controlled synthesis and gas sensing properties of hollow sea urchin like

Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb ELSEVIE Controlled synthesis and gas-sensing propertie

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Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

journal homepage: www.elsevier.com/locate/snb

ELSEVIE

Controlled synthesis and gas-sensing properties of hollow sea urchin-like

a-Fe203 nanostructures and a-Fe203 nanocubes

Fenghua Zhang, Heqing Yang*, Xiaoli Xie, Li Li, Lihui Zhang, Jie Yu, Hua Zhao, Bin Liu

Key Laboratory of Macromolecular Science of Shaanxi Province, School of Chemistry and Materials Science, Shaanxi Normal University, Xi’an 710062, China

Article history:

Received 4 September 2008

Received in revised form 28 June 2009

Accepted 30 June 2009

Available online 9 July 2009

Hollow sea urchin-like a-Fe,03 nanostructures were successfully synthesized by a hydrothermal approach using FeCl; and NazSO, as raw materials, and subsequent annealing in air at 600°C for 2h The hollow sea urchin-like a-Fe203 nanostructures with the diameters of 2-4.5 um consist of well-aligned o-Fe203 nanorods with an average length of about 1 um growing radially from the centers of the nanostructures, have a hollow interior with a diameter of about 2 um a-Fe.03 nanocubes with a diameter of 700-900 nm were directly obtained by a hydrothermal reaction of FeCl; at 140°C for 12 h The response

Keywords: Sr (Sp =Ra/Rg) of the hollow sea urchin-like a-Fe2O03 nanostructures reached 2.4, 7.5, 5.9, 14.0 and 7.5 to Hollow sea urchin-like nanostructure 56 ppm ammonia, 32 ppm formaldehyde, 18 ppm triethylamine, 34 ppm acetone, and 42 ppm ethanol, Nanocubes respectively, which was excess twice that of the a-Fe203 nanocubes and the nanoparticle aggregations Gas sensors Our results demonstrated that the hollow sea urchin-like w-Fe203 nanostructures were very promising

for gas sensors for the detection of flammable and/or toxic gases with good-sensing characteristics

1 Introduction

Ammonia and various volatile organic compounds have been

released in high quantities into the atmosphere as aresult of human

activities, and have generated environmental risks Chemical sen-

sors for the detection of these flammable and/or toxic gases play

a very important role in chemical industries, environmental pro-

tection, public safety and human health Metal oxides [1-4], such

as SnO2, ZnO, Fe203 and V2Os, function as gas-sensitive materials

by changing their resistance due to exposure to oxidizing or reduc-

ing gases Nevertheless, there are still some critical limitations to

be overcome for the commercial sensors based on particulate or

thin-film semiconductor metal oxides, such as limited maximum

sensitivity, high working temperatures and lack of long-term sta-

bility [5] Recently, nanorods, nanowires and nanoribbons of metal

oxides were used to fabricate sensors; the results indicate that one-

dimensional (1-D) nanomaterials are promising for highly sensitive

chemical sensors [5] Therefore, the fabrication of the metal oxide

nanomaterials with a defined size and shape for gas sensor applica-

tion is currently a major focus of nanoscience and nanotechnology

Hematite (a-Fe203), the most stable iron oxide with n-type

semiconducting properties (Eg = 2.1 eV) under ambient conditions,

is widely used as catalysts, pigments, gas sensors, and electrode

materials [3], owing to its low cost, high resistance to corro-

* Corresponding author Fax: +86 29 85307774

E-mail address: hqyang@snnu.edu.cn (H Yang)

doi:10.1016/j.snb.2009.06.049

sion, and environmentally friendly properties The previous studies mainly focused on the a-Fe203 films [6-11] and powders [12,13] Stimulated by both the promising applications of iron oxides and the novel chemical and physical properties of nanoscale materials, considerable efforts have been made in the synthesis of a-Fe,03 nanostructured materials with different morphologies

Up to now, a variety of a-Fe,0O3 nanostructured materials in various geometrical morphologies have been successfully fabricated, such as nanoparticles [14], nanotubes [15], nanowires [16], nanobelts [16], nanocubes [17], nanorods [18], spindles [19], hollow spheres [20,21], nanoplates [22], nanorings [23], rhombohedra [24] and complex hierarchical structures constructed with nanoscale building blocks [25-30] In particular, three-dimensional (3-D) superstructures assembled with one-dimensional nanorods have attracted much attention because of their unique properties and potential applications [28-30] However, to our knowledge, the synthesis of hollow sea urchin-like a-Fe,03 nanostructures has not been reported until now The hollow nanostructures have widespread potential applications in catalysts, gas sensors, drug delivery, etc., owing to their higher specific surface area and lower density

Recently, the nanorods [3], nanotubes [15], hollow spheres with

a mesoporous shell [20], porous nanospheres [21], nanorings [23] and flutelike porous nanorods and hexapod-like nanostructures [31] of a-Fe203 have been used to fabricate gas sensors for the detection of ethanol, acetone, 92* gasoline, heptane, hydrogen, formaldehyde, toluene, acetic acid and ammonia However, they did not investigate the stability of the sensors even though stability

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is one of the most important parameters of sensors In addition, the

sensing properties of hollow sea urchin-like a-Fe,03 nanostruc-

tures and a-Fe203 nanocubes have not been studied until now

In this paper, we report on controlled synthesis of hollow sea

urchin-like a-Fe203 nanostructures and a-Fe203 nanocubes Con-

trol over the both a-Fe203 nanostructures were achieved by adding

different anions in the Fe?*-H2O hydrothermal system To the

best of our knowledge, this is the first report of the selective

synthesis of a-Fe203 superstructures and nanocubes The gas-

sensing properties of the a-Fe,03 superstructures, nanocubes and

nanoparticle aggregations with irregular morphology in detect-

ing ammonia, formaldehyde, triethylamine, acetone, ethanol and

hydrogen were studied The sensitivities of the as-prepared a-

Fe203 superstructures are higher than that of nanocubes and

nanoparticle aggregations of a-Fe203 with irregular morphology

2 Experimental

2.1 Synthesis

Hollow sea urchin-like a-Fe,03 nanostructures: In a typical syn-

thesis, 10 mL of 0.1 M sodium sulfate (Na2SO,4) aqueous solution was

added to 2 mL of 0.5M iron chloride (FeCl3) solution under mag-

netic stirring After stirring for 10 min, 8 mL of deionized water was

added under constant stirring to form a homogeneous solution The

mixed solution was sealed into a Teflon-lined stainless steel auto-

clave of 50 mL capacity and heated at 140°C for 12 h After reaction,

the autoclave was cooled to room temperature naturally The yel-

low product was isolated by centrifugation, washed with deionized

water and absolute ethanol several times, and finally dried in air at

room temperature The as-prepared product was heated to 600°C

with a rate of 1.0°C min~! and then was maintained at 600°C for

2 hin air The red powder was obtained, which was used for further

analysis and characterization

a-Fe203 nanocubes and a-Fe203 nanoparticle aggregations

with irregular morphology: 10mL of deionized water was

employed instead of 0.1M NazSO4 aqueous solution, a-Fe203

nanocubes were directly obtained in the same hydrothermal con-

ditions When Fe(NOQ3)3 was used as Fe source instead of FeCls,

a-Fe203 nanoparticle aggregations with irregular morphology

were directly obtained in the same hydrothermal conditions

2.2 Characterization

The as-obtained samples were characterized by X-ray diffrac-

tion (XRD, Rigaku D/MAX-IIIC X-ray diffractometer with Cu Ka

radiation, 4=1.5406A), scanning electron microscopy (SEM, FEI

Quanta 200, 20kV), and transmission electron microscopy (TEM,

JEOL JEM-3010, 300kV) The samples for TEM were prepared

by dispersing a-Fe203 powders on carbon-coated copper grids

The Brunauer-Emmett-—Teller (BET) specific surface area was per-

formed by N> gas adsorption using a ST-03A surface analytical

instrument (Beijing Analysis Instrument Factory, China)

2.3 Gas-sensing properties test

Measurements on gas sensitivity were performed with a

WS-30A system (Weisheng Instruments Co., Zhengzhou, China)

Schematic diagram of the typical gas sensor system is shown in

Fig 1 First, the as-prepared a-Fe,03 nanostructures were mixed

with terpineol to form a slurry, the slurry was coated as a thin film

on a ceramic tube with a pair of previously printed gold electrodes

that were connected by four platinum wires (the outer diameter

of the ceramic tube is 1.34 mm, and the distance between the both

gold electrodes is 1.40 mm) The thickness of the sensing thin film

is about 0.06 mm After drying at 60°C in air, the ceramic tube was

7

V heating

W)

thin film of the products

Pt wire Ni-Cr resistance heater

Au electrode ceramic tube

2

=

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Vicaipat

3:

tư

>}

reference

sensor

7 O

V working

Fig 1 Schematic diagrams of the gas sensor measurement system

heated to 600°C at a rate of 1°Cmin~! in air and kept for 2h

A Ni-Cr resistor wire was crossed through the ceramic tube as a heater allowing us to control the working temperature by adjusting the heating voltage (Vpeating) Then the electrical contact was made through connecting the four platinum (or Ni—Cr resistor) wires with the instrument base by Sn paste The as-prepared a-Fe203 sensors were aged at 350°C for 7 days to improve the stability of the devices Finally, a reference resistor was put in series with the sensor to form

a complete measurement circuit Test gases were injected into the

18 L-testing chamber directly by a microinjector and mixed with air immediately The air was used as a reference gas In the test process, the voltage (Voutput ) across the reference resistor changes with the sensor’s resistance, which responds to the types and concentrations of the test gases Thus, the response of the sensor in clean air

or in the test gas can be measured by monitoring Voutput, Here, the sensor response (S;) to a test gas is defined as Ra/Rg, where Ra and

Rg are the resistance of the sensor in clean air and in the test gas, respectively Our gas-sensing measurements were carried out at a working temperature of 350°C and relative humidity of 5-38%

3 Results and discussion 3.1 Microstructures of hollow sea urchin-like a-Fe203 nanostructures and a@-Fe203 nanocubes

Fig 2 shows the XRD patterns of the a-FeOOH precursors prepared by the hydrothermal reaction of FeCls with NazSOa at 140°C for 12 h and a-Fe203 products obtained by calcining the precursors

at 600°C in air for 2h As can be seen in Fig 2a, all the diffraction peaks of a-FeOOH precursors can be indexed to the pure orthorhombic a-FeOOH, which are consistent with the values in the literature (Joint Committee on Powder Diffraction Standards, JCPDS No: 81-0462) All the strong and sharp diffraction peaks shown

in Fig 2b can be indexed to a-Fe203 with a hexagonal structure, which are consistent with the values in the literature (JCPDS No:

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a ơ-FeOOH

26(degree) Fig 2 XRD patterns of (a) the precursors prepared by the hydrothermal process and

(b) the products obtained by calcining the precursors at 600°C in air for 2 h

33-0664) In addition, no peaks from other phases are found, sug-

gesting high purity of the as-synthesized a-Fe,03 The SEM images

of the œ-FeOOH precursors prepared by the hydrothermal process

are shown in Fig 3a and b Fig 3a and b shows that the as-obtained

a-FeOOH precursors consist of a large quantity of microspheres

with typical diameters in the range of 2-4.5 1m The SEM image

at high magnifications (inset of Fig 3b) reveals that the micro-

sphere is constructed from one-dimensional nanorods with the

diameters of about 150nm Fig 3c and d shows SEM images of

a-Fe,03 products obtained by calcining the a-FeOQOH precursors

As can be seen in Fig 3c and d, when œ-FeOOH precursors were

_““

J 0 NON

decomposed to form a-Fe203, the spherical morphologies of the products were almost maintained Interestingly, the solid microspheres became into the hollow ones The hollow sea urchin-like nanostructures with a diameter of about 2 um are built from a single layer of radially oriented nanorods with a diameter of about

120 nm, self-wrapping to form hollow interior (inset of Fig 3d) TEM

is employed to study the structural characteristics of the hollow sea urchin-like a@-Fe,03 nanostructures in details, and the results are given in Fig 4 Fig 4a shows a typical TEM image of an individual a-Fe,03 superstructure The central portion of the superstructure

is lighter than that of the edge, further confirming the hollow interiors of the unique self-wrapped nanorod arrays Interestingly, an important phenomenon is found in the TEM observations: a sheaf

of tiny nanorods with diameters of about 15 nm are attached side- by-side into the external sharp end of the constituent “mother” nanorods, as shown in Fig 4b The high-resolution transmission electron microscopy (HRTEM) image of the constituent nanorod is displayed in Fig 4c We observed that the constituent nanorod is assembled from nanorods with diameters of 13-20 nm in Fig 4c, the measured lattice spacings of 0.25, 0.37 and 0.22 nm are consistent with the d values of the (1 10), (012) and (1 13) planes of a-Fe203 with a hexagonal structure, respectively These results suggest that hollow sea urchin-like a-Fe203 nanostructures constructed with a- Fe203 nanorods can be fabricated by the hydrothermal reaction of FeCl3 and Na2SO4 and subsequent annealing In addition, we can clearly see some brighter areas on the HRTEM image of Fig 4c, which indicates that there are some pits on the nanorod surfaces The diameter of these pits is 5-9 nm The presence of the pits may

be due to the decomposition of a-FeOOH and release of H20 Fig 5a and b show SEM image and XRD pattern of the products obtained via a hydrothermal reaction of FeCl3 with HzO at 140°C for 12h The SEM and XRD results indicated that FeCl; reacts with H20O in the absence of Na2SO, to form monodispersive

Fig 3 Typical SEM images of (a and b) a-FeOOH precursors synthesized by the hydrothermal process and (c and d) a-Fe203 products prepared by calcining of a-FeOQOH precursors at 600°C in air for 2h.

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Fig 4 (a and b) Typical TEM and (c) HRTEM images of the hollow sea urchin-like a-Fe203 nanostructures

a-Fe203 nanocubes with a hexagonal structure (JCPDS No: 33-

0664) instead of sea urchin-like a-FeOOH nanostructures The

edges of the nanocubes are 700-900 nm Fig 5c and d displays SEM

image and XRD pattern of the products obtained via a hydrothermal

reaction of Fe(NO3)3 with H20 at 140°C for 12 h The SEM image and

XRD pattern reveal that the as-synthesized products consist of a-

Fe,03 with a hexagonal structure (JCPDS No: 33-0664) nanoparticle

aggregations with irregular morphology

It is well known that an aggregation process involving the for-

mation of larger crystals by greatly reducing the interfacial energy

of small primary nanocrystals is energetically favored However, the

interaction between unprotected building units with nanoscale size

is generally not competent to form stable and uniform microstruc-

tures [29], such as the sea urchin-like a-FeOOH nanostructures

and a-Fe,03 nanocubes discussed here According to Korchef et

al [32], Xie et al [33] and Sun et al [34], in the presence of

SO,42~ ions the iron precipitates obtained from aqueous solution

are a-FeOOH instead of a-Fe203 The SO42~ ions play an impor-

tant role in the formation and self-assembly of a-FeQOH nanorods

into sea urchin-like nanostructures They serve as ligand to Fe?*,

and may adsorb on the facets parallel to the c-axis of a-FeOOH

nuclei by a monodentate structure (Fe—O-SOQ3) to obtain a-FeOOH

nanorods [34] These nanorods gradually assemble into 3-D urchin-

like congeries because that bidentate (Fe-O-SO2—O-Fe) structure

is formed between FeOOH nanorods [34] The a-FeOOH nanostructures changed into hollow urchin-like a-Fe203 nanostructures

by calcining in air The presence of Cl~ ions was believed to be crucial for the formation of Fe,03 nanocubes [34] The primary a- Fe,03 nanocrystals with a hexagonal structure were capped with Cl- ions acting as ligand According to Zheng et al [17], the density of the iron atom on the low-index crystal planes of {110}, {111}, and {100} of a-Fe203 with a hexagonal structure is 10.1, 7.9, and 4.1 atoms/nm7?, respectively The {1 1 0} planes with a relatively higher density of iron atom adsorb more Cl~ ions, thus, they would grow slower during the oriented attachment of primary nanocrystals than the other planes, tending to form nanocubes enclosed by {110} exposure planes [17] However, NO3~ ions do not serve as ligand to Fe?*, primary a-Fe203 nanocrystals randomly aggregate into aggregations with irregular morphology

3.2 Gas-sensing properties The transient response characteristics towards ammonia, formaldehyde, triethylamine, acetone, ethanol and hydrogen of the sensors based on the hollow sea urchin-like nanostructures, nanocubes and nanoparticle aggregations were investigated at

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œ-Fe,O;

20 (degree)

d = S ơ Fe,O;

a

S

L L 1 L L

29 (degree)

Fig 5 (a) SEM image and (b) XRD pattern of the products synthesized via a hydrothermal reaction of FeCl; at 140°C for 12h, (c) SEM image and (d) XRD pattern of the products prepared by a hydrothermal reaction of Fe(NO3)3 at 140°C for 12h

350°C and relative humidity of 5-18%, and the results are dis-

played in Fig 6 Fig Ga-f shows typical response curves on

cycling between increasing concentration of ammonia, formalde-

hyde, triethylamine, acetone, ethanol and hydrogen and ambient

air, respectively It can be seen that Voutput values increased abruptly

with the injection of ammonia, formaldehyde, triethylamine, ace-

tone or ethanol then decreased rapidly and recovered their initial

value after the test gas was released In particular, the change

Of Voutput Values for the sensor based on hollow sea urchin-like

a-Fe2O3 nanostructures is the sharpest However, the Voutput val-

ues hardly change with the injection and release of hydrogen

From Ohm’s law, the electric resistance of the sensors accord-

ingly underwent a decreasing and increasing process when the

test gas was injected and released, respectively, which is consis-

tent with the sensing behavior of n-type semiconductor sensors

After many cycles between the test gas and clean air, the voltage

of the reference resistor and the resistance of the sensor could

recover their initial states, which indicates that the sensors have

good reversibility The response time (defined as the time required

to reach 90% of the final equilibrium value) of the hollow sea

urchin-like nanostructure-based sensor is 5-8, 17-50, 8-18, 5-10

and 7-21s towards ammonia, formaldehyde, triethylamine, ace-

tone and ethanol, respectively The recovery time (taken as the

time necessary for the sensor to attain a conductance 10% above

the original value in air) is 10-21, 12-20, 10-20, 9-20 and 11-14s,

respectively

Fig 7a shows the response of the sensors based on the hol-

low sea urchin-like nanostructures, nanocubes and nanoparticle

aggregations to 56 ppm ammonia, 32 ppm formaldehyde, 18 ppm

triethylamine, 34 ppm acetone and 42 ppm ethanol at 350°C The

responses of the a-Fe,O3 superstructures are about twice, five-

fold, twice, twice and three-fold higher than that of the a-Fe203 nanocubes, twice, six-fold, twice, twice and four-fold higher than that of the a-Fe,03 nanoparticle aggregations, respectively The results indicate that the response performance of the hollow sea urchin-like nanostructures is better than that of the nanocubes and nanoparticle aggregations, and the performance of nanoparticle aggregations is the worst, whichever gas was tested

The response of the a-Fe,03 gas sensors can be empirically represented as R=1 + Ag(Pg)6, where Pg is the target gas partial pressure, which is in direct proportion to the gas concentration,

Ag is a prefactor, and is the exponent on Pg Generally, 8 has an ideal value of either 0.5 or 1, which is derived from surface interaction between chemisorbed oxygen and reducing gas to the n-type semiconductors [35] So, logarithm of the response should be linear with logarithm of gas concentration Fig 7b-f displays chart of the logarithm of response of the three kinds of sensors versus the logarithm of gas concentration The linear equations and correlation factor, R, were given in Table 1 The value of 6 for the hollow sea urchin-like nanostructures towards ammonia, formaldehyde, triethylamine, acetone and ethanol, is about 0.28, 0.44, 0.69, 0.72 and 0.75, respectively, determined by the fit using the empirical formula The deviation of the 6 may be due to the disorder and some insensitive area (vacancy between the hollow microspheres) existing in the sensors [35] The lowest detection limit for ammonia, formaldehyde, triethylamine, acetone and ethanol is about 17,

2, 2,3 and 5 ppm, respectively

The response of the sensor based on a-Fe203 superstructures towards 560 ppm ammonia, 160 ppm formaldehyde, 90 ppm triethylamine, 170 ppm acetone and 210ppm ethanol at different working temperatures were studied, and the results are given in Fig 8 It can be seen that the responses of the sensor to these gases

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AWG nu TU Uk

0 4 L 4 L + L 4 L + L + 0 4 1 1 L 1 L L

0 100 200 300 400 500 600 0 200 400 600 800 1000

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0 L L 1 L 0 1 L 1 L 1 L 4 L 4 L L

0 100 200 300 400 500 600 700 0 100 200 300 400 500 600 790

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0 1 L 1 1 1 1 4 1 5 1 5 0 L L 4 1 L ¿ L 4

0 100 200 300 400 500 600 0 100 200 300 400 500 600

Fig 6 Typical transient response curves of the sensors based on (I) the hollow sea urchin-like a-Fe203 nanostructures, (II) nanocubes and (III) nanoparticle aggregations to (a) ammonia, (b) formaldehyde, (c) triethylamine, (d) acetone, (e) ethanol and (f) hydrogen of different concentrations at 350°C and relative humidity of 5-18%

Table 1

The linear equations and correlation factor for the chart of the logarithm of response of the three kinds of sensors versus the logarithm of gas concentration

Gas

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F Zhang et al / Sensors and Actuators B 141 (2009) 381-389 387

nanoparticle aggregations (a) AB nanocubes

ga hollow sea urchin-like nanostructure:

ammonia formaldehyde TEA acetone alcohol (36 ppm) (32 ppm) (18 ppm) (34ppm) (42 ppm)

Gases 2.0

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Fig 7 (a) The response comparison between the three kinds of sensors; (b-f) the logarithm of response of hollow sea urchin-like a-Fe2O3 nanostructures (-C)-), nanocubes (-™-) and nanoparticle aggregations (-@-) sensors versus the logarithm of ammonia, formaldehyde, triethylamine, acetone and ethanol concentration at 350°C and relative humidity of 5-18%

increase with an increase in the working temperature, and reaches

to maximum at 350°C It is possibly related to the chemical reac-

tion kinetics between gas molecules and oxygen ions adsorbed on

the surface of the a-Fe203 superstructures The chemical reaction

rate is lower at lower temperature, leading to a lower response of

the sensor

Stability is also one of the most important characteristics for the

sensors To investigate the time stability of the hollow sea urchin-

like a-Fe,03 nanostructure-based sensor, the sensor was stored in

air and kept working at 350 °C for subsequent sensing property tests

after the first measurement A series of tests were carried out at the

times of 1, 5, 6, 7, 17 and 47 days after the sensor fabrication and

aging for 7 days, with a 42 ppm ethanol concentration at a working

temperature of 350°C and relative humidity of 5-18% The chart

of the sensor response versus the storing time is shown in Fig 9 It

was found that the response of the sensor to ethanol is lower during

the initial 5 days and reached subsequently a nearly constant value,

showing that the sensor exhibited good long-term stability after the sensor storing for 6 days in air

It is generally accepted that for metal oxide-based sensors the change in resistance is mainly caused by the adsorption and des- orption of gas molecules on the surface of the sensing structure

To understand the origin of the difference in the sensing performance of the three kinds of sensors, the BET surface area of the nanocubes, hollow sea urchin-like nanostructures and nanoparticle aggregations of a-Fe,03 was measured It was found that their BET surface area is 3.63, 18.8 and 28.0 m2 g~!, respectively Further- more, the sensor response is also determined by the quantity of active sites on the surfaces of a-Fe203 gas sensors Fig 10 shows the XRD patterns of the nanocubes, hollow sea urchin-like nanostructures and nanoparticle aggregations of a-Fe203 The XRD indicates that their crystallinity reduces in file Although surface area of the a-Fe,03 nanoparticle aggregations is the maximal, their crystallinity is the worst According to Li et al [36] and Hyodo et al [37],

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25

—— ammonia

20 |—?7 formaldehyde

n —O— triethylamine

» lŠ5Ƒ |—®—_ cthanol

a

S

=

Ø 10

s2

5 1

0 "

1 1 L + 1 1 1 4 1 + L 1 1

0 50 100 150 200 250 300 350 400

temperature (°C)

Fig 8 Response of the sensor based on the as-prepared hollow sea urchin-like o-

acetone (170 ppm, -v-), triethylamine (90 ppm, -(1-) and ethanol (210 ppm, -@-) at

different temperatures and relative humidity of 23-38%

“> 10 F

cổ

a

6

~ OT

a—a

0 4 L 1 1 4 L * 1 1

Time (days) Fig 9 Variations in response of the sensor based on the hollow sea urchin-like o-

Fe203 nanostructures to 42 ppm ethanol at 350°C and relative humidity of 5-18%

after storage in air for different time periods

œ-Fe,Q;

2theta (degree) Fig 10 XRD patterns of (a) nanoparticle aggregations, (b) hollow sea urchin-like

nanostructures and (c) nanocubes of the a-Fe203

grain-boundaries or grain-junctions are considered as the active

sites and they act positively on the sensor response, whereas sec-

ondary grains, in which many grain-boundaries have disappeared

during their formation, act negatively on the sensor response The

a-Fe203 nanoparticle aggregations have serious agglomeration,

which was typical large secondary grains with a fewer amount

of grain-boundaries So the gas response of the sensor based on

a-Fe,03 nanoparticle aggregations is the lowest The amount of grain-boundaries on the surface of a-Fe.03 nanocubes is the most due to its best crystallinity, thus its response is higher than that

of a-Fe203 nanoparticle aggregations The as-synthesized a-Fe203 superstructures with a large specific area are composed of many small well-aligned nanorods, and many nanorods/nanorods grain- boundaries could be formed The hollow interiors and interspaces between the nanorods can facilitate the diffusion of the test gas and improve the kinetics of both the reaction of the test gas with surface-adsorbed oxygen and the replacement of the latter from the gas phase [21] In addition, SO42~ ions on the surface of a-Fe,03 nanorods in the superstructures may contribute to the enhance- ment of gas sensitivity [38] Therefore, the sensor based on the a-Fe203 superstructures exhibits excellent sensing performances

in detecting ammonia, formaldehyde, triethylamine, acetone and ethanol

4 Conclusions

In summary, hollow sea urchin-like a-Fe,03 nanostructures have been successfully fabricated via the hydrothermal reaction of FeCl3 and Na2SO,4 and subsequent annealing in air Sensor based

on the a-Fe,03 superstructures shows high gas-sensing responses, short response and recovery time and long-term stability in detecting ammonia, formaldehyde, triethylamine, acetone and ethanol, indicating that these hollow sea urchin-like a-Fe203 nanostructures could be promising candidates as the building blocks for the fabrication of gas sensors for the detection of ammonia and various flammable and/or toxic volatile organic compounds in air Acknowledgments

This work was supported by National Natural Science Foun- dation of China (Grant No 20573072) and Specialized Research Fund for the Doctoral Program of Higher Education (Grant No 20060718010)

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Biographies

Fenghua Zhang was born 1982, and she is now a master student at the School

of Chemistry and Materials Science under the supervision of prof Heqing Yang Her research project now focuses on “synthesis and characterization of a-Fe203 nanostructures”

Heqing Yang received his PhD degree from Xi’an Jiaotong University in 1999 He did his post-doctorial research in Fudan University for two years from 2000 to 2002 Now, he is doing research in the School of Chemistry and Materials Science, Shaanxi Normal University He is currently involved in research on using nanostructured materials as gas sensors, catalysts and biosensors

Xiaoli Xie received her MS degree in physical chemistry in 2008 from Shaanxi Normal University, Xi’an, China

Li Li received her MS degree in inorganic chemistry in 2002 from Northwest Uni- versity, China She is now a PhD student at the School of Chemistry and Materials Science under the supervision of prof Heqing Yang

Lihui Zhang received her MS degree from the School of Chemistry and Materials Science, Shaanxi Normal University, in 2004 She is now a PhD student at the School

of Chemistry and Materials Science under the supervision of prof Heqing Yang

Jie Yu was born in 1982, and he is now a master student at the School of Chemistry and Materials Science under the supervision of prof Heqing Yang

Hua Zhao was born in 1982, and he is now a master student at the School of Chem- istry and Materials Science under the supervision of prof Heqing Yang

Bin Liu received his MS degree from the School of Chemistry and Materials Sci- ence, Shaanxi Normal University, in 2004 He is now a PhD student at the School of Chemistry and Materials Science under the supervision of prof Heqing Yang.

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