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Highly aligned SnO2 nanorods on graphene sheets for gas sensors†

Zhenyu Zhang,‡ Rujia Zou,‡ Guosheng Song, Li Yu, Zhigang Chen and Junqing Hu*

Received 28th June 2011, Accepted 15th August 2011

DOI: 10.1039/c1jm12987b

Highly aligned SnO2 nanorods on graphene 3-D array structures were synthesized by a straightforward

nanocrystal-seeds-directing hydrothermal method. The diameter and density of the nanorods grown on

the graphene can be easily tuned as required by varying the seeding concentration and temperature. The

array structures were used as gas sensors and exhibit improved sensing performances to a series of gases

in comparison to that of SnO2 nanorod flowers. For nanorod arrays of optimal diameter and

distribution, these structures were proved to exert an enhanced sensitivity to reductive gases (especially

H2S), which was twice as high as that obtained using SnO2 nanorod flowers. The improved sensing

properties are attributed to the synergism of the large surface area of SnO2 nanorod arrays and the

superior electronic characteristics of graphene.

Introduction

For gas sensing materials, considerable effort has been made to

achieve better sensitivity and higher selectivity towards low

concentrations of pollutant gases under low operating tempera-

tures. One-dimensional (1-D) nanomaterials of SnO2 have

attracted most attention for the detection of a range of harmful

gases due to the high mobility of their conducting electrons, and

to their good chemical and thermal stability under the operating

conditions of sensors.1,2 Nanocomposites can effectively improve

materials’ performances because of their potential to combine

the desirable properties of different nanoscale building blocks to

improve mechanical, chemical or electronic properties.3 So, in

order to obtain improve the sensitivity and selectively of sensing

materials, approaches, such as properly choosing dopants and

additives4–6 or modifying the SnO2 surface with catalysts,7–10

have been reported. However, these products are usually

randomly oriented and cannot be made into nanostructured

devices. Compared to polycrystalline films or crystalline nano-

wires, arrays are particularly advantageous as building blocks for

the fabrication of functional devices,11–13 because the oriented

geometry provides direct conduction paths for carriers to

transport from the junction to the external electrode.14–16

Attributing to their high surface-to-volume ratio and ordered

arrangement, 3-D array structures are preferable for the detec-

tion of pollutant gases.17 On the one hand, highly ordered N-type

semiconductor SnO2 nanorods provide larger effective surface

areas, which is of great benefit for gas diffusion and mass

State Key Laboratory for Modification of Chemical Fibers and PolymerMaterials, College of Materials Science and Engineering, DonghuaUniversity, Shanghai, 201620, China. E-mail: hu.junqing@dhu.edu.cn

† Electronic supplementary information (ESI) available: See DOI:10.1039/c1jm12987b

‡ These authors contributed equally to the work

17360 | J. Mater. Chem., 2011, 21, 17360–17365

transport in sensor materials.18 On the other hand, a conductive

substrate forms a Schottky contact with metal oxide nanorods,

which is favorable for the capture and migration of electrons

from the conduction band. Although there have been a few

successful syntheses of aligned 1-D SnO2 arrays on Si or

SiO2,19–21 metal or alloy substrates,22,23 these substrates are unfit

for gas sensing electronic devices due to a lack of sufficient

conductivity, toughness and chemical stability. Graphene is

a fascinating two-dimensional carbon material that is worth

evaluating for chemical sensing and biosensing due to its

outstanding physical and chemical properties.24 Graphene sheets

with extremely high mobility25 and high mechanical elasticity26

are an excellent material for conducting substrates of gas

sensors,27 which is beneficial for the fabrication of individual gas

sensor devices.

In this paper, we integrated 1-D SnO2 nanorods and 2-D

graphene sheets to produce heterogeneous 3-D array structures

(SnO2/G 3-D array structures) via a so-called ‘‘nanocrystal-seeds-

directing’’ hydrothermal approach. The diameter and density of

the SnO2 nanorods grown on the graphene sheets can be easily

tuned as required by varying the seeding concentration and

temperature. The novel array structures exhibit enhanced gas

sensing sensitivity to gaseous pollutants; different distributions

of these array structures for gas sensing were compared. It was

proved that enhanced sensitivity to H2S, over twice that obtained

with pure SnO2 nanorod flowers, was obtained using SnO2

nanorod arrays with moderate nanorod diameters and uniform

distribution, large surface area and appropriate pore size.

Experimental section

Material synthesis

All chemicals were purchased from Sinopharm Chemical

Reagent Co. (Shanghai, China) and were used without further

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purification. Graphene sheets were prepared by a chemical vapor

deposition (CVD) method,28 as described in detail in the ESI.†

The SnO2/G 3-D array structures were synthesised via two steps,

hydrolytically arranging nanocrystal-seeds on the graphene

sheets and submissive hydrothermal growth of the SnO2/G 3-D

array structures. In a typical synthesis, microgrammes of gra-

phene were ultrasonically dispersed into SnCl4 aqueous solution

to give a designated concentration (0.0025 M, 0.005 M, 0.05 M,

and 0.5 M). The suspension was hydrolyzed for 12 h, subse-

quently, the precipitate was centrifuged and dried or calcined at

a designated temperature (25 �C, 100 �C, 200 �C, and 300 �C).The precursor aqueous solution containing 0.1 M SnCl4, 1 M

NaOH, and 0.33 M SDS (sodium dodecyl sulfate) as surfactant

was stirred, forming an homogeneous solution. After the gra-

phene sheets bestrewed with SnO2 nanocrystal-seeds was injected

into the homogeneous solution, it was transferred into a Teflon-

lined autoclave (50 mL) with a stainless-steel shell, and the

reaction system was kept at 220 �C for 20 h and naturally cooled

to room temperature. The precipitate was washed with deionized

water and pure alcohol several times to remove any possible

residues.

Characterization

Powder X-ray diffraction experiments were conducted by

a D/max-2550 PC X-ray diffractometer (Rigaku, Japan). The

morphologies and structures of the products were characterized

by a field-emission scanning electron microscope (S-4800), and

a transmission electron microscope (JEM-2100F). Raman

spectra of the graphene sheets were tested by Raman spectros-

copy (LabRam-1B). The surface area, pore size, and pore-size

distribution of the products were determined by Brunauer–

Emmett–Teller (BET) nitrogen adsorption-desorption and

Barett–Joyner–Halenda (BJH) methods (Quantachrome, Auto-

sorb-1MP).

Fig. 1 (a) SEM image of SnO2/G 3-D array structures. (b) Higher

magnification SEM image view from overhead and lateral (inset). (c)

TEM image of an individual SnO2 nanorod. (d) HRTEM image of SnO2

nanorod showing the [001] growth direction; inset: the ED pattern along

the [110] axis. (e) XRD pattern of the product.

Gas sensor fabrication and response test

The gas sensing test was operated in an HW-30A measuring

system (Hanwei Electronics Co. Ltd., PR China). The products

calcined at 400 �C for 2 h were mixed with terpineol forming

a paste and then coated onto an alumina tube-like substrate with

a pair of Au electrodes on each end. A small Ni–Cr alloy coil was

placed through the tube as a heater to provide the working

temperature. In order to improve the long-term stability of the

sensors, the sensors were maintained at a working temperature

for several days. A stationary state gas distribution method was

carried out for gas response testing. Detected gases, such as H2S,

were injected into a test chamber and mixed with air (air

humidity: 37%). In the measurement electric circuit, a 1 MU load

resistor was connected in the series with the gas sensors. The

circuit voltage was 4.5 V, and the output voltage (Vout) was the

terminal voltage of the load resistor. The working temperature of

the sensors was adjusted by varying the heating voltage. In this

study, the optimal operating temperature (at which the highest

response value was exhibited) was selected to be 260 �C. Theresistance of the sensor in air or testing gas was measured by

monitoring the Vout. The gas response of the sensor in this paper

was defined as S ¼ Ra/Rg, where Ra and Rg were the resistance in

This journal is ª The Royal Society of Chemistry 2011

air and in the test gas, respectively. The response or recovery time

was estimated as the time taken for the sensor output to reach

90% of its saturation after applying or switching off the gas in

a step function.

Results and discussion

The characterization of the as-synthesized graphene sheets via

the CVD method, including Raman spectra, TEM images and

electron difference (ED) patterns, are given in Fig. S1.† The low-

magnification SEM image in Fig. 1a clearly shows the typical

morphology of the as-obtained product, and demonstrates the

large scale uniformity of the substrate with tens of micrometres

dimension. In Fig. 1b, high-magnification SEM images viewed

from overhead and laterally (inset) are presented, the narrow size

distribution of these nanorods with a square cross section as well

as the vertical and dense alignment in the substrate are clearly

shown. The density of the nanorods on the plane is statistically

counted to be ca. 285 mm�2. The width of the square cross section

was approximately 50 nm, and the axial length of the nanorods

was estimated to be in the range of 300–400 nm. The TEM image

(Fig. 1c) of an individual SnO2 nanorod shows that the length

and diameter of the nanorods were 300 nm and 50 nm, respec-

tively. It also indicates that the SnO2 nanorods have a smooth

surface and an obtuse angle top but not a flat top. The HRTEM

image of the SnO2 nanorod edge in Fig. 1d shows that the growth

direction is parallel to [001] crystalline orientation. As seen from

this image, the lattice fringes of the {001} and {�110} have

a d spacing of 0.32 nm and 0.335 nm, respectively, for the

tetragonal rutile structure SnO2 crystal. In the SAED pattern,

upper right inset, the spots can be indexed as the [110] zone axis.

The XRD pattern, Fig. 1e, reveals the crystal structure and phase

purity of the as-grown products. All of the diffraction peaks can

be indexed to the tetragonal structure of the SnO2 material with

lattice constants of a¼ 4.75 and c¼ 3.20 �A, which agree with the

values (a ¼ 4.738 and c¼ 3.187 �A) of the JCPDS card (41–1445).

The existence of the graphene sheet and its role as a substrate

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were verified by the TEM images of the crystal-seeded graphene

sheets and the 3-D array structure (Figs. S2, S3†).

It is well known that the size and density of nanorod arrays

affect their properties such as optical,29,30 field emission31–33 and

photovoltaic34 properties, surely, including gas sensing. For the

purpose of investigating the contribution of nanorod array

morphology to gas sensing, different appearances of SnO2/G 3-D

array structures were synthesized. A hydrothermal growth

nanorods process was generally carried out, what the crucial

points and differences need to be emphasized is the seeding

crystal seeds on the graphene sheets. Different concentrations

(0.0025, 0.005, 0.05, and 0.5 M) of SnCl4 hydrolysis aqueous

solution were tried. It was proved that as the quantity of Sn4+

increases, the amount of seeds on the graphene sheets increase

too, thus the number and diameters of SnO2 nanorods increase,

creating more crowded and inhomogenous arrays. As Fig. 2a–d

show, the diameters of the nanorods on the graphene sheets are

highly dependent on the concentration of Sn4+, they are esti-

mated on average to be 20, 30, 70, 200 nm, respectively. Their

densities are statistically counted to be ca. 258, 241, 180, 43 mm�2,

respectively. Correspondingly, the BET surface area is measured

to be 13.966, 21.348, 22.568, 7.237 m2g�1, respectively. The thin

rods are standing aslant on the graphene, while the thick ones are

too crowded and not uniform in size. After hydrolysis lasting for

12 h, a raised temperature calcination procedure was imple-

mented to help the formation of the crystal grain. It can be

indicated that a higher temperature contributes to densification

of the array structures, which is attributed to the dense sintering

of the crystal seeds on the graphene sheets. The diameter of the

nanorods increases substantially as the temperature is raised in

the range of 25, 100, 200, and 300 �C. Analogously, the diameters

and densities of nanorods on graphene are about 25, 50, 70,

200 nm, and 512, 285, 302, 40 mm�2, according to Fig. 2e–h,

respectively. Similarly, the BET surface area corresponding to

these four structures is measured to be 19.413, 25.261, 20.281,

5.614 m2g�1, respectively. The surface area figures proved that the

diameter or density of the nanorods varies in accordance with

their BET surface areas. A moderate diameter (�50 nm) and

density (�285 mm�2) of the SnO2 nanorods will contribute to the

largest surface area (25.261 m2g�1) of the SnO2/G 3-D array

Fig. 2 SnO2/G 3-D array structures prepared via seeding crystal seeds on

hydrolysis concentration (0.0025 M, 0.005 M, 0.05 M, and 0.5 M). (e–h) su

300 �C). Scale bars: 500 nm.

17362 | J. Mater. Chem., 2011, 21, 17360–17365

structures. In Fig. 2f, the nanorods have the most uniform

diameter and optimal gap size. It should be remarked here that

the experiment parameters of optimal array architecture for the

gas sensing test are hydrolysis concentration of 0.05 M, calci-

nation temperature at 100 �C, and the same condition for the

SnO2 radial flowers except the participation of the graphene

sheets.

On the basis of the experimental results presented above, we

speculate that the SnO2/G 3-D array structures were formed by

the hydrolytic growth of nanocrystal seeds on the graphene

sheets, which strongly depended on conditions such as the

hydrolysis concentration and sintering temperature, followed by

a second hydrothermal growth of SnO2 nanorods on the gra-

phene substrates. The two steps correspond to the nucleation and

growth process, respectively. In general, there are more or less

dangling bonds or defects on the edge or surface of the graphene

sheets.35 These randomly distributed bonds or defects can act as

anchor sites for Sn4+ ions deposition and consequently lead to the

in situ formation of SnO2 nanocrystals on the surface and edges

of the graphene sheets. The reaction is fairly straightforward, as

described by this chemical equation: SnCl4 + 2H2O ¼ SnO2 +

4HCl. Due to the sustaining deposition of ions and the subse-

quent sintering, stabilizing crystal seeds formed. Sintering is

essential for the adequate contact of the crystal seeds with the

graphene rather than mechanically stacking on it, which is of

significance to its gas sensing properties. The crystal seeds on the

graphene then promote the occurrence of anisotropic growth and

induce the formation of regular nanostructures. Because of the

intrinsic anisotropic nature of rutile SnO2, crystal growth along

the [001] c-axis and with a square cross-section was favorable.36,37

As it turned out, the nanocrystals seeded graphene has a decisive

effect on the formation of array structures. If the nanocrystal

seeding on the graphene sheets was omitted, the morphology of

the product was radial flowers composed of nanorods instead of

an aligned nanorods array structure (see Fig. S4†).

To demonstrate the enhanced performance of the as-obtained

SnO2/G 3-D array structures as a sensing material, the responses

to a series of gases was investigated at an operating temperature

of 260 �C. As shown in Fig. 3a, the responses of the SnO2/G 3-D

array structures to all of the gases tested are almost twice as much

graphene sheets under different conditions. (a–d) successive increase of

ccessive raising of calcination temperature (25 �C, 100 �C, 200 �C, and

This journal is ª The Royal Society of Chemistry 2011

Fig. 3 (a) The sensitivity (Sr ¼ Ra/Rg) to a series of gases with concentration of 50 ppm, contrasting between the SnO2/G 3-D array structures (the inset

left) and SnO2 flowers (the inset right), scale bars: 1 mm. (b) The dynamic responses of sensors constructed with SnO2/G 3-D array structures to H2S with

different concentrations at 260 �C.

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as those obtained using pure SnO2 flowers. Especially, the array

structures appear highly selectivity to H2S gas. In addition, the

sensitivities to formaldehyde, ethanol, methanol, acetone, and

ammonia also increased significantly compared to pure SnO2

flowers. Fig. 3b displays the dynamic response–recovery curves

of the sensor towards H2S gas with a concentration sequence of

1, 5, 25, and 50 ppm. It is evident that the response amplitude is

highly dependent on gas concentration. The novel architecture

has a low detection limit, which exhibits a sensitivity of 2.1 to

H2S with a concentration as low as 1 ppm. It indicates that H2S

sensing is a fast response–recovery process, the response and

recovery time for 1 ppm H2S is as short as 5 s and 10 s,

respectively.

Better sensitivity and selectivity is foreseen due to the opti-

mized architecture such as nanorod array structures with

a moderate nanorods diameter and uniform distribution, which

are also reflected by their surface area and pore size. In Fig. 4, the

dynamic sensitivity curves of four different morphologies of

SnO2/G 3-D array structures and the SnO2 flowers were tested

simultaneously. As it turned out, array structures with too thick

(diameter greater than 200 nm, Fig. 4d) or too thin (diameter

Fig. 4 The dynamic sensitivity of different morphologies of the SnO2/G 3-D a

at 260 �C, the four lines are corresponding to the four SEM images marked

This journal is ª The Royal Society of Chemistry 2011

smaller than 20 nm, Fig. 4b) nanorods are adverse to the sensing

properties. The optimal sensitivity practically reached 130 with

a modest diameter of about 50 nm, which has the highest

nanorods density of ca. 285 mm�2 (Fig. 4a), neither sparse (thin

nanorods with density of ca. 241 mm�2, Fig. 4b) nor compact

(thick nanorods with density of ca. 40 mm�2, Fig. 4d). The

response time and recovery time are 7 s and 20 s, respectively. It

was also proved that if the graphene substrate was not intro-

duced, i.e. for the SnO2 flowers (Fig. 4c), the response was

relatively weak. Moreover, it took longer time to reach a steady

state in response and recovery process.

In order to further confirm the relationship between the array

architectures and gas sensing performances, nitrogen adsorption

and desorption measurements of the above four products were

carried out to estimate the properties. As shown from the

nitrogen adsorption and desorption cyclic curves in Fig. 5, the

adsorbed quantity of the four morphologies marked a, b, c, d in

Fig. 4 decrease successively. In fact, the BET surface area of the

four structures was calculated to be 25.261, 21.348, 15.338 and

5.614 m2g�1, respectively, indicating a downtrend of the active

surface among them. So, it can be concluded that the moderate

rray structures and the SnO2 flowers to H2S with concentration of 50 ppm

by (a, b, c), and (d), respectively. Scale bars in all images: 500 nm.

J. Mater. Chem., 2011, 21, 17360–17365 | 17363

Fig. 5 N2 adsorption–desorption cyclic curves of the SnO2/G 3-D array

structures and the SnO2 flowers product, inset showing BJH pore size

distribution of these structures. The curves with different colors and

symbols corresponds to the four products’ morphologies marked a, b, c,

d in Fig. 4.

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nanorods’ diameter and a uniform distribution contribute to

a large surface area, and hence lead to high sensitivity. Pore size

distribution curves (inset of Fig. 5) of the four products suggest

that the moderate nanorod diameter (�50 nm) and density

(�285 mm�2) link to uniform and proper pore size, which are

important factors for mass transport and effective surface area.

The average pore size of the four structures was calculated to be

3.539, 3.531, 5.667 and 4.4 cm3g�1nm�1, respectively. For the

former two, thinner nanorods (morphology b) have a similar

average pore size as that of the moderate one (morphology a),

but it is not adequate to generate as large a surface area as the

moderate nanorods. Comparing the SnO2/G 3-D array struc-

tures, thin and sparse nanorods make a larger pore volume but

a less effective surface area, while thick and dense nanorods make

a small pore space and surface area. Only the moderate diameter

and uniform distribution of these nanorods lead to appropriate

pore space and a large surface area, and thus are of benefit to

their sensing performance. It is noted that even if the SnO2/G 3-D

array structures and SnO2 flower structure have nearly the same

surface area and average pore size, the former shows higher gas

sensing response than the later (Fig. S5†). Therefore, these results

demonstrate that the surface area and pore size of structures also

affect their gas sensing performance, which is in agreement with

our earlier arguments.

In general, gas sensing on the surface of SnO2 is an adsorp-

tion–oxidation–desorption process that leads to a change in the

electrical resistance of the sensing material.38,39 Since SnO2 is an

n-type semiconductor, the electron depletion layer generated

because of the existence of surface oxygen species (O2�, O� and

O2�), resulting in the increase in surface potential barrier.

Exposed to reductive gas such as H2S, the surface oxygen ions are

consumed and additional electrons generated, thus reducing the

resistance of the materials. The present experimental results

clearly show that the sensing performances of composite SnO2/G

3-D array structures are significantly preferable than that of

SnO2 flowers. Two main reasons satisfactorily account for this:

17364 | J. Mater. Chem., 2011, 21, 17360–17365

First, gas molecules adsorbing on the active surface benefit from

the structures with larger surface area, which will facilitate

molecular absorption, gas diffusion and mass transport.

Moderate diameter and uniform distribution of nanorods highly

aligned on the plane completely meet this demand. Owing to the

direct growth of the nanorods onto the graphene substrates, their

large surface area and appropriate pore size offer quantities of

active centers and efficient electron pathways for amperometric

sensors. Second, as graphene exhibits outstanding electrical

conductivity and chemical sensitivity, employing graphene sheets

as substrates results in better gas sensing behavior. Graphene

substrates not only enhanced the conductivity of the sensor

component, but also created a Schottky contact at the interface

with SnO2. As metallic nature of the graphene, the graphene–

SnO2 interface is a forward-biased Schottky barrier, thus

resulting in the easy capture and migration of electrons from the

conduction band.40,41 In addition, due to the high charge carrier

mobility of graphene, the electrical signal linked closely and

propagated rapidly. These are all the reasons for the excellent

performance, including high sensitivity, fast response and

recovery, and low detection limit, of highly aligned SnO2 nano-

rods on graphene sheets forming 3-D array structures for use as

gas sensors.

Conclusions

In summary, we have succeeded in synthesizing highly aligned

SnO2 nanorods on graphene sheets forming 3-D array structures

by a straightforward crystal-seeds-directing hydrothermal

method, in which the diameter and density of the SnO2 nanorods

grown on the graphene sheets can be easily tuned as required by

varying the seeding concentration and temperature. The ordered

3-D array structures were exploited as gas sensors and exhibited

improved sensing performances to a series of gases. The sensi-

tivity to H2S when SnO2 nanorod arrays of moderate diameter

and uniform distribution were used was over twice that obtained

with comparable SnO2 flowers. The enhanced sensing properties

are attributed to the synergism of the large surface area of SnO2

nanorod arrays and the superior electronic characteristics of

graphene. The large scale and high yield of the composite array

structures with flexible substrates will be beneficial for the

fabrication of individual gas sensor devices. Strategies for

combining various metal oxides and nanoscale building blocks

into integrated 3-D array structures will open new opportunities

for designing and synthesizing multifunctional nanocomposite

materials.

Acknowledgements

This work was supported by the National Natural Science

Foundation of China (Grant No. 21171035, 50872020 and

50902021), the Program for New Century Excellent Talents of

the University in China, the ‘‘Pujiang’’ Program of Shanghai

Education Commission (Grant No. 09PJ1400500), the ‘‘Dawn’’

Program of the Shanghai Education Commission (Grant No.

08SG32), the Science and Technology Commission of Shanghai-

based ‘‘Innovation Action Plan’’ Project (Grant No.

10JC1400100), the ‘‘Chen Guang’’ project (Grant No. 09CG27)

supported by the Shanghai Municipal Education Commission

This journal is ª The Royal Society of Chemistry 2011

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and Shanghai Education Development Foundation, and the

‘‘Dawn’’ Program of Shanghai Education Commission in China

(Grant No. 08SG32).

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