;β is the estimated full width at half maximum(FWHM);θ is the actual Bragg angle.Thus,the theoretical calculated average particle size of as-prepared MoO3products that correspond to the(021)plane diffraction peak is about 26.697 nm,showing a normal grain thickness of this crystal plane.

Fig.1 XRD patterns of the precursor(a)and the product calcined at 350℃in air for 2 h(b)

Fig.2 SEM images(a,b)and typical EDS spectra(c)of MoO3nanoleaves

The morphology and structure of the obtained dark blue MoO3powders were analyzed through SEM,EDS and TEM.From the perspective of SEM panorama(Fig.2a and 2b),one can see that the typical MoO3sample is leaf-like and consists of massive stacked nanowires.According to Fig.2c,the EDS spectra measurement of as-prepared material showed that the mass fraction of element Mo and O reached 90%,which is indicative of pristine MoO3by successfully constructed.For the purpose of further surveying the single microstructure of the nanoleaves,TEM was employed to achieve a higher magnification image.Visually,Fig.3a and 3c show that the multichip leaf-like individual MoO3nano-structure were assembled from the partial overlaying of abundant irregular 1D nanowires with rough surfaces and an average width of about 50 nm.Fig.3b can clearly show the single leaf size,and its composition diameter is about 457 nm.In addition,high quality and clear inter plane lattice fringes of individual MoO3nanoleaves are accurately observed in designated A,B region.However,it is clear that the lattice spacing at two different adjacent lattice locations of the MoO3nanoleaves is not evenly distributed in the area A.Clear lattice spacing(DLS)was determined to be 0.374 2 and 0.348 1 nm,respectively,corresponding to the(110)and(040)crystal planes of MoO3.Additionally,most of the other MoO3nanoleaves exhibit the narrower DLSas shown in area B.The DLSis approximately 0.329 6 nm,which can be indexed to the(021)lattice plane(marked blue).These findings prove that the preferential growth lattice plane is(021),which is also consistent well with above-mentioned XRD data that the intensity of the(021)shrill peak is stronger than other peaks.

Fig.3 Low(a)and high(b)magnification TEM image of MoO3nanoleaves;Corresponding HRTEM image and DLSof labeled areas A,B(c)

Fig.4 (a)UV-Vis diffuse reflectance spectrum of MoO3nanoleaves;(b)(αhν)1/2vs hν(photon energy)curve

For another important representation,the UV-Vis diffuse reflectance absorption of MoO3nanoleaves in a wavelength range of 300～800 nm is shown in Fig.4a.It indicates a strong absorption edge has taken place in the visible light region(380～780 nm).Moreover,a notable absorption peak around 425 nm could be seen in the spectrum,which was also measured by Alsaif in reported literature[32].As exhibited in Fig.4b,the optical properties indirect band gap(Eg)of MoO3nanoleaves was calculated to be 2.85 eV via the extrapolating equation:(αhν)1/2=hν-Eg.Obviously,we found it was indeed marginally lower than the theoretical value of distorted octahedral orthorhombic MoO3(2.9 eV).Through the common sense of photocatalysis,the larger bandgap energy of semiconductor materials is quite difficult to generate electron holes.Hence,this leaf-like structure of α-MoO3is expected to impart ranking gas sensitive properties.

To demonstrate the formation principle of specimens,the conversion of precursors during annealing treatment was also studied by TG and DSC at a programmable controlled temperature elevation rate of 10℃·min-1in air.As shown in Fig.5,the MoO3precursor underwent an obvious mass loss below 200℃,which is probably owing to the loss of deliquescent moisture and the decomposition of residual organic reagent on the surface of the leaf-like materials.In a later stage(350～760℃),a dramatic changes in energy and slight changes in weight,as well as two sharp exothermic peaks at 767 and 812℃were observed,indicating that a stable α-MoO3phase gradually formed due to the highly crystalline MoO3precursor in the air,which can be approved by the XRD and EDS results.Simultaneously,there were no detectable changes observed in the TG curve,indicating formation of thermally stable MoO3phase.So,when these samples were calcined at 350 ℃ in this experiment,it′s structure and chemical activity retained steadily,which is beneficial to the gas transfer and ultimately facilitate gas sensing properties.There was further abrupt weight loss above 800℃,corresponding to the melting and the sublimation behavior of MoO3.

Fig.5 TG and DSC curves of the MoO3precursor before sintering

Fig.6 (a)N2adsorption-desorption isotherm of MoO3nanoleaves;(b)Corresponding BJH pore size distribution curve

Specifically,the specific surface area and porosity of the materials are necessary for their gas sensing properties.In order to survey the surface adsorption properties,the N2adsorption isotherms and corresponding Barret-Joyner-Halenda(BJH)pore-size distribution measurements[33]are shown in Fig.6.According to the IUPAC classification criteria,the leaf-like MoO3showed a typeⅣisotherm with a slightly skewed H3 hysteresis loop observed at a p/p0range of 0.4～1(Fig.6a).The molecules adsorbed under a certain pressure couldn′t be desorbed due to the presence of surface tension,resulting in a non-overlapping of the curves.Fig.6b shows the corresponding pore size distri-bution,and the diameter of the hole was mostly 2.45 nm(mesoporous range).Correspondingly,the Brunauer-Emmett-Teller(BET)specific surface area was calculated to be approximately 110 m2·g-1.These results coincide with the transmission image shown in Fig.3,which indicates the potential application of α-MoO3multichip leaf-like nanopore structures as gas sensors.

2.2 Gas sensing properties and mechanism

It is proverbial that actual operating temperature play a vital role in MOS gas sensors on account of their heavily influences on microscopic state of sensing materials,along with the physicochemical adsorption reaction that occurs during gas testing.Consequently,temperature dependent experiment of the sensors to 2.25 g·m-3triethylamine vapors was measured under varying temperatures in a range of 240～340℃ and the results are shown in Fig.7.From this dotted line chart,it could be clearly understood that the response value of the MoO3sensors gradually varied along with the increased temperature and reached the maximum value of 4 460 at 300℃,which demonstrates that the manufactured sensor possesses ultra-sensitivity to TEA.The temperature dependence phenomenon can be interpreted in the chemical redox interaction between the nanodevice and target test gas.At a low temperature(＜300 ℃ ),as the temperature increased,more and more TEA molecules were adsorbed on the surface of α-MoO3.But it still has no enough thermal energy to break the shackles and combine with the adsorbed oxygen species(Oδ+),which causes the relatively high trend response from 240 to 300℃.With the increase again of the operating temperature(300～340 ℃),the desorption rate of triethylamine molecules would be greater than adsorption rate,revealing a fluctuation in the reduced response.Therefore,the thermal energy which TEA molecules provide by absorbing from the surrounding environment is large enough to surmount the activation energy barrier of the α-MoO3surface reaction at 300℃.Simultaneously,the conversion of adsorbed oxygen can also attract more electrons from the conduction band of sensing materials,promoting the change in the carrier concentration,resulting in a sharp drop in resistance.According to the definition of sensitivity,the response value to TEA maximum increases at this time[34-35].

Fig.7 MoO3sensor response to TEA at different operating temperatures

On the other hand,Fig.8a depicts the correlation between TEA concentration and response value of the MoO3sensors operated at 300℃.Apparently,the leaflike MoO3could attain 12.4 at the lower detection level(4.5 mg·m-3).The resistance of the sensors declined sharply as soon as they were placed in the TEA reducing vapors,while exposed to air,the resistance value could be easily restored to its initial state.As the concentration increased, this dynamic reversibility endowed a higher sensitivity to the sensor.Furthermore,the responses of TEA sensors were an increase of greater magnitude with the gas concentration increasing in the entire concentration range of 4.5～2 250 mg·m-3,which fitted next equation of α-MoO3response(y)and TEA concentration(x)was y=2.848 01x-13.936 75,and the correlative coefficient R2was 0.995 95 at the low concentration (4.5～225 mg·m-3),as shown in Fig.8b.Thereout,on the basis of above analysis and the formula of theoretical detection limit:DL=3(Nrms/S),where Nrmsis root mean square of noise and S is slope,we calculated the theoretical TEA minimum detection concentration of α-MoO3sensor to be 0.009 mg·m-3.Accordingly,the time of the response and recovery(Tres/Trec)are also key parameters for evaluating the performance of the gas sensor in application.Fig.8c shows the typical response and recovery transients of α-MoO3sensor toward 2.25 mg·m-3triethylamine gas under their optimum working temperature.The response time was 5 s and the recovery time was 62 s,which demonstrates the performance of fast detection.Fig.8d illustrates four dynamic cycle-stability curves of the α-MoO3sensor to 2.25 mg·m-3TEA at 300 ℃,indicating that the amplitude and optimal response are similar to maintain about 4 460.These results conclude that the sensorhasanexcellentreproducibilityandrepeatability.

Fig.8 Dynamic response recovery curve of concentration(a),corresponding linear relationship between response and concentration(b),Tresand Trec(c),and repeatability(d)of MoO3nanoleaves

Fig.9 Selectivity of sensor to various gases(a)and long-term stability test(b)of MoO3nanoleaves

To quantify the excellently selectivity and stability of α-MoO3based sensors,the real responses of the sensors to 2.25 g·m-3of xylene,toluene,acetone,and formaldehyde interfering gases are investigated at 300℃ and summarized in Fig.9a.Obviously,its responses are negligible compared to smaller units(4.5 mg·m-3)for TEA.Fig.9b more fully demonstrates the repeatability and reversibility of its practical application,and no apparent variation trend in response upon exposed to 2.25 g·m-3TEA at 300 ℃ for a period of 10 d.Noteworthily,the oxidation process of the probe gases is actually the result of product dehydrogenation,so the enthalpy change data of each substance is particularly important.From consulting the literature,the experimental measurements for acetone,toluene,formaldehyde,xylene and triethylamine are 393,379,364,157.8 and 99.64 kJ·mol-1,respectively.Those results can be known as minimal enthalpy change required for triethylamine decomposition.Because TEA has three ethyl groups,while the nitrogen atom has a long pair conjugate electrons that can donate to form bonded key energy,it is preferably adsorbed on Lewis-acid Moδ+sites.Hence,it indicates the greatly palmary selectivity,reversible and stable response advantages of the sensors in quantitative testing.

The complicated mechanism of surface control for triethylamine gas sensing can be explained in detail by the resistance change due to gas particles adsorption and desorption,and the varied thickness of electronic depletion layer(EDL)model[36-38].The specific details are shown in Fig.10.According to the related literature[39-40],the final oxidation product of TEA molecule under photocatalysis is thought to be functional ethylene amine monomers.Therefore,the triethylamine sensing principle over α-MoO3participation is interpreted as the constantly transformation of TEA to ethylene amine monomers and eventually evolved into N2,CO2and H2O.Then,when the α-MoO3device is placed in atmospheric environment,the chemical oxygen molecules would be adsorbed on the surface of MoO3nanoleaves combining with free electrons to form reactive oxygen species(e.g.O2-,O-,O-2).Due to rough surface and inter-nanowire channels in α-MoO3leaf-like structure,the carrier transport is more convenient while relatively high resistance and thick space charge layer would be produced in dynamic equilibrium of the chemisorption process(defined as Ra).But when exposed to TEA with high concentration at a moderate temperature,the active oxygen of α-MoO3is catalytically oxidized,simultaneously accompanying with the partial reduction of metal molybdenum ion,more quantity of Mo6+would be reduced to Mo5+or other valence,which causesthatthe carrierconductivity ofthe nonstoichiometric α-MoO3is comparatively higher than stoichiometric α-MoO3.In the process,leaf-like structure plays an exceptional role in the large effective specific surface area.Furthermore,it can provide more active sites to help the C-H diffusion of the triethylamine methyl group,which is dehydrogenated to amine and hydrogen.They react with the dissociative oxygen ions and release the captured electrons back into the conduction band of α-MoO3,resulting in a decrease in resistance(Rg)and thinned depletion layer.Therefore,the response of α-MoO3sensor is obtained according to the sensitivity formula.Fig.10 gives a graphical description where the value of(Ec-Ef)is widened and the value of(Ef-Ev)is narrowed.The overall oxidationreduction reaction in the sensitive process can be simplified to the following equation:where Oδand V2+represent the chemical oxygen and homologous oxygen vacancy at the atomic lattice level;(s)and(ads)indicate surface sites and types of adsorption state,separately.Taking all the above factors into account,the consequents also can be better to conclude super sensitive response.

Fig.10 Schematic illustration for gas sensing mechanism of MoO3nanoleaves towards triethylamine

Taking into account the gas sensitive properties of different single metal oxides,a comparison between our present experimental data and literature published about TEA based on typical pure MOSs is summarized in Table 1[40-44].Palpably,our sensor indicates a satisfactory TEA gas sensing ability from some aspects(such as limit of detection,sensor response,and nanostructures).

Table 1 Comparison of triethylamine sensing performance among the gas sensors based on typical pure MOSs

3 Conclusions

In summary,the multichip leaf-like MoO3with nanowire channel structure were successfully prepared by a simple solvothermal route and characterized for the accurate triethylamine sensing at lower(mg·m-3)detection level.Themanufactured sensor devices exhibit an unexpected response recovery and excellent selectivity,which highlights the practical advantages of fast triethylamine detection.In the meantime,we discuss in depth surface-controlled model sensing mechanism of this structure in detail,which indicates a great significance to explore the application of other morphologies of α-MoO3nanomaterials in gas sensing field.