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Cite this: J. Mater. Chem., 2012, 22, 11009

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Tuning gas-sensing properties of reduced graphene oxide using tin oxidenanocrystals†

Shun Mao, Shumao Cui, Ganhua Lu, Kehan Yu, Zhenhai Wen and Junhong Chen*

Received 18th January 2012, Accepted 15th April 2012

DOI: 10.1039/c2jm30378g

We report a novel and selective gas-sensing platform with reduced

graphene oxide (RGO) decorated with tin oxide (SnO2) nano-

crystals (NCs). This hybrid SnO2 NC–RGO platform showed

enhanced NO2 but weakened NH3 sensing compared with bare

RGO, showing promise in tuning the sensitivity and selectivity of

RGO-based gas sensors.

Graphene, an atomic-thick layer of carbon atoms, has drawn

significant attention due to its unique structure and properties.1–3

Graphene and reduced graphene oxide (RGO)-based nanostructures

have been widely studied for gas sensor applications due to their large

specific surface area (2630 m2 g�1)4 and high sensitivity to electrical

perturbations from gas molecule adsorption due to their ultra-small

thickness.5,6 So far, many graphene/RGO-based gas sensors have

been reported7 and various gas species, including NO2,8–12 NH3,

13–16

CO,17 CO2,18 O2,

17 and H2,19 can be sensitively detected by the new

sensing platforms. Although graphene and RGO present excellent

sensitivities to gas molecules, the performance of the sensors should

be further improved tomeet the requirements of practical gas sensors,

e.g., a low detection limit and high selectivity. To fulfil these

requirements, several studies reported the incorporation of nano-

crystals/nanoparticles in the graphene/RGO-based gas sensors, which

could improve the sensor performance in terms of sensitivity/detec-

tion limit, response time, or recovery time.20–22 However, very few

reports study the selectivity improvement by using nanocrystals in

graphene/RGO-based gas sensors.

Tin oxide is an n-type semiconducting material widely used in gas-

sensing applications;23–27 however, the study on SnO2–graphene/

RGO hybrid nanostructure gas sensors is limited,28–30 and electrical

field-effect transistor (FET) gas sensors based on SnO2 NC–gra-

phene/RGO have not been reported. Typically, the working principle

of graphene/RGO-based gas sensors is based on the charge/electron

transfer between the adsorbed gas molecules and the graphene/RGO

sheet. While graphene and RGO exhibit ambipolar and almost

symmetric behavior in the electron and hole doping regions under

Department of Mechanical Engineering, University ofWisconsin-Milwaukee, 3200 N Cramer Street, Milwaukee, WI 53211,USA. E-mail: jhchen@uwm.edu; Fax: +1-414-229-6958; Tel: +1-414-229-2615

† Electronic supplementary information (ESI) available: Detailedexperimental methods and supporting figures. See DOI:10.1039/c2jm30378g

This journal is ª The Royal Society of Chemistry 2012

vacuum, under ambient conditions, they have p-dominant conduct-

ing properties because of the adsorbed water and oxygenmolecules.31

By creating a hybrid nanostructure with n-type nanocrystals on

a graphene/RGO sheet, similar to that in a SnO2–carbon nanotube

(CNT) structure,32,33 a p–n junction is formed, and it is anticipated

that this novel structure will modify the gas-sensing performance of

the graphene/RGO sensor, especially on sensitivity and selectivity.

In this report, a novel and selective gas-sensing platformwith SnO2

NC–RGO was studied. Fig. 1 shows a schematic of the gas-sensing

platform of an RGO sheet decorated with SnO2 NCs. Direct

adsorption of target gas molecules (NO2 and NH3) onto the SnO2–

RGO surface induces electron transfer between gas molecules and

RGO, thereby changing the sensor conductivity. The fabricated

sensors show excellent response to target gases at room temperature

(detection limit of 1 ppm for NO2), and the decoration of SnO2 NCs

on an RGO sheet can enhance the sensor signal to NO2 and weaken

the signal toNH3. The reported findings present promising directions

in tuning the sensitivity and selectivity of RGO-based gas sensors and

further improving the sensor performance for practical applications.

The RGO sheets were prepared using a procedure previously

reported (see details in the ESI†).34 SnO2 NCs were synthesized using

a mini-arc reactor and deposited onto RGO sheets through an

Fig. 1 (a) Schematic of the novel gas-sensing platform of an RGO sheet

decorated with SnO2 NCs. (b) Schematic of the sensor testing system.

J. Mater. Chem., 2012, 22, 11009–11013 | 11009

Fig. 3 (a) XPS survey spectra of RGO with and without SnO2 NCs.

High-resolution XPS and curve fit of (b) C1s and (c) Sn3d in SnO2 NC–

RGO hybrid structures.

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electrostatic force-directed assembly (ESFDA) process (see details in

the ESI†).35,36 The fabricated hybrid nanostructure was characterized

by transmission electron microscopy (TEM) imaging. In Fig. 2a,

a single RGO sheet is found hanging on the carbon lattices. SnO2

NCs ranging from 3–6 nm were uniformly distributed on the surface

of the RGO sheet. The high-resolution TEM (HRTEM) image

(Fig. 2b) confirms the crystalline structure of the nanocrystal, and the

measured lattice spacings of the nanocrystals are 0.33 and 0.27 nm,

which correspond to those of rutile SnO2 from the (110) and (101)

reflections, respectively. The RGO sensor was designed with a FET

structure, in which RGObridges the source and drain electrodes, and

acts as the sensing channel. Sensor fabrication details are found in our

previous report (see details in the ESI†).11,37 Fig. 2c shows the scan-

ning electron microscopy (SEM) image of RGO sheets bridging

a pair of Au electrode fingers. After SnO2 NCs were deposited as

shown in Fig. 2d, the hybrid nanostructures consisting ofRGO sheets

and SnO2 NCs were created and worked as the sensing channel in

the sensor.

The surface chemical composition of the SnO2 NC–RGO hybrid

nanostructure was further characterized by X-ray photoelectron

spectroscopy (XPS) analysis (Fig. 3). From the survey spectra of

RGO with and without SnO2 NCs (Fig. 3a), C1s, O1s, and Sn with

different valance states are evidenced. Based on the high-resolution

and curve fit C1s spectrum (Fig. 3b), three peaks centered at 284.5,

286.6, and 288.7 eV are observed, corresponding to C–C, C–O, and

–COO– groups, respectively. The C–O and –COO– peaks indicate

the existence of oxygen-containing groups in the RGO, e.g.,

hydroxyl, epoxide, and carbonyl, which is consistent with our

previous report.34The Sn3d spectrum inFig. 3c shows two peakswith

a binding energy of 486.3 and 494.7 eV corresponding to Sn3d5/2 and

Sn3d3/2, respectively,25 which are attributed to SnO2 NCs. The XPS

results indicate that the sensor consists of SnO2NC andRGOhybrid

nanostructure and there are residue oxygen groups in the RGO.

The fabricated sensor was then characterized by direct current (dc)

and FET measurements, which helps to understand the electrical

Fig. 2 (a) TEM and (b) HRTEM images of the RGO sheet decorated

with SnO2 NCs. The inset shows the SAD patterns of the RGO sheet and

SnO2 NCs. (c) SEM image of RGO sheets bridging a pair of Au electrode

fingers. (d) SEM image of the RGO sheets decorated with SnO2 NCs

bridging a pair of Au electrode fingers.

11010 | J. Mater. Chem., 2012, 22, 11009–11013

properties of the SnO2 NC–RGO sensor and determine the semi-

conducting type of the sensor. GO sheets are non-conductive due to

the extensive presence of saturated sp3 bonds, the high density of

electronegative oxygen atoms bonded to carbon, and other ‘‘defects’’

that create an energy gap in the electron density of states;38 therefore,

the conductivity of the RGO sheet must be verified before the gas-

sensing test. From the dc measurement results of RGO, as shown in

Fig. 4a, device resistance of the RGO is around�103 to 104 U, which

is several orders of magnitude lower than that of GO (resistance

around 1010 U),34 and confirms that the RGO sheet was successfully

reduced and could work as the sensing channel in the sensor. Also,

the measured I–V curve is linear, which indicates that the contact

between the RGO sheets and Au electrode is Ohmic. After SnO2 NC

decoration, the RGO sensor resistance increased from 6.6 � 103 to

7.4 � 103 U. This could be explained by the fact that the deposited

SnO2NCs result in a hole depletion region of theRGO sheet near the

interface with n-type SnO2 NCs, which leads to a decrease in the

RGO electrical conductivity. To study the transistor properties of

SnO2 NC–RGO sensors, FET measurements were carried out. The

gate voltage dependence of the drain current Id of the sensor shows

that the SnO2 NC–RGO nanohybrid sensor was a p-type semi-

conductor (Fig. 4b); and with Vg ramping from negative to positive,

the drain current slowly decreased. The Dirac point of the transistor

was beyond +40 V, mainly due to the large number of adsorbed

water and oxygen molecules on the RGO sheet. The on–off ratio of

the SnO2 NC–RGO FET is small, mainly because the band gap of

the RGO is small. The decoration of SnO2 NCs does not change the

semiconducting type of the RGO (RGO FET curve, Fig. S1, ESI†),

and this is because the amount of deposited SnO2 NCs is limited and

the nanocrystals are individual discrete particles.

The gas-sensing tests of as-fabricated sensors were characterized

against low-concentration NO2 (1–100 ppm) and NH3 (1%) diluted

in dry air at room temperature (see details in the ESI†). Fig. 4c and

d show typical dynamic responses (normalized drain current vs. time,

Ia is the device current in air and Ig is the device current in target gas)

of anRGOdevice for detecting (c)NO2 and (d)NH3 before and after

This journal is ª The Royal Society of Chemistry 2012

Fig. 4 (a) Direct current measurement results of RGO before and after

SnO2 NC decoration. (b) FET measurement result (Vds ¼ 2.0 V) of the

SnO2 NC–RGO sensor. (c and d) Gas sensing signals of NO2 and NH3

from RGO sensors with and without SnO2 NCs. The sensing signal is

normalized by the measured sensor current in air (base line, Ig/Ia ¼ 1). (e)

SnO2 NC–RGO sensor response to NO2 at various concentrations. (f)

The sensitivity of the SnO2 NC–RGO sensor vs. NO2 concentration.

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the decoration of SnO2 NCs. Upon injecting 100 ppm NO2, the

normalized sensor drain current increased, which indicated that the

sensor resistance decreased in NO2. When NO2 flow was turned off

and the air flow was restored, the device started to recover. The bare

RGO sensor (Fig. 4c, blue line) sensitivity to 100 ppm NO2 was

�2.16 (ratio of device resistance in air to that in target gas), which is

close to the thermally reduced GO sensor (�2.56) that our group

previously reported.37 Upon the 1% NH3 exposure (Fig. 4d, blue

line), the sensor resistance increased and the device sensitivity (now

defined as the ratio of device resistance in target gas to that in air) was

�1.46, whereas the sensitivity of thermally reduced GO to NH3 was

�1.3.11 For the gas molecule adsorption, NO2 typically serves as an

electron acceptor while NH3 acts as an electron donor in the sensing

reaction.6 Since the RGO exhibits a p-type semiconducting behavior

in air, the sensing response of RGO is most likely due to the

adsorption of NO2 (NH3) that extracts (injects) electrons in RGO,

thereby increasing (decreasing) the RGO conductivity. For ammonia

sensing, prior reports have shown that ammonia can react with

functional groups on GO in an ambient environment.39–42 For

instance, Bandosz and co-workers have investigated the ammonia

adsorption on graphite oxides and shown that ammonia could react

with epoxy, leading to the formation of amines and hydroxyl func-

tionalities.39,42 These functional groups greatly modify the surface

chemistry of GO and may also lead to changes in the GO conduc-

tivity. It is plausible that both doping and chemical reactions could

contribute to the observed ammonia sensing signal. To understand

the exact relation between the GO structure modification and its

conductivity change, density functional theory (DFT) modeling may

be useful and related work will be carried out in the future.

This journal is ª The Royal Society of Chemistry 2012

To compare the sensing properties of SnO2 NC–RGO with bare

RGO sensors, SnO2 NC–RGO sensors were tested with NO2 and

NH3 by using exactly the same procedure as for bare RGO sensors.

Based on the sensing results (Fig. 4c and d, red lines), the response of

the sensor is similar to that of the bare RGO sensor; however, the

sensor sensitivity to NO2 increased (from 2.16 to 2.87) while the

sensor sensitivity toNH3 decreased (from 1.46 to 1.12). Rutile SnO2 is

an n-type semiconductor and the sensing mechanism of the SnO2

sensor is based on the fact that adsorbed oxygen (O2� or O) on the

SnO2 surface will interact with target gas molecules and result in the

SnO2 conductivity change.32,33 Normally, nanocrystal decoration on

the RGO surface will increase the overall active sensing surface area

and the subsequent gas molecule adsorption during the sensing

process. And the sensing signal can be enhanced by adding nano-

crystals. However, in our case, the sensor sensitivity to NO2 increased

but decreased with NH3. Therefore, the sensor sensitivity change

cannot be solely attributed to the increased adsorption of gas mole-

cules. In fact, the experimental observation of the conductivity

decrease of the RGO sensor after SnO2 NC decoration suggests the

formation of a hole depletion region of the RGO sheet near the

interface with n-type SnO2 NCs. At the interface between the SnO2

(n-type) and the RGO (p-type), due to the p–n junction and the

depletion zone, more electrons are attracted from the RGO toward

SnO2 in NO2.33 This electron transfer shifts the Fermi level of the

RGO towards the valence band and enhances theRGO conductivity.

On the other hand, in the case of NH3, because of the p–n junction

and hole depletion zone, fewer electrons are injected into the RGO;

therefore, the RGO conductivity decrease is smaller than that of the

pureRGO.Another possible reason for the sensitivity decrease is that

SnO2 NCs cover a large portion of the surface area of RGO and

prohibit the direct adsorption of NH3 on RGO sheets and screen the

chemical reactions between the ammonia and functional groups on

GO. Nevertheless, the mechanism of the gas-sensing from the SnO2

NC–RGO sensor is intricate because of the hybrid nanostructure and

needs further investigation before the exact mechanism is drawn.

To study the sensor dynamic range, sensors were tested against

NO2 with different concentrations at room temperature, and the

results are shown in Fig. 4e and f. The sensor sensitivity decreases

with the decrease of the NO2 concentration (linear dependent on the

concentration) and the detection limit of the sensor is around 1 ppm

with a sensitivity of 1.11. Such a sensor detection limit is pretty low at

room temperature. For a practical gas sensor, the response time of

a sensor is also critical. For the SnO2 NC–RGO sensor, the response

time is about 65 seconds for NO2 and 30 seconds for NH3. Note that

the sensor is treated here as a first-order dynamic system with

a constant input (target gas with a fixed concentration) and the

response time is defined as the time required for a 63.2% change

(corresponding to one time constant in a first-order dynamic system)

in device resistance from its base value in air to the minimum/

maximum value in target gases. The recovery of the sensor was slow,

mainly because the high-energy binding sites on the RGO, e.g.,

graphitic carbon atoms, vacancies, and defects, could delay the

recovery.

To study the impact of gate voltage (Vg) on the NO2 sensing

performance, SnO2 NC–RGO sensors were tested at positive

(+35 V), neutral (0 V), and negative (�35 V) Vg and the results are

shown in Fig. S2, ESI†. The sensor shows similar sensitivity, response

time, and recovery time to NO2 at different gate voltages. A FET

sensor can be p-type (holes as the majority carriers) or n-type

J. Mater. Chem., 2012, 22, 11009–11013 | 11011

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(electrons as the majority carriers) by adjusting Vg, resulting in

opposite sensing signal trends (increase or decrease in sensor resis-

tance). In our previous report,16 RGO sensors showed opposite

sensing signal trends to NH3 at positive and negative gate voltages, in

which the RGO was ambipolar in air. However, in this study the

SnO2 NC–RGO FET sensor is p-type (unipolar) for Vg from �40 V

to +40 V, so the sensing trends do not depend on Vg. The difference

in transistor properties of the RGO and SnO2 NC–RGO FET is

likely to be a result of different reducing agents (hydrazine mono-

hydrate vs. hydroxylamine hydrochloride) used to reduce GO. The

similar sensor sensitivity is because that the on–off ratio of the FET

sensor is low (1.18) and the carrier density difference in the sensor is

small at different Vg. Our previous report showed that the work

function of graphene could be lowered by increasing the Vg, which

improves the response of the sensor. In addition, Coulomb interac-

tion between gas molecules (polar molecules) andVg-induced charges

on the FET sensor surface could affect the gas molecule desorption,

resulting in different recovery times.16,43 However, in our case, no

obvious change in response and recovery time was evidenced, which

implies that the work function difference in RGO and the Coulomb

interaction between gas molecules and Vg-induced charges may be

too small to be registered in the sensing signal. Nevertheless, the SnO2

NC–RGO sensor shows a low detection limit to NO2 at room

temperature and the selective detection of NO2 using SnO2 NC

decoration presents a promising route for tuning the gas sensor

selectivity in practical applications.

The repeatability of the sensor was studied by repeating sensing

experiments with several similar sensors. In our study, the RGO

sheets were randomly dispersed onto the electrodes, the number and

area of the RGO sheets may vary for each sensor, so the fabricated

sensor resistance lies in a large range from 103 to 107 U. From our

previous study, it was found that sensors had larger sensitivity with

larger sensor resistance44 and the results in this study further confirm

this trend. The repeatability results show that the SnO2 NC–RGO

sensors have NO2 (100 ppm) sensitivity of 1.29–2.87 and sensitivity

enhancement (percentage of the sensitivity increase) of 3.0–33.4%

from pure RGO; on the other hand, the sensors have an NH3 (1%)

sensitivity of 1.20–1.47 and a sensitivity reduction (percentage of the

sensitivity decrease) of 13.4–23.8% from pure RGO. The stability of

the sensor was also studied by testing the sensor after storage in an

ambient environment for eight months. Results (Fig. S3, ESI†) show

that there was no significant change in the sensing abilities after the

storage, which indicates that our sensor is stable in air with a long

lifetime.

In our study, to understand the impact of SnO2 on the RGO

sensor, no thermal treatment was carried out after the nanocrystal

decoration; since the thermal treatment may further reduce the RGO

sheets and complicate the comparison of the sensing performance.

However, for a practical sensor, thermal treatment could be applied

to further improve the sensor performance and stability, since it helps

to remove the residual solvent from the RGO suspension and

improve the binding of the SnO2 NCs and RGO sheets. Another

possible direction to improve the sensor performance is to further

reduce the RGO, and highly reduced (hydrazine-reduced) GO

sensors were reported to have high sensitivities to NO2.12

In summary, gas sensors with RGO sheets decorated with SnO2

NCs were reported. SnO2 NCs can enhance the sensor response to

NO2 and weaken the sensor response to NH3. The introduction of

semiconducting nanomaterials onto the RGO presents a promising

11012 | J. Mater. Chem., 2012, 22, 11009–11013

route to tune the sensitivity and selectivity of the sensor. In addition,

the hybrid SnO2 NC–RGO sensor shows low detection limit and

repeatable performance to target gases at room temperature, which

help to bring nanotechnological advances from the laboratory to

a product, and to speed technology development for the gas sensor

industry.

Financial support for this work was provided by the U.S. NSF

(CMMI-0900509) and the U.S. DOE (DE-EE0003208). The authors

thank Professor Marija Gajdardziska-Josifovska for TEM access at

the HRTEM Laboratory at UWM. The SEM imaging was con-

ducted at the Electron Microscope Laboratory of UWM. The XPS

was conducted at Advanced Analysis Facility of UWM. Sensor

electrodes were fabricated at CNM of Argonne National Labora-

tory, which is supported by U.S. DOE (DE-AC02-06CH11357).

Notes and references

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