synthesis of hematite (r-fe2o3) nanorods diameter-size and shape effects on their

Synthesis of Hematite r-Fe2O3 Nanorods: Diameter-Size and Shape Effects on TheirApplications in Magnetism, Lithium Ion Battery, and Gas Sensors Changzheng Wu, Ping Yin, Xi Zhu, Chuanzi O

Synthesis of Hematite (r-Fe2O3) Nanorods: Diameter-Size and Shape Effects on Their

Applications in Magnetism, Lithium Ion Battery, and Gas Sensors

Changzheng Wu, Ping Yin, Xi Zhu, Chuanzi OuYang, and Yi Xie*

Department of Nano-materials and Nano-chemistry, Hefei National Laboratory for Physical Sciences at Microscale, UniVersity of Science & Technology of China, Hefei, Anhui 230026, China

ReceiVed: June 1, 2006; In Final Form: July 16, 2006

We demonstrated in this paper the shape-controlled synthesis of hematite (R-Fe2O3) nanostructures with a gradient in the diameters (from less than 20 nm to larger than 300 nm) and surface areas (from 5.9 to 52.3

m2/g) through an improved synthetic strategy by adopting a high concentration of inorganic salts and high temperature in the synthesis systems to influence the final products of hematite nanostructures The benefits

of the present work also stem from the first report on the <20-nm-diameter and porous hematite nanorods,

as well as a new facile strategy to the less-than-20-nm nanorods, because the less-than-20-nm diameter size meets the vital size domain for magnetization properties in hematite Note that the porous and nonporous hematite one-dimensional nanostructures with diameter gradients give us the first opportunity to investigate the Morin temperature evolution of nanorod diameter and porosity Evidently, the magnetic properties for nanorods exhibit differences compared with those for the spherical particle counterparts Hematite nanorods are strongly dependent on their diameter size and porosity, where the magnetization is not sensitive to the size evolution from submicron particles to the 60-90 nm nanorods, while the magnetic properties change significantly in the case of <20 nm In other words, for the magnetic properties of nanorods, in a comparable size range, the porous existence could also influence the magnetic behavior Moreover, applications in formaldehyde (HCHO) gas sensors and lithium batteries for the hematite nanostructures with the diameter/ surface area gradient reveal that the performance of electrochemical and gas-sensor properties strongly depends

on the diameter size and Brunauer-Emmett-Teller (BET) surface areas, which is consistent with the crystalline point of view Thus, this work not only provides the first example of the fabrication of hematite nanostructure sensors for detecting HCHO gas, but also reveals that the surface area or diameter size of hematite nanorods can also influence the lithium intercalation performances These results give us a guideline for the study of the size-dependent properties for functional materials as well as further applications for magnetic materials, lithium-ion batteries, and gas sensors

1 Introduction

Developing new methods for the preparation of nanomaterials

as well as the modification of their size, morphology, and

porosity, has been intensively pursued not only for their

fundamental scientific interest but also for many technological

applications Nanoparticles (zero-dimensional (0-D)) and

nano-wires/nanorods (one-dimensional (1-D)) with controlled size and

shape are of key importance because their electrical, optical,

and magnetic properties strongly depend on their size and

shape.1Hematite (R-Fe2O3), the most stable iron oxide, with

n-type semiconducting properties under ambient conditions, is

of scientific and technological importance because of its usage

in catalysts, pigments, magnetic materials, gas sensors, and

lithium-ion batteries.2Its size and shape effect on corresponding

properties has attracted much attention For example, the Morin

transition temperature (TM) of R-Fe2O3 nanoparticles (0-D)

decreases with decreasing spherical particle size according to a

1/d dependence.3 Additionally, 1-D R-Fe2O3 nanostructures,

such as nanorods,4nanowires,5nanobelts,6and nanotubes7have

also been synthesized and used for investigating their peculiar properties For example, Woo et al synthesized R-Fe2O3

nanorods by a sol-gel mediated reaction of ubiquitous Fe3+

ions in reverse micelles.8Zhang et al managed to grow R-Fe2O3

nanowires out of the oxidized surface of iron substrates.9

Recently, R-Fe2O3hollow nanowires with outer diameters of

ca 50 nm have been synthesized through a vacuum-pyrolysis route fromβ-FeOOH nanowires.10Nevertheless, it still remains

a challenge to develop simple and versatile approaches to synthesize 1-D nanostructures of R-Fe2O3with slimmer diam-eters, which will then facilitate our understanding of the shape-and size-dependent properties of R-Fe2O3

Since the Morin temperature of R-Fe2O3spherical particles was found to be strongly dependent on particle size and tends

to disappear (<5 K) below a diameter of 8-20 nm,11 their counterpart nanorods/nanowires with a diameter of <20 nm are then significantly necessary for further understanding of the magnetic properties Currently, because of limited studies on

R-Fe2O3nanorods/nanowires with a diameter of <20 nm and because their <20 nm and porous nanorods have not been obtained so far, their subsequent applications in magnetization fields and investigations on the size-dependent properties of iron oxides are significantly delayed Herein, we demonstrate the synthesis of R-Fe2O3nanorods with a gradient in the surface

* Corresponding author Address: Department of Nano-materials and

Nano-chemistry Hefei National Laboratory for Physical Sciences at

Microscale, University of Science and Technology of China, Hefei, Anhui

230026, P R China Tel: 86-551-3603987 Fax: 86-551-3603987

E-mail: yxie@ustc.edu.cn.

10.1021/jp0633906 CCC: $33.50 © 2006 American Chemical Society

Published on Web 08/19/2006

areas and diameter sizes via an improved strategy: first, by

adopting high-concentration salts and high temperature, the

as-produced R- orβ-FeOOH nanorods are slimmer than usual in

our synthetic system Then with the influence of inorganic salt

ions, the samples pyrolyze at a slower rate, and then the

corresponding well-defined hematite nanostructures can be

achieved Note that the hematite crystal structure is a

rhombo-hedrally centered hexagonal structure of the corundum type with

a closed-packed lattice; there are no tunnels or interlayer spacing

for accommodating the inorganic ions any more, as is the case

for R-FeOOH orβ-FeOOH crystal structures (see Supporting

Information) Therefore, the inorganic ions can be easily

removed after calcining, and the pure hematite (R-Fe2O3) could

be obtained after rinsing with water Here, the hematite

morphology with different sizes and shapes could be well

controlled by simply choosing different kinds of inorganic salts

This work presents not only a new strategy to produce R-Fe2O3

nanorods with diameters of <20 nm, but also the first report

on the synthesis of porous R-Fe2O3nanorods with diameters of

<20 nm Evidently, the magnetic properties were strongly

dependent on the size of their diameter and the porosity in the

present work, while the lithium intercalation and HCHO gas

sensor properties were significantly dependent on the surface

area Therefore, the present work provides not only the first

example of investigating the magnetic property evolution of

nanorods/nanowire diameters and porosity, but also the first

example of the fabrication of hematite nanostructure sensors

for detecting HCHO gas

2 Experimental Section

To prepare FeOOH nanostructure precursors, 50 mL of 0.06

M iron chloride (FeCl3) aqueous solution, with/without the

addition of 0.300 mol of inorganic salts (NH4Cl, KCl, and Na2

-SO4) was put in a conical flask and stirred with a magnetic

stirrer for 30 min The homogeneous solution was then

transferred into a 60 mL Teflon-lined stainless steel autoclave,

sealed, and then heated to 120°C After the autoclaves were

maintained at 120 °C for 12 h, the resulting yellow product

was centrifuged, rinsed with distilled water, and finally dried

at 40°C in a vacuum The obtained yellow solid products were

collected for the following experiments and characterization

To prepare hematite nanostructures, the as-prepared FeOOH

nanostructures were heated to 520°C with a ramping rate of

10 °C min-1 and then maintained at 520 °C for 8 h The

decomposition was performed in air, and the synthetic conditions

are summarized in Table 1, where the as-obtained R-Fe2O3

nanostructures have been named S1, S2, S3, and S4 The

as-collected R-Fe2O3products were rinsed with distilled water and

finally dried at 40°C in a vacuum

The samples of as-prepared FeOOH and R-Fe2O3

nanostruc-tures were characterized by X-ray powder diffraction (XRD)

with a Philips X’Pert Pro Super diffractometer with Cu KR

radiation (λ ) 1.54178 Å) The transmission electron

micros-copy (TEM) images for both FeOOH and R-Fe2O3 were

obtained on a Hitachi Model H-800 instrument with a tungsten

filament at an accelerating voltage of 200 kV The selected-area electron diffraction patterns and high-resolution transmis-sion electron microscopy (HRTEM) images were obtained on

a JEOL-2010 TEM at an acceleration voltage of 200 kV The porosity and adsorption performance of R-Fe2O3 were deter-mined via a Micromeritics ASAP-2000 nitrogen adsorption apparatus The magnetic properties of R-Fe2O3were measured using a vibrating sample magnetometer and superconducting

quantum interference device The performance of the R-Fe2O3

as a cathode was evaluated using a Teflon cell with a lithium metal anode The cathode was a mixture ofβ-FeOOH/acetylene

black/poly(vinylidene fluoride) with a weight ratio of 85/10/5 The electrolyte was 1 M LiPF6in a 1:1 mixture of ethylene carbonate/diethyl carbonate, and the separator was Celgard 2500 The cell was assembled in a glovebox filled with highly pure argon gas (O2and H2O levels < 5 ppm) A galvanostatic charge/ discharge experiment was performed between 3.0 and 0.5 V at

a current density of 0.2 mA cm-2 Gas sensing measurements were performed with a WS-30A system (Weisheng Instruments Co., Zhengzhou, China), and the system integral error for the WS-30A system was less than

(1% The sensors of the as-prepared samples were fabricated

on ceramic tubes with the connection of gold electrodes that were connected by four platinum wires Here, the sensor structure and the testing principle were similar to that for previous reports.12In this case, the mixture of the as-prepared hematite nanomaterials and ethanol was coated as a thin film spanning across the two Au electrodes After drying at 150°C for 2 h in air to improve stability, the electrical contact was made through connecting the four platinum wires with the instrument base by silver paste Before analysis, the sensors-settled chamber was kept under a continuous flow of fresh air for 30 min During the measurement, the sensors were hosted

in a closed plastic tube equipped with appropriate inlets and outlets for gas flow A given amount of formaldehyde (HCHO) was injected into the chamber by a microinjector The sensitivity could be measured when the detecting gas was mixed with air

homogeneously Here, the response magnitude, S, is defined as

Rs(air)/Rs(gas), where Rs(air) and Rs(gas) are the resistance of the sensor in clean air and in detected gas, respectively

3 Results and Discussion 3.1 Morphology, Characterization, and Formation Mech-anism of the As-Obtained Hematite Nanostructures The

hematite nanostructures could be originated from the well-controlled FeOOH nanostructure precursors (see Supporting Information), and all the synthetic conditions are summarized

in Table 1 Furthermore, the phase and morphology information for the as-obtained products are revealed by Figure 1, where panels c, f, i, and m are the corresponding XRD patterns for panels a-b, d-e, g-h, and j-l, respectively All the XRD patterns in Figure 1 show characteristics of pure hexagonal

R-Fe2O3(JCPDS card 33-664, a ) 5.035 Å and c ) 13.74 Å).

No characteristic peak was observed for other impurities such

as β-FeOOH, FeO, γ-FeO, and other inorganic ions As

TABLE 1: Synthetic Conditions for Different r-Fe2O3 Nanostructures.

R-Fe 2 O 3

average diameter (nm)

reaction condition for FeOOH precursors S1 submicron particles 300-500 direct hydrolysis systems

porosity: 20-50

FeCl 3 -KCl systems

porosity: 2-16

FeCl 3 -NH 4 Cl systems

is shown in Figure 1a,b, the appearance of S1 is the

hematite submicrometer particles with the diameter range of

300-500 nm, and no hollow structures were found even from

the amplified TEM image for the direct hydrolysis system The

calcined products (S2) from the FeCl3-KCl system have the

regular pores (20-50 nm) distributed along the hematite

nanorods with a diameter-size range of 60-90 nm, as shown

in Figure 1d,e The calcining products (S3) obtained in the

FeCl3-Na2SO4system were mostly solid nanorods with

diam-eters of <15 nm, as shown in Figure 1g The HRTEM image

of a single-crystalline solid nanorod in Figure 1h shows a clear

interplanar distance of 0.25 nm, matching well with the d110

spacing of pure hexagonal hematite Figure 1j displays that heating the products prepared in the FeCl3-NH4Cl system yielded the nanoporous crystalline R-Fe2O3nanostructures (S4) without altering the morphology of the 1-D and even the nanorod bundles, and many holes with sizes of 8-30 nm form

in the nanorods (Figure 1k) The TEM image with higher magnification shows that the porous appearance can be clearly observed in all the visible nanorods, with uniform pores of <10

Figure 1 Representative TEM and HRTEM images of the as-obtained R-Fe2O 3 nanostructures with different diameter sizes for S1 (a-b), S2 (d-e), S3 (g-h), and S4 (j-l) The corresponding XRD patterns for the samples of S1, S2, S3, and S4 are shown in panels c, f, i, and m, respectively (n) The magnified (110) peaks for these four samples, where the (110) peak becomes narrower in the sequence of S4-S1, revealing the size evolution.

nm (Figure 1l) Evidently, the above TEM images were

consistent with the analysis of the magnified (110) peak, as

shown in Figure 1n, where the (110) peak becomes narrower

in the sequence of S4-S1, and the sharper peaks for S1 indicate

its good crystallinity and greater grain size than the other three

products (S2-S4) whose precursors grew under the control On

the basis of the combination analysis of TEM images and XRD

patterns, the as-obtained R-Fe2O3nanostructures possess

diameter-sized gradients in the sequence from S4 to S1 with increasing

sizes

As described above, the systems with the addition of inorganic

salts retained the morphology of the FeOOH precursors, and

regular nanopores formed along the nanorods, while the direct

hydrolysis system produced only particles under air conditions

The phenomenon can be explained by the difference in

thermalstability behavior based on DrTGA, as shown in Figure

2 From DrTGA curves, one can see that there is a broad

exothermal peak with a coexistence of feeble shoulder peaks

in the temperature range of 200-400°C for each of S2-S4,

showing that their weight loss is much more lagged The

intensity of the exothermal peak for S1 (Figure 2a) without salt

addition in the temperature range of 200-400°C is evidently

stronger than the corresponding peaks for the other three samples

with the addition of high-concentration salt ions, indicating that

the weight loss is much quicker Compared with the direct

hydrolysis systems (S1), the final products of FeOOH in the

other three systems will possess more ions to bind with the

tunnel structures or the surface sites of R- orβ-FeOOH crystal,

and the total amounts of the residual ions that can be removed

in the calcination process13are too small to be reflected by the

XRD pattern Thus, the small amount of residual inorganic salt

ions seems to be responsible for the formation of hollow

structures in our synthetic conditions

In fact, the addition of inorganic salt ions enables the samples

to pyrolyze at a slower rate As for the products ofβ-FeOOH

with the absence of adequate ions in the direct hydrolysis

systems, this high-rate pyrolysis process will produce too much

energy in a short time to be effectively released from the

systems, as indicated by a stronger peak in the curve Most of

the energy was adsorbed in the systems, resulting in the collapse

of the nanorods to form larger aggregated particles, as shown

in Figure 1a,b, whereas, for the inorganic salt systems, the

existence of salt ions binding with the FeOOH precursors impedes the samples to pyrolyze at the higher rates, and the as-produced energy could have enough time to be effectively released from the systems, and the final products of R-Fe2O3

obtained possess the regular pores and the morphology remi-niscent of their precursors (β-FeOOH) In a word, the pyrolysis

rates of β-FeOOH seem to be a significantly influencing

parameter for the formation of porous R-Fe2O3and reminiscent

of the orientation-ordered nanostructures for R-Fe2O3 in air conditions based on the combined analysis of DrTGA and TEM results

Additionally, it is interesting that when the precursors are prepared in high-concentration Cl-ions, the calcined products have the appearance of porosity (S2 and S4), while in high-concentration SO42-, S3 only has the solid appearance These results indicate that the existence of large amounts of Cl-ions held in the tunnels inβ-FeOOH might favor the appearance of

porosity in the calcined products such as S2 and S4 However, the high-concentration large anions such as SO42-, which existed

in the surface sites,14 might favor the formation of a solid morphology for S3

3.2 BET Surface Areas of r-Fe 2 O 3 Nanostructures The

Brunauer-Emmett-Teller (BET) surface areas of S1 and S2 were found to be 5.9 and 15.8 m2/g, respectively, by calculating from the results of N2adsorption The BET values of S3 and S4 show relatively higher surface areas of 32.5 and 52.3 m2/g, respectively Considering the factors that affect specific surface area, we can conclude that, in a comparable size range, it is the pores that increase specific surface area, according to the comparison between S3 and S4, whereas, when there is a wide size discrepancy, it is the size that determines the specific surface area, according to the comparison between S2 and S3 It is worth

noting that the as-obtained R-Fe2O3 nanostructures with the gradient in BET surface area and diameter size, provide a fine example to study the size-dependent properties of magnetization, lithium batteries, and gas sensors

3.3 Magnetic Properties for r-Fe 2 O 3 Nanostructures.

Owing to their gradient in the BET surface area and the diameter

size, the magnetic behavior of as-obtained R-Fe2O3 nanostruc-tures, which is of importance for practical applications, was investigated for samples S1-S4 Figure 3 shows the curves for the temperature dependence of zero-field-cooled (ZFC) and field-cooled (FC) magnetizations from 4 to 300 K, under an applied field of 500 Oe The insets are the corresponding differential ZFC curves

As for the submicron solid particle sample S1, the FC and ZFC magnetization curves overlap in the entire concerned temperature range, as shown in Figure 3a, displaying the characteristic behavior for R-Fe2O3 with a Morin transition

temperature (TM) of 255 K, which is determined by the sharp peak in the differential ZFC curve (inset in Figure 3a) Normally, bulk hematite has a Morin transition from the low-temperature antiferromagnetic phase to a weakly ferromagnetic phase at 263

K.15 Here, the TM value for the submicron R-Fe2O3 solid particles is approaching that for bulk hematite As for the sample with 60-90 nm nanorods with porosities of 20-50 nm (S2),

as shown in Figure 3b, the characteristics of the ZFC and FC magnetization curves show the same trend as those of S1, except that these two curves slightly split in the temperature ranges of 4-100 and 260-300 K Notably, the Morin transition temper-ature of S2 remains 255 K, showing no change compared to that of S1, which indicates that the magnetization is not sensitive

to the size evolution from submicron particles to the 60-90

nm nanorods As for the 5-16 nm nanorods (S3), as shown in

Figure 2 DrTGA curves for the samples of the FeOOH precursors

obtained by the direct hydrolysis system (a), the FeCl 3 -KCl system

(b), the FeCl 3 -Na 2 SO 4 system (c), and the FeCl-NH 4 Cl system (d) at

heating rate of 10 ° C min-1, from which the calcining influence

parameters for the formation of S1, S2, S3, and S4 could be discussed,

respectively The relation between these system and the final products

of S1, S2, S3, and S4 mentioned in this work can be seen in Table 1.

Figure 3c, the FC and ZFC magnetization curves split

signifi-cantly; the FC magnetization rises significantly, while the ZFC

curve decreases slowly The split between the FC and ZFC

curves reflects the existence of a large size distribution of

magnetic units resulting from the decrease in effective size,

whose moments block progressively with decreasing

tempera-ture.16Additionally, the Morin transition temperature for S3 (235

K) is lower than that for S1 and S2, which may be related to

the decrease in diameters for 1-D nanohematite, agreeing with

the theory that TMdecreases with decreasing particle size Since

the temperature of the overlap point is much higher than 100

K, the character of this curve should stem from a pilling center

effect, rather than spin glass freezing.17As for the sample with

5-19 nm nanorods with porosities of 2-16 nm (S4) in Figure

3d, the FC and ZFC magnetization curves split significantly,

and the Morin transition disappears in the concerned temperature

range of 4-300 K, indicating that the porous nanorods with

diameters less than 20 nm exhibit no Morin transition

In summary, first, although our nanorods are grown

prefer-entially along (110) as single crystalline, the wire diameter

greatly restrains the maximal volume of the domain Evidently,

TMdecreased to 235 K when the diameters are less than 20 nm

for S3, while it seems to be insensitive to the size effect in the

case of diameters larger than 60 nm (S1 and S2) Second, the

experimental results show that the decreasing diameter of

nanorods could lead to the split in the ZC and ZFC

magnetiza-tion curves, which may result from the decrease in effective

size, whose moments block progressively with decreasing

temperature Third, the porosity could also influence the

magnetic behavior in a comparable size range For example,

for the case of S3 and S4, the porous product of S4 exhibits

no Morin transition in the concerned temperature range of 4-300 K

3.4 r-Fe 2 O 3 Nanostructures in a Lithium-Ion Battery It

is found that the lithium intercalation performance is related to the intrinsic crystal structure, where the lithium ions can intercalate into the interlayer, the tunnels, and the holes in the crystal structure.18As for the hematite crystal structure, each

Fe atom is surrounded by six O atoms, whereas each O atom is bound to four Fe atoms in a typical hematite crystal unit A hematite crystal has a rhombohedrally centered hexagonal structure of the corundum type with a closed-packed lattice in which two-thirds of the octahedral sites are occupied by Fe3+

ions (see Supporting Information) As seen from the hematite structure along [001], [100], and [110], there are no interlayer spacings and tunnels through the crystal structure (See Sup-porting Information) Upon careful observation of the hematite surface structure, the holes could be observed in the first octahedral layer projected along [001] and [100]; however, the tunnels could not be seen as the layer number was increased,

as shown in Figure 4 That is to say, holes existed in the surface hematite crystal, which allowed foreign atoms or molecules to

be introduced, for example, Li+ions When the introduction of lithium ions to the holes in the hematite surface is concerned,

it gives us the impression that the lithium intercalation perfor-mance will improve by increasing the surface area or the porosity of the hematite crystals Therefore, the synthesis of hematite nanocrystals with higher surface area or porosity structures is much needed because of the intercalation capacities and affinities for Li+to the more exposed holes in the hematite

Figure 3 Temperature dependence of ZFC and FC magnetization for an applied field of 500 Oe for S1 (a), S2 (b), S3 (c), and S4 (d) Insets are

their corresponding differential ZFC curves.

surface with higher surface area, which could then shorten the

diffusion length of lithium ions.19

Evidently, the electrochemical performance of lithium ions

strongly depends on the diameter size and BET surface areas,

which agrees with the above considerations As mentioned

above, the sample possesses a surface area with the sequence

of S4 > S3 > S2 > S1 The electrochemical performance of

the as-prepared hematite R-Fe2O3samples of S1-S4 in the cell

configuration of Li/R-Fe2O3 was evaluated Figure 5a shows

the comparison discharge curves for the concerned four samples

of S1-S4 on the first cycle with a cutoff voltage of 0.6 V at a

current density of 0.2 mA cm-2, which is similar to that of the

R-Fe2O3particles.20The S4 electrode exhibited a high discharge

capacity of 1151 mAh/g, corresponding to 6.8 Li per R-Fe2O3,

while the S3, S2, and S1 electrodes exhibited 1088, 981, and

894 mAh/g, corresponding to 6.5, 5.8, and 5.3 Li per R-Fe2O3,

respectively According to the results presented above, it is

evident that the electrochemical properties of the first discharge capacity possess the sequence of S4 > S3 > S2 > S1, which

is consistent with that of the surface areas for the as-obtained

R-Fe2O3nanostructures in this case

3.5 The r-Fe 2 O 3 Nanostructures in Formaldehyde (HCHO) Gas Sensors As a toxic chemical component to our health,

formaldehyde (HCHO) widely exists in building materials and

in the combustion gas of organic materials Thus, finding a way

to fabricate effective sensors for detecting the existence of

HCHO is much needed R-Fe2O3, an n-type semiconductor with

an electrical conductivity highly sensitive to gaseous environ-ments, has been used as a sensor for ethanol and H2.21Inspired

by this, we suspected that the as-obtained R-Fe2O3with different BET surface areas should also be useful for the fabrication of the HCHO sensors From the crystalline point of view, there are no interlayer spacings and tunnels through the crystal structure, revealing that increasing the surface area could then produce more activity sites for the HCHO sensors The gas-sensing characteristics of the as-obtained products from S1 to S4 in response to HCHO are shown in Figure 6, in which the curves are the plot of the gas sensitivity versus HCHO

concentration The gas sensitivity, Sg, is defined as Rair/Rgas,

where Rairand Rgasare the electrical resistances for sensors in air and in gas.22 Although the sensitivity of all the FeO

Figure 4 Schematic hematite structure projected along either [001]

(a) or [100] (b), where holes can be observed in the first octahedral

layer No tunnels can be found as the layer number is increased.

Figure 5 First charge-discharge curves of hematite (R-Fe2 O 3 ) samples (S1-S4) at a current density of 0.2 mA cm-2.

Figure 6. Room-temperature sensor sensitivity to formaldehyde (HCHO) of the as-prepared hematite (R-Fe 2 O 3 ) nanostructures for S1 (a), S2 (b), S3 (c), and S4 (d).

nanostructures (S1-S4) gradually increases with an increase

in HCHO gas concentration, as indicated in Figure 6, it can be

seen that the sensitivity of the as-obtained Fe2O3nanostructures

follows the sequence S4 > S3 > S2 > S1 under a given HCHO

concentration and testing temperature Notably, this sensitivity

sequence is consistent with that for the BET surface area,

indicating the sensitivity for the nanostructures is coherent with

its corresponding surface area These results verify the generally

accepted opinion that, for R-Fe2O3-based sensors, the change

in resistance is mainly caused by the adsorption and desorption

of gas molecules on the surface of the sensing structure For

example, the superior sensing properties for S4 could be due to

its porous structure associated with the small grain size, which

enables HCHO gas to access more surfaces of the

porous-nanorod structures contained in the sensing unit Therefore, the

higher surface area for the R-Fe2O3nanostructure provides more

chances to adsorb and desorb HCHO gas molecules, thus leading

to higher sensitivity at room temperature This will give us a

guideline to devise the R-Fe2O3 sensors for detecting the

concentration of HCHO gas, which is certainly scientifically

and technically interesting

4 Conclusions

In summary, we have described in this paper the

shape-controlled synthesis of hematite (R-Fe2O3) nanostructures with

a gradient in the diameters (from less than 20 nm to larger than

300 nm) and surface areas (from 5.9 to 52.3 m2/g) through an

improved synthetic strategy The benefits of the present work

also stem from the first report on the porous hematite nanorods

with diameters of <20 nm, as well as a new facile strategy to

the less-than-20-nm nanorods, because the less-than-20-nm

diameter meets the vital size domain for magnetization

proper-ties in hematite Here, the first systematic investigation on the

Morin temperature evolution of nanorod/nanowire diameter or

porosity found that hematite nanorods are strongly dependent

on the diameter size and porosity of the nanorod products The

magnetization is not sensitive to the size evolution from

submicron particles to 60-90 nm nanorods, while the magnetic

properties change significantly in the case of <20 nm nanorods

In other words, in a comparable size range, the porous existence

could also influence the magnetic behavior Moreover,

applica-tions in lithium battery and formaldehyde (HCHO) gas sensors

for the hematite nanostructures with diameter/surface area

gradients reveal that the performance of the electrochemical and

gas-sensor properties strongly depends on the BET surface areas,

which can be well understood by the crystalline analysis Note

that this work not only provides the first example of the

fabrication of hematite nanostructure sensors for detecting

HCHO gas, but also reveals that the nanorod diameter size or

porosity can also influence the lithium intercalation

perfor-mances Further work is under way to further study the size-dependent properties for other functional materials as well as further applications for magnetic materials, lithium-ion batteries, and gas sensors

Acknowledgment This work was financially supported by

the National Natural Science Foundation of China (No 20321101) and the state key project of fundamental research for nanoma-terials and nanostructures (2005CB623601)

Supporting Information Available: Crystal structural

analy-sis, syntheanaly-sis, characterization, and discussion about the forma-tion mechanism for R- and β-FeOOH, as well as the crystal

structural analysis for R-Fe2O3 This material is available free

of charge via the Internet at http://pubs.acs.org

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