facile synthesis of porous a - fe2o3 nanorods and their application in ethanol sensors

The as-prepared porous R-Fe2O3nanorods have a tiny crystal size 5 nm and a pore size distribution of 1-10 nm, resulting in a high specific surface area of 221.9 m2· g-1.. Due to the fact

Facile Synthesis of Porous r-Fe2O3 Nanorods and Their Application in Ethanol Sensors

Yan Wang, Jianliang Cao, Shurong Wang, Xianzhi Guo, Jun Zhang, Huijuan Xia,

Shoumin Zhang, and Shihua Wu*

College of Chemistry, Nankai UniVersity, Tianjin 300071, P.R China

ReceiVed: June 2, 2008; ReVised Manuscript ReceiVed: September 12, 2008

A facile solution approach was employed to synthesize R-FeOOH nanorods by using FeSO4· 7H2O and

CH3COONa without templates at low temperature (40°C) The porous R-Fe2O3nanorods were successfully obtained by calcining the R-FeOOH precursors at 300°C for 2 h The as-prepared products were characterized

by thermogravimetry-differential thermal analysis, X-ray powder diffraction, transmission electron microscopy (TEM), high-resolution TEM, and N2 adsorption-desorption analysis techniques The as-prepared porous R-Fe2O3nanorods have a tiny crystal size (5 nm) and a pore size distribution of 1-10 nm, resulting in a high specific surface area of 221.9 m2· g-1 A possible growth mechanism of the porous R-Fe2O3nanorods was proposed The gas-sensing measurement results demonstrated that the porous R-Fe2O3nanorods presented a much higher response than the R-Fe2O3nanoparticles and showed excellent selectivity and stability to ethanol vapor Due to the fact that it has exciting gas-sensing properties and can be obtained easily, the as-prepared porous R-Fe2O3nanorod would be an ideal candidate for application in ethanol sensors

1 Introduction

In recent years, the fabrication of nanostructure materials with

a desired size, morphology, and porosity has received steadily

growing interest owing to their special electrical, optical,

magnetic, and physicochemical properties that are superior to

those bulk materials.1-4 Currently, one-dimensional (1-D)

nanostructures, such as nanorods, nanowires, nanobelts, and

nanotubes, have become the focus of intensive research not only

for their peculiar properties but also for many potential

applications in catalysis, electronics, photonics, drug delivery,

medical diagnostics, sensors, and magnetic materials.5-8

Hematite (R-Fe2O3) is the most stable iron oxide with n-type

semiconducting properties (Eg ) 2.2 eV) under ambient

conditions It has been intensively investigated because of its

wide applications in catalysts, pigments, magnetic materials, gas

sensors, and lithium ion batteries.9-15For its excellent properties,

much attention has been directed to the controlled synthesis of

one-dimensional (1-D) R-Fe2O3, such as nanospindles,16,17

nanofibers,18,19 nanorods,20,21 nanowires,22,23 nanobelts,24and

nanotubes25,26by a variety of techniques and methods Wang

et al prepared R-Fe2O3nanobelts and nanowires via a gas-solid

reaction process under 700 and 800°C.20Mann et al synthesized

R-Fe2O3nanotubes by using biomacromolecules as templates.27

Yi-Xie et al and Bo-Tong et al prepared R-Fe2O3 nanorods

through a hydrothermal process at 120 and 100 °C,

respec-tively.28,29The preparation of R-Fe2O3nanotubes with alumina

membranes as the substrates was also employed by many

researchers.30-33 However, the gas-solid reaction usually

requires special equipment and high temperatures, the methods

employing templates or substrates often suffer from

disadvan-tages related to the high cost and the removal of impurities,

and the hydrothermal process usually needs tedious reaction

times It is still a challenge to develop simple, low-cost, and

environmentally friendly approaches for the synthesis of 1-D

structural R-Fe2O3

Recently, the concern over environmental protection and increasing demands to monitor hazardous gases in industry and the home has attracted considerable attention to developing gas sensors for various polluting and toxic gases Due to its low cost, good stability, and reversibility, R-Fe2O3has been proved

to be an important semiconductor gas sensor The gas sensors based on R-Fe2O3nanoparticles have been widely investigated

by many researchers in the past decades.11,12However, so far, there are only a few reports on the gas-sensing properties of 1-D nanostructural R-Fe2O3 Generally, the properties of a gas sensor are strongly dependent on its surface area The relatively low ratio of surface to volume of the conventional bulk R-Fe2O3

materials leads to their poor gas-sensing properties Hence, developing the 1-D nanostructure R-Fe2O3 with high surface area is very important for increasing their applications on gas sensors

Herein, we report a facile route for the preparation of porous R-Fe2O3nanorods without any templates via a low-temperature (40 °C) solution approach First, the precursor of R-FeOOH nanorods was prepared by using FeSO4· 7H2O as the iron source material in the presence of CH3COONa in an aqueous solution The CH3COONa was used as a source of hydroxide ions during the hydrolysis of iron salts to form iron oxyhydroxide (FeOOH) Then the porous R-Fe2O3 nanorods were obtained by the calcination of as-prepared R-FeOOH at 300 °C for 2 h The as-obtained porous R-Fe2O3nanorods have a tiny crystal size (5 nm) and a high surface area (221.9 m2· g-1) The gas-sensing properties of the sensor based on the porous R-Fe2O3nanorods

to ethanol were systematically investigated Meanwhile, the gas-sensing properties of the porous R-Fe2O3nanorods were com-pared with those of R-Fe2O3nanoparticles

2 Experimental Section All chemicals were of reagent

grade and used without further purification

In a typical synthesis procedure of the R-FeOOH nanorods, 2.78 g of FeSO4· 7H2O and 3.28 g of CH3COONa were dissolved in 50 mL of deionized water under magnetic stirring After stirring vigorously for a period at 40°C, a yellow slurry was formed The products were collected and washed with

* Corresponding author Phone: +86 22 2350 5896 Fax: +86 22 2350

2458 E-mail: wushh@nankai.edu.cn.

10.1021/jp806430f CCC: $40.75 2008 American Chemical Society

Published on Web 10/23/2008

distilled water several times by vacuum extraction filtering with

two sheets of medium speed qualitative filter paper (pore

diameter 30-50 µm) and then dried at 40°C under vacuum

for 2 h The porous R-Fe2O3 nanorods were obtained by

calcining the as-prepared R-FeOOH nanorods precursor at 300

°C for 2 h in air The color of the samples changed from yellow

to red The whole preparation process for the porous R-Fe2O3

nanorods can be finished in no more than 6 h The short

production process would be helpful for the large-scale industrial

manufacture of porous R-Fe2O3nanorods

Thermogravimetry-differential thermal analysis (TG-DTA) of

the as-prepared R-FeOOH precursor was conducted on a

ZRY-2P thermal analyzer Ten milligrams of an R-FeOOH sample

was heated from room temperature to 600°C in air at a heating

rate of 10 °C min-1 X-ray diffraction (XRD) analysis was

performed on a D/MAX-RAX diffractometer with Cu KR

radiation (λ ) 0.154 18 nm) operating at 40 kV and 100 mA.

Diffraction peaks of crystalline phases were compared with those

of standard compounds reported in the JCPDS data file

Transmission electron microscopy (TEM) and high-resolution

transmission electron microscopy (HRTEM) analysis were

carried out on a Philips-T20ST electron microscope operating

at 200 kV N2adsorption-desorption isotherms were collected

at liquid nitrogen temperature using a Quantachrome Nova

2000e sorption analyzer The pore diameter and the pore size

distributions were determined by the Barret-Joyner-Halenda

(BJH) method The specific surface areas (SBET) of the samples

were calculated following the multipoint

Brunauer-Emmett-Teller (BET) procedure

The gas-sensing performance was systematically investigated

by a HW-30A gas-sensing measurement system (Henan Hanwei

Electronical Technology Co., Ltd.) The fabrication and testing

principle of the gas sensor are similar to that described in our

previous reports.34,35The porous R-Fe2O3nanorod samples were

mixed with terpineol to form a paste and then coated onto the

outside surface of an alumina tube 4 mm in length The thickness

of the coated sensing layer is around 50 µm A small Ni-Cr

alloy coil was placed through the tube to supply the operating

temperatures from 100 to 500°C Electrical contacts were made

with two platinum wires attached to each gold electrode To

improve their stability and repeatability, the gas sensors were

sintered at 300°C for 10 days in air Here, the sensing properties

of the gas sensors were measured under a steady-state condition

in a chamber with a volume of 15 L at a working temperature

of 250 °C and 40% relative humidity (RH) An appropriate

amount of ethanol vapor was injected into the closed chamber

by a microinjector, and the sensor was exposed to air again by

opening the chamber when the test was completed

3 Results and Discussion

TG-DTA measurement was performed to study the

conver-sion process of the as-prepared R-FeOOH during calcination

in air, and the result is shown in Figure 1 From the TG curve

of Figure 1, it can be seen that the total weight loss is about

12%, which is a little larger than the theoretical value (10.1%),

indicating that about 2% adsorbed water is present in the

as-prepared R-FeOOH The abrupt weight loss (about 10.5%) that

occurred at the temperature range of 250-300°C is attributed

to the decomposition of R-FeOOH precursors Correspondingly,

there are an endothermic peak and an exothermic peak on the

DTA curve which may be ascribed to the removal of the

structural water molecules and the crystallization process of

R-Fe2O3, respectively Above 300°C, the weight of the

pre-cursor no longer changes, which indicates that the stable residue

can reasonably be ascribed to the pure R-Fe2O3phase This result can also be confirmed by the following XRD analysis results

As expected, porous R-Fe2O3nanorods have been prepared by the calcination of the as-prepared R-FeOOH precursors at 300

°C in air

Figure 2 shows the XRD patterns of the samples The deflection peaks of the as-prepared precursor (Figure 2a) can

be perfectly assigned to the standard value of the R-FeOOH phase (JCPDS No 29-0713) When the R-FeOOH precursor were calcined in air at 300°C for 2 h, all the deflection peaks

of the product (Figure 2b) were in agreement with the standard data of R-Fe2O3(JCPDS No 33-0664) No characteristic peaks are observed for impurities such as γ-Fe2O3 and Fe3O4, indicating that the R-FeOOH precursor was completely trans-formed into hematite at 300°C, which is also consistent with the results of TG-DTA

Figure 1 TG-DTA curves of as-prepared FeOOH nanorods.

Figure 2 XRD patterns of (a) R-FeOOH and (b) R-Fe2 O 3 nanorods.

The morphologies of the as-prepared R-FeOOH and R-Fe2O3

nanorods were further investigated by TEM and HRTEM Figure

3a and b shows the representative TEM and HRTEM

micro-graphs of the as-prepared R-FeOOH sample, respectively The

images clearly demonstrate that the sample has a smooth, rodlike

morphology with average diameter of about 10-15 nm and a

length of about 200 nm After being calcined at 300°C for 2 h,

the sample still maintains the rodlike 1-D morphology, as is

shown in Figure 3c and d Compared with the smooth surface

of the as-prepared R-FeOOH nanorods, it is interestingly found

that the calcined sample possesses a pore structure These pores

are 1-10 nm in size, open to the outer surface, and almost

isolated from each other The formation of the pores may be

due to the removal of H2O from the as-prepared R-FeOOH

nanorods during the calcination process Some shorter rods were

also found in Figure 3c, which may be the rudiments of the

nanorods or the broken ones destroyed by high temperature

Figure 3e presents the corresponding electron diffraction pattern

of 300 °C calcined products (R-Fe2O3 nanorods); the shape

diffraction ring indicates the product is highly crystallized The

HRTEM image of a typical R-Fe2O3nanorod (Figure 3f) shows

regular lattice fringes with a spacing of 0.37 nm, which

corresponds to the (012) plane of R-Fe2O3

To investigate the formation processes of the R-FeOOH

nanorods and the porous R-Fe2O3 nanorods, time-dependent

experiments were carried out, and the resultant products were

investigated by TEM (see Figure S1 in the Supporting

Informa-tion) At a shorter reaction time of only 5 min, there are almost

no nanorods formed, and the average diameter of the

nanopar-ticles is about 5 nm As the reaction time increased to 10 min,

part of the nanoparticles began to combine with each other, and

the rodlike structure appeared Upon prolonging the reaction

time to 30 min, the products were totally transformed to rodlike

nanostructures If the reaction time was further increased to 2 h,

as seen in Figure 3, well-structured nanorods were obtained,

and the length of the nanorods increased with the increase in

the reaction time On the basis of the above results, a growth

mechanism of the porous R-Fe2O3nanorods can be proposed

The schematic diagram of the process is described in Figure 4

In the first stage, the R-FeOOH crystal nucleus formed by the

reaction of Fe2+with O2and OH-produced by the hydrolysis

of CH3COO- Then these R-FeOOH particles further assembled

into rodlike structures by combining together with OH groups

Finally, the porous R-Fe2O3nanorods formed with the removal

of H2O after being calcined at 300°C in air The equations of the reactions in the synthetic process are as follows:

CH3COO-+H2O f CH3COOH + OH- (1) 4Fe2++8OH-+O2f4FeOOH + 2H2O (2) 2FeOOH f Fe2O3+H2O (3)

The porosity of the porous R-Fe2O3 nanorods was further confirmed by nitrogen adsorption-desorption analysis, and the results are shown in Figure 5 The isotherm indicates that the R-Fe2O3 nanorods have a porosity of type IV with a distinct

hysteresis loop in the range of 0.5-1.0 P/P0.36 The curve of pore size distribution of the porous R-Fe2O3nanorods is shown

in the inset figure The curve exhibits that the sample has relatively small pores with a size distribution of 1-10 nm and centered at 2 nm This is in good agreement with the TEM images Calculated by multipoint the BET method, the porous R-Fe2O3nanorods have a high surface area of 221.9 m2· g-1, whereas, the surface area of the R-Fe2O3nanoparticles is only 18.31 m2· g-1, which is reported in our previous work.37The high surface area of the porous R-Fe2O3 nanorods may be attributed to their tiny crystal size and porosity structure Prompted by the high specific surface area, we forecast that the sensor based on the as-prepared porous R-Fe2O3nanorods should have enhanced gas sensitivity

As an n-type semiconductor, one of the most important applications of R-Fe2O3material is in gas sensors It has been reported by many researchers that an R-Fe2O3sensor exhibits

an excellent gas-sensing property to some combustible or toxic gases.38-42It is generally accepted that the sensing mechanism

of the R-Fe2O3-based sensor belongs to the surface-controlled type The gas-sensing properties of an R-Fe2O3-based sensor are coherent with its surface area The higher the surface area the sensor has, the more test gas and oxygen molecules it adsorbs, and the better sensitivity it exhibits Therefore, the design of sensing materials with a high specific surface area should be useful for enhanced gas-sensing performance In addition, it has been demonstrated that a decrease in the size of

Figure 3 (a, b) TEM and HRTEM images of R-FeOOH nanorods.

(c, d) TEM and HRTEM images of porous R-Fe 2 O 3 nanorods (e) The

corresponding ED pattern and (f) HRTEM image of a single R-Fe 2 O 3

nanorod.

Figure 4 Schematic diagram of the formation mechanism of the

porous R-Fe 2 O 3 nanorods.

Figure 5 N2 adsorption-desorption isotherm and BJH pore-size distribution plot (inset) of porous R-Fe 2 O 3 nanorods.

the crystallites in the sensing layer can result in a considerable

increase in sensitivity.43Thus, the as-prepared porous R-Fe2O3

nanorods, which possess a tiny particle size (5 nm) and a high

surface area (221.9 m2· g-1), are expected to have a good

gas-sensing performance

Figure 6 illustrates the typical response-recovery

character-istics of the porous R-Fe2O3nanorods to ethanol vapor with

concentrations of 50, 100, 200, 500, and 1000 ppm The sensing

properties of R-Fe2O3 nanoparticles with an average particle

size of about 30 nm and a surface area of 18.31 m2/g, reported

in our previous work,37is also shown in Figure 6 for comparison

purposes It can be seen from Figure 6 that the response of the

sensor based on the porous R-Fe2O3nanorods increases

dramati-cally with the increase in the ethanol vapor concentration and

is much higher than that of the R-Fe2O3nanoparticles under

the same ethanol concentration This result indicates that the

gas-sensing property of the as-prepared porous R-Fe2O3

nano-rods is much better than that of the previously reported R-Fe2O3

nanoparticles A comparison study between the nanorods and

the nanoparticles in sensitivities to ethanol of different

concen-trations is shown in Table 1 From Table 1, we can see that the

sensitivities of the porous R-Fe2O3nanorods are almost several

decade times greater than that of R-Fe2O3nanoparticles for all

the ethanol vapor of different concentrations The gas sensitivity

is defined as the resistance ratio Rair/Rgas, where Rair and Rgas

are the electrical resistance for the sensor in air and in gas When

the sensor is in air, the surface of R-Fe2O3is covered by plenty

of oxygen adsorbates, such as O2-, O-, and O2- The formation

of the oxygen adsorbate layer leads to a decrease in the electron

density on the sensor surface due to the transfer of electrons

from the sensor surface to the adsorbate layer When the sensor

is exposed to ethanol vapor, the ethanol gas reacts with the

oxygen ions on the surface, which results in the release of free

electrons to the sensor This leads to the change in resistance

of the R-Fe2O3sensor The amount of oxygen and test gas on

the surface of materials is strongly dependent on the

micro-structure of the materials; namely, the specific area, particle size,

and the porosity The main reason for the above result is that

the conventional R-FeO nanoparticle sensor has a poor surface

area and a relatively large particle size, whereas the sensor based

on porous R-Fe2O3nanorods has a high surface area and tiny crystal size, which can provide more adsorption-desorption sites for gas molecules Moreover, the abundant pores on the surface

of the R-Fe2O3nanorods can facilitate the diffusion of the gas molecules and enable them to access all surfaces of the nano-particles contained in the sensing unit

As is known, response and recovery times, which are defined

as the time required to reach 90% of the final resistance, are the basic parameters for gas sensors It can also be seen from Figure 6 that the porous R-Fe2O3nanorod sensor still shows a short response/recovery time, even to high-concentration ethanol vapor, indicating a good response/recovery capability for prac-tical application

For practical use, the selectivity of the sensor is a necessary consideration Hence, we also examined the gas-sensing of the same sensor on the basis of the response of the porous R-Fe2O3

nanorods to methanol, NH3, H2S, H2, and CO The results are shown in Figure 7 It can be seen clearly from Figure 7 that the sensor exhibits the highest response to ethanol and very low responses to other gases In addition, the sensor was totally insensitive to CO and H2 According to the experimental results, the as-prepared porous R-Fe2O3nanorod sensor can selectively detect ethanol gas with the interference of other gases The effect of humidity on the gas sensitivity of the sensor has also been investigated The sensitivity of the sensor to ethanol at different relative humidities is shown in Figure S2

in the Supporting Information The result reveals that it is no problem for the sensor of porous R-Fe2O3nanorods to detect ethanol under 60% relative humidity Furthermore, the sensor exhibited a nearly constant response to ethanol under the same conditions, even after 6 months, illustrating the good reversibility

of the porous R-Fe2O3nanorod sensor

4 Conclusions

In summary, we have presented a facile route for preparing porous R-Fe2O3nanorods via a template-free solution approach

at low temperature (40°C) This method is feasible for large-scale industrial manufacture of porous hematite nanorods due

to the advantages of the simple production process, low cost, and environmental friendliness The as-prepared porous R-Fe2O3

nanorods have a tiny crystal size (5 nm) and a porosity structure, resulting in a high surface area of 221.9 m2· g-1 On the basis

of the experimental results, a possible growth mechanism of the porous R-Fe2O3 nanorods has been proposed The gas-sensing measurements demonstrated that the sensor based on

Figure 6. Responses of porous R-Fe 2 O 3 nanorods and R-Fe 2 O 3

nanoparticles to ethanol of different concentrations.

TABLE 1: Sensitivities of the Two Sensors to Ethanol of

Different Concentration

S to ethanol (ppm)

sample

SBET

porous R-Fe 2 O 3 nanorods 221.9 43.6 60.7 82.8 127.3 174.9

Figure 7 Sensitivities of porous R-Fe2 O 3 nanorods to various gases

of 50-1000 ppm.

porous R-Fe2O3nanorods exhibited a much higher sensitivity

to ethanol vapor than the sensor based on R-Fe2O3nanoparticles

This is possibly due to the fact that the porous R-Fe2O3nanorods

have a high surface area and plentiful pores to adsorb and react

with gas molecules Moreover, the sensor also presented

ex-cellent selectivity to ethanol and good stability for a rather long

time (6 months) Hence, it is expected that this facile route

prepared porous R-Fe2O3nanorods would be an ideal candidate

for applications in ethanol sensors Other properties and

app-lications, such as catalysts and fuel cells, may also be found

Acknowledgment The authors thank the National Nature

Science Foundation of China (20871071), the 973 Program

(2005CB623607), and the Applied Basic Research Programs

of Science and Technology Commission Foundation of Tianjin

(08JCYBJC00100) for financial support

Supporting Information Available: TEM images of the

products obtained at different reaction times and the sensitivities

of porous R-Fe2O3 nanorods to ethanol at different relative

humidity This material is available free of charge via the

Internet at http://pubs.acs.org

References and Notes

(1) Poizot, P.; Laruelle, S.; Grugeon, S.; Dupont, L.; Tarascon, J M.

Nature 2000, 407, 496.

(2) Kong, J.; Franklin, N R.; Zhou, C W.; Chapline, M G.; Peng, S.;

Cho, K.; Dai, H J Science 2000, 287, 622.

(3) Hamley, I W Nanotechnology 2003, 14, 39.

(4) Wan, Y.; Shi, Y F.; Zhao, D Y Chem Comm 2007, 897.

(5) Ding, Y.; Gao, P X.; Wang, Z L J Am Chem Soc 2004, 126,

2066.

(6) Liu, J F.; Wang, X.; Peng, Q.; Li, Y D AdV Mater 2005, 17,

764.

(7) Pan, Z W.; Dai, Z R.; Wang, Z L Science 2001, 291, 1947.

(8) Wang, X.; Li, Y D J Am Chem Soc 2002, 124, 2880.

(9) Zhang, Y P.; Chu, Y.; Dong, L H Nanotechnology 2007, 18,

435608.

(10) Weiss, W.; Zsccherpel, D.; Schlogl, R Catal Lett 1998, 52, 215.

(11) Fukazawa, M.; Matuzaki, H.; Hara, K Sens Actuators, B 1993,

13, 521.

(12) Neri, G.; Bonavita, A.; Galvagno, S.; Siciliano, P.; Capone, S Sens.

Actuators, B 2001, 82, 40.

(13) Bondioli, F.; Ferrari, A M.; Leonelli, C.; Manfredini, T Mater.

Res Bull 1998, 33, 723.

(14) Mitra, S.; Das, S.; Mandal, K.; Chaudhuri, S Nanotechnology 2007,

18, 275608.

(15) Feldmann, C AdV Mater 2001, 13, 1301.

(16) Zeng, S Y.; Tang, K B.; Li, T W.; Liang, Z H.; Wang, D.; Wang,

Y K; Zhou, W W J Phys Chem C 2007, 111, 10217.

(17) Liu, X M.; Fu, S Y.; Xiao, H M.; Huang, C J J Solid State

Chem 2005, 178, 2798.

(18) Yan, X H.; Liu, G J.; Liu, F T.; Tang, B Z.; Peng, H.; Pakhomov,

A B.; Wang, C Y Angew Chem., Int Ed 2001, 40, 3593.

(19) Zhu, Y.; Zhang, J C.; Zhai, J.; Jiang, L Thin Solid Films 2006,

510, 271.

(20) Jing, Z H.; Wu, S H Mater Lett 2004, 58, 3637.

(21) Woo, K.; Lee, H L.; Ahn, J P.; Park, Y S AdV Mater 2003, 15,

1761.

(22) Chueh, Y L.; Lai, M W.; Liang, J Q.; Chou, L J.; Wang, Z L.

AdV Funct Mater 2006, 16, 2243.

(23) Fu, Y Y.; Chen, J.; Zhang, H Chem Phys.Lett 2001, 350, 459.

(24) Wang, H Z.; Zhang, X T.; Liu, B.; Zhan, H L.; Li, Y C.; Huang,

Y B.; Du, Z L Chem Lett 2005, 34, 184.

(25) Wen, X G.; Wang, S H.; Ding, Y.; Wang, Z L.; Yang, S H J.

Phys Chem B 2005, 109, 215.

(26) Liu, L.; Kou, H Z.; Mo, W L.; Lin, H J.; Wang, Y Q J Phys.

Chem B 2006, 110, 15223.

(27) Archibald, D D.; Mann, S Nature 1993, 364, 430.

(28) Wu, C Z.; Yin, P.; Zhu, X.; Ouyang, C Z.; Xie, Y J Phys Chem.

B 2006, 170, 17806.

(29) Tang, B.; Wang, G L.; Zhuo, L H.; Ge, J C.; Cui, L J Inorg.

Chem 2006, 45, 5196.

(30) Chen, J.; Xu, L N.; Li, W Y.; Gou, X L AdV Mater 2005, 17,

582.

(31) Sun, Z Y.; Yuan, H Q.; Liu, Z M.; Han, B X.; Zhang, X R.

AdV Mater 2005, 17, 2993.

(32) Steinhart, M.; Wendorff, J H.; Greiner, A.; Wehrspohn, R B.

Science 2002, 296, 1997.

(33) Schuth, F Angew Chem., Int Ed 2003, 42, 3604.

(34) Wang, Y.; Wang, S R.; Zhao, Y Q.; Zhu, B L.; Kong, F H.;

Wang, D.; Wu, S H.; Huang, W P.; Zhang, S M Sens Actuators, B 2007,

125, 79.

(35) Wang, Y.; Kong, F H.; Zhu, B L.; Wang, S R.; Wu, S H.; Huang,

W P Mater Sci Eng., B 2007, 140, 98.

(36) Brunauer, S.; Deming, L.; Deming, W.; Teller, E J Am Chem.

Soc 1940, 62, 1723.

(37) Wang, Y.; Wang, Y M.; Cao, J L.; Kong, F H.; Xia, H J.; Zhang,

J.; Zhu, B L.; Wang, S R.; Wu, S H Sens Actuators, B 2008, 131, 183.

(38) Huo, L H.; Li, Q.; Zhao, H.; Yu, L J.; Gao, S.; Zhao, J G Sens.

Actuators, B 2005, 107, 915.

(39) Tan, O K.; Cao, W.; Zhu, W.; Chai, J W.; Pan, J S Sens.

Actuators, B 2003, 93, 396.

(40) Dai, G R Sens Actuators, B 1998, 53, 8.

(41) Malyshev, V V.; Eryshkin, A V.; Koltypin, E A.; Varfolomeev,

E A.; Vasiliev, A A Sens Actuators, B 1994, 19, 434.

(42) Poghossian, A S.; Abovian, H V.; Aroutiounian, V M Sens.

Actuators, B 1994, 18, 155.

(43) Yamazoe, N Sens Actuators, B 1991, 5, 7.

JP806430F