Synthesis and gas-sensing characteristics of x-Fe2O3 hollow balls

The interspace between nanoparticles and hollow structure of the materials facilitate the fast diffusion of analytic gas molecules into the sensing layer and adsorption on the total surf[r]

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Original article

Synthesis and gas-sensing characteristics of a-Fe2O3 hollow balls

Chu Manh Hung, Nguyen Duc Hoa**, Nguyen Van Duy, Nguyen Van Toan, Dang Thi Thanh Le, Nguyen Van Hieu*

International Training Institute for Materials Science, Hanoi University of Science and Technology, No Dai Co Viet Street, Hanoi, Viet Nam

a r t i c l e i n f o

Article history: Received March 2016 Accepted 21 March 2016 Available online 11 April 2016 Keywords:

a-Fe2O3hollow balls

Hydrothermal Gas sensors

a b s t r a c t

The synthesis of porous metal-oxide semiconductors for gas-sensing application is attracting increased interest In this study, a-Fe2O3 hollow balls were synthesized using an inexpensive, scalable, and

template-free hydrothermal method The gas-sensing characteristics of the semiconductors were sys-tematically investigated Material characterization by XRD, SEM, HRTEM, and EDS reveals that single-phasea-Fe2O3hollow balls with an average diameter of 1.5mm were obtained The hollow balls were

formed by self assembly ofa-Fe2O3nanoparticles with an average diameter of 100 nm The hollow

structure and nanopores between the nanoparticles resulted in the signiﬁcantly high response of thea -Fe2O3hollow balls to ethanol at working temperatures ranging from 250C to 450C The sensor also

showed good selectivity over other gases, such as CO and NH3promising signiﬁcant application

1 Introduction

Chemical and gas sensors are attracting increased interest worldwide because of the growing demand for monitoring gaseous molecules in various applications [1,2] Various wide-bandgap metal oxide semiconductors, such as SnO2, TiO2, ZnO, In2O3,

Fe2O3, and WO3, have been synthesized for gas-sensing

applica-tions[3] The synthesis of earth-abundant metal oxides, such as Fe2O3, with a three-dimensional conﬁguration and a porous

structure for advanced applications has been the topic of interest in recent years [4e6] a-Fe2O3 is a nontoxic, stable, and

earth-abundant transition metal oxide [7,8] This compound has been used as a sensing material for the detection of various gases[9], such as CO[10], xylene[11], and acetone[12], among others[13] A hollow spherical structure has been reported to show signiﬁcantly faster response and recovery times, as well as higher response to analytic gases, compared with other structures Thus, recent studies have focused on the synthesis of this material for sensing

applications [14,15] Hollow balls are typically fabricated by a template-assisted method, in which the scarified template is pre-paredfirst, then the desired materials are coated, and finally the template is removed [16] For instance, hollow sphere Fe2O3

composed of ultrathin nanosheets were prepared by

template-assisted method, in which monodispersed Cu2O spheres were

used as scariﬁed template to synthesize FeOOH, which was sub-sequently converted intoa-Fe2O3nanospheres[17] Wang et al.[15]

prepared Fe2O3 hollow spheres using ZnS-cyclohexylamine as a

template-assisted agent However, the use of template in the syn-thesis of hollow balls has some limitations, such as multiple-step processes and contamination by foreign elements[17]

In this study, we synthesizeda-Fe2O3 hollow balls using a facile,

inexpensive, and scalable hydrothermal method using glucose and ferric chloride hexahydrate as precursors for gas-sensing applications The

a-Fe2O3hollow balls were formed by the aggregation of single-crystal a-Fe2O3nanoparticles with an average diameter of 100 nm The

inter-space between aggregated nanoparticles facilitates the entry of the gas molecules into the hollow balls and adsorption on the total surface of thea-Fe2O3nanoparticles, thus enhancing the sensing performance

2 Experimental

Large-scalea-Fe2O3hollow balls were synthesized using a facile

and template-free hydrothermal method with glucose and ferric chloride as precursors In a typical synthesis, 2.7 g of ferric chloride hexahydrate (99%, SigmaeAldrich) and 3.7 g of glucose (99.5%, * Corresponding author International Training Institute for Materials Science

(ITIMS), Hanoi University of Science and Technology (HUST), No.1, Dai Co Viet Road, Hanoi, Viet Nam Tel.:ỵ84 38680787; fax: ỵ84 38692963

** Corresponding author International Training Institute for Materials Science (ITIMS), Hanoi University of Science and Technology (HUST), No.1, Dai Co Viet Road, Hanoi, Viet Nam Tel.:ỵ84 38680787; fax: ỵ84 38692963

E-mail addresses:ndhoa@itims.edu.vn(N.D Hoa),hieu@itims.edu.vn(N Van Hieu)

Peer review under responsibility of Vietnam National University, Hanoi

Contents lists available atScienceDirect

Journal of Science: Advanced Materials and Devices

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d

http://dx.doi.org/10.1016/j.jsamd.2016.03.003

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SigmaeAldrich) were dissolved in 50 ml of deionized water at room temperature to obtain a clear solution Then, ammonium hydroxide (25%) was added dropwise to adjust the pH level to pH to obtain a milky solution The milky solution was then poured into a Te ﬂon-lined autoclave for hydrothermal treatment at 180 C for 24 h before cooling to room temperature naturally The precipitates were washed several times with deionized water and ethanol and then collected by centrifugation at 4000 rpm Finally, the collected products were air dried at 60C for 24 h and then calcined at 600C for h prior to use in sensor fabrication and characterizations The morphology and crystal structure of the synthesized materials were characterized usingﬁeld-emission scanning electron micro-scopy (FESEM, JSM, 7600F), transmission electron micromicro-scopy (TEM, Tecnai, G20, 200 kV, FEI), and X-ray powder diffraction (XRD, Bruker D8 Advance)[18]

The gas sensor was fabricated by dispersing the obtained powders in dimethyl formamide solution and then coating the mixture onto a pair of comb-type Pt electrode deposited on ther-mally oxidized silicon substrate The gas-sensing characteristics

were measured by aﬂow-through technique with a standard ﬂow

rate of 400 sccm for both dry air and balanced gas using a home-made sensing system Details of the sensing system can be found in our recent publication[19] Gas-sensing characteristics were tested using ethanol, CO, and NH3at temperatures ranging from 250C to

450 C The sensor response S was deﬁned as S ¼ Rair/Rgas for

reducing gases, where Rgasand Rairare the sensor resistances in the

presence of test gas and dry air, respectively

3 Results and discussion 3.1 Material characterization

The morphology of the synthesized materials was characterized by FESEM [Fig 1] The as-hydrothermal products have a spherical shape with an average diameter of approximately 1.5mm [Fig 1(A) and (B)] The glucose may have decomposed and grown into carbon spheres under the hydrothermal treatment[20] The ferric particles were then aggregated on the surface of carbon spheres to form the coreeshell materials[21] The carbon cores were burned out after calcination at 600 C, forming the a-Fe2O3 hollow spheres

[Fig 1(C)e(F)] Thea-Fe2O3 hollow balls were formed from the

aggregated nanoparticles with an average diameter of 100 nm The shell of the hollow balls is not a dense material, but porous as a result of the nanoparticle aggregation The shell thickness of the hollow sphere from the broken area is estimated to be approxi-mately one layer of nanoparticles [Fig 1(E)]

TEM images and elemental analytical results by EDS of thea -Fe2O3hollow balls are shown inFig The hollow structure of the a-Fe2O3balls is clearly shown inFig 2(A), in which the central part

is brighter than the surrounding region The HRTEM image of the sample demonstrates the high crystallinity of thea-Fe2O3phase

where the gap between two adjunction fringes is approximately 0.25 nm, corresponding to the interspace of (110) planes[22] The inter-grain boundary between nanoparticles can also be seen in the HRTEM image Selective area electron diffraction of the selected

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area marked by white square in the HRTEM image exhibits diffraction spots revealing the single crystallinity ofa-Fe2O3 The

EDS analytical results of the sample shown inFig 2(C) demonstrate the peaks of C, O, Fe, and Cu Elements C and Cu came from the carbon-coated Cu grid used for TEM characterization, whereas O and Fe were from the sample The ratio [O]/[Fe]¼ 1.64 is higher than the composition of stoichiometric Fe2O3, possibly because of

contamination of some OH groups on the surface of sample Crystal structures of the as-hydrothermal and calcined materials characterized by XRD are shown inFig 3(A) and (B), respectively The XRD patterns of the as-hydrothermal product shown in

Fig 3(A) illustrate unresolved peaks The metastable phase, such as Fe(OH)2or Fe(OOH), may have been formed after the hydrothermal

process[6] The metastable phase was then converted to Fe2O3by

thermal oxidation at high temperature The XRD pattern of the calcined sample [Fig 3(B)] demonstrates that the materials have a rhombohedral crystal structure, with the main peaks indexed to the standard proﬁle ofa-Fe2O3phase (JCPDS No 86e0550)[22] No

detectable peaks of FeOOH or Fe3O4impurities and other phases

were observed, indicating the formation of single-phasea-Fe2O3

No template was used in the fabrication of hollow balls, thus the products were not contaminated by any foreign element[15] 3.2 Gas-sensing characteristics

Gas-sensing characteristics of the synthesizeda-Fe2O3hollow

balls were tested using ethanol at different temperatures ranging from 250C to 450 C [Fig 4] Fig 4(A) shows that the initial resistance of the a-Fe2O3 hollow ball sensor measured in air at

250C, 300C, 350C, 400C, and 450C were approximately 85, 58, 43, 31, and 18 kU, respectively The decrease in the initial resistance ofa-Fe2O3sensor with increasing operating temperature

reveals the semiconducting nature of metal oxide, that is, the thermal energy excites electrons from valence band to conduction

band to contribute to the conductivity of the material[23] Thea -Fe2O3hollow balls showed n-type semiconducting characteristics

at all measured temperatures The sensor resistance decreased signiﬁcantly upon exposure to reducing gases (ethanol, NH3, and Fig (A, B) Transmission electron micrographs and (C) EDS results ofa-Fe2O3hallow balls; inset of (B) is the corresponding FFT

Fig X-ray diffraction patterns of the (A) as-hydrothermal and (B) calcineda-Fe2O3

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CO) [6] The semiconducting characteristics of metal oxide are determined by the deﬁciency or excess of material composition The excess or deﬁciency of oxygen in the crystal structure ofa -Fe2O3generally leads to p-type or n-type semiconducting

charac-teristics[24,25] Long et al.[26]demonstrated that the polyhedral

a-Fe2O3 particles showed p-type gas-sensing characteristics, in

which the sensor resistance increased upon exposure to reducing gases, such as H2, CO, and C2H5OH They reported that the p-type

characteristic of materials was due to the incorporation of Na into

a-Fe2O3oxide Heat treatment temperature signiﬁcantly inﬂuences

the electrical properties of a-Fe2O3, such that high-temperature

treatment can result in p-type characteristics[25] In this study, the synthesizeda-Fe2O3hollow balls were heat-treated at a

rela-tively low temperature of approximately 600C for h, so the balls exhibited n-type characteristics This result is consistent with other reports, where the n-type nature of metal oxide semiconductor was attributed to the presence of oxygen vacancies[12,27] The effect of temperature heat treatment on the ethanol-sensing characteristics ofa-Fe2O3hollow balls was determined by annealing the sample at

800C for h However, high-temperature heat treatment led to the distortion of sensor response [Fig S1, Supplementary] Sensor response as a function of ethanol concentration measured at different temperatures is shown inFig 4B The sensor response increases with increasing working temperature from 250C and

reaches a maximum value at 400 C Further increase in the

working temperature results in a slight decrease in the sensor response At 400C, the sensor response also increases from 1.77 to 4.29 with increasing ethanol concentration from 50 ppm to 500 ppm Fast response and recovery times of the sensor are also important in real-time measurements of the device[18] The 90%

response and recovery times of the sensor at different

Fig Ethanol-sensing characteristics ofa-Fe2O3hollow balls: (A) transient resistance versus time of sensor upon exposure to different concentrations of ethanol at various

temperatures; (B) sensor response as functions of ethanol concentrations, (C) response and recovery times; (D) short-term stability of sensors

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temperatures were calculated [Fig 4(C)] The response and recov-ery times to 50 ppm ethanol were approximately 16/30, 7/20, 4/15, and 3/12 s at working temperatures of 250 C, 300C, 350 C, 400C, and 450C, respectively The response and recovery times decrease with increasing working temperature because of the ac-celeration of thermal energy for the adsorption and desorption processes[18] The fast response recovery times of less than is sufﬁcient for practical application[27] The transient stability of the fabricated sensor was also tested at 450C for several cycles switching from air to analytic gas and back to air [Fig 4(D)] A slight deviation from the baseline resistance is observed after several cycles possibly due to the poor adhesion of sensing layer and substrate The experiment was repeated for a day, and negligible distortion in response was found, indicating sufﬁcient stability.

Selectivity of the sensor to CO and NH3 was also tested at

different temperatures [Fig 5(A) and (B), respectively] The sensor

resistance decreased upon exposure to CO and NH3 gases The

response and recovery characteristics to CO gas improve with in-crease in temperature At 450C, the sensor response to 25, 50, and 100 ppm CO is very low, that is, approximately 1.22, 1.30, and 1.33,

respectively The sensor also shows weak response to NH3

(50÷500 ppm) [Fig 5(B)] At 450C, the sensor responses to 50, 100, 250, and 500 ppm NH3are 1.04, 1.11, 1.18, and 1.27, respectively The

response ofa-Fe2O3hollow balls to ethanol (500 ppm) is 3.38 times

higher than that to NH3(500 ppm), at low working temperature,

suggesting the possibility of using this material for sensing ethanol 3.3 Gas-sensing mechanism

The gas-sensing mechanism of the fabricated sensor can be explained by the spaceecharge layer mode[28] The gas-sensing characteristics were measured under a continuousﬂow of dry air Thus, the oxygen molecules in air can capture the free electron from

a-Fe2O3crystals to form the electron-depletion region The oxygen

molecules adsorb on the surface of the sensing layer in the form of O2 ,Oand O2, as follows[29].

O2gasị ỵ e4O2adsị (1)

O2gasị ỵ e42Oadsị (2)

Oadsị ỵ e4O2 (3)

The analytic molecules interact with the pre-adsorbed oxygen upon exposure to ethanol gas, according to the following equations:

C2H5OHỵ 3O242CO2ỵ 3H2Oỵ 3e (4)

C2H5OHỵ 6O42CO2ỵ 3H2Oỵ 6e (5)

C2H5OHỵ 6O242CO2ỵ 3H2Oỵ 12e (6) The interactions between analytic ethanol molecules and pre-adsorbed oxygen release electrons back to the crystals and reduce the spaceecharge layer, resulting in decreased sensor resistance. The porosity of the sensing layer is also very important in con-trolling the sensitivity of the device because it decides the diffusion rate of analytic gas molecules into the sensing layer The diffusion constant (DK) can be calculated based on the Knudsen diffusion

model as DK¼ 4r/3(2RT/pM)1/2, where r is the pore size, R is the

universal gas constant, T is the temperature, and M is the molecular weight of the diffusing gas[30] In this study, the shell of the hollow balls was formed by the aggregation of the monolayer nano-particles with approximately 100 nm in diameter The interspace

between nanoparticles acted as diffusion path for analytic gas molecules to enter deeply into the balls to be adsorbed on the total surface of sensing materials, thereby enhancing sensing perfor-mance[31]

4 Conclusion

The synthesis ofa-Fe2O3hollow balls by a facile hydrothermal

method for gas-sensing application is introduced The a-Fe2O3

hollow balls were formed by the aggregation of highly crystallinea -Fe2O3 nanoparticles The average diameters of a-Fe2O3

nano-particles and hollow balls were 100 nm and 1.5mm, respectively The interspace between nanoparticles and hollow structure of the materials facilitate the fast diffusion of analytic gas molecules into the sensing layer and adsorption on the total surface of sensing materials These characteristics ensured the high sensitivity of materials Thus, thea-Fe2O3hollow balls were found to be suf

ﬁ-cient for ethanol sensor application Acknowledgment

The present study was funded by the Vietnam Ministry of Ed-ucation and Training under Code No KB2015e01e100

Appendix A Supplementary data

Supplementary data related to this article can be found athttp:// dx.doi.org/10.1016/j.jsamd.2016.03.003

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