Characterization of a hybrid composite of SnO₂ nanocrystal-decorated reduced graphene oxide for ppm-level ethanol gas sensing application

Abstract

This paper demonstrated a hybrid composite of tin oxide–reduced graphene oxide (rGO) for ppm-level detection of ethanol vapour. A facile and low-cost method of hydrothermal synthesis was used to construct the SnO₂–rGO hybrid film on a PCB substrate with coil-like interdigital microelectrodes. The presence of small-sized SnO₂ nanocrystals on rGO sheets was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), an X-ray diffractometer (XRD) and Brunauer–Emmett–Teller (BET) measurements. The sensing properties of the SnO₂–rGO hybrid film sensor, including the sensitivity, response–recovery times and repeatability, were investigated by exposing it to various concentrations of ethanol gas varying from 1 ppm to 100 ppm at room temperature. As a result, the presented sensor exhibited a low detection limit of 1 ppm at room temperature, as well as a fast response–recovery time and good repeatability, which outstripped that of pure rGO film sensors. Finally, the possible mechanism of ethanol gas-sensing for the above presented sensor was discussed in detail.

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1. Introduction

Portable, reliable and low-cost gas sensors are of great significance in various fields, such as industrial production, environmental monitoring, and people's daily life.^1–4 Ethanol as one of the most commonly and widely used chemical compounds, is applied in the chemical industry, medicine, traffic safety, and the food industry. Precise quantitative detection of ethanol vapors at ppm levels is required for many applications.^5–7 So far, several techniques such as chromatography,⁸ electrochemistry,⁹ optical fibre,¹⁰ potentiometry,¹¹ surface acoustic wave (SAW)¹² and spectrophotometry,¹³ have been developed for detecting ethanol gas. However, these methods have some disadvantages such as time consuming, not suitable for portable use, and require expensive equipment. And recently, considerable attentions have been devoted to metal-oxide semiconducting (MOS) materials-based gas sensor for ethanol gas determination. Various metal-oxide semiconductors, such as V₂O₅,¹⁴ ZnO,¹⁵ CuO,¹⁶ In₂O₃,¹⁷ NiO,¹⁸ Fe₂O₃,¹⁹ Cr₂O₃ (ref. 20) and Co₃O₄,²¹ are widely used for sensing ethanol gas. These sensors exhibited unique properties such as small-size, low-cost, fast response and recovery, but they suffered from an obvious drawback of high operating temperature, resulting in high power consumption and difficulty in integration.²²

Graphene or reduced graphene oxide (rGO) has aroused tremendous attentions owing to its unique features including overlarge specific surface area of 2600 m² g⁻¹ for molecular adsorption, outstanding electrical properties and high carrier mobility of 1.5 × 10⁵ cm² (V⁻¹ s⁻¹) at room temperature.^23–25 However, despite graphene has great potential and promising prospect, challenges in its synthesis and processing constitute the main obstacles towards practical device applications.^26,27 Up to now, some methods such as micromechanical exfolication,²⁸ chemical vapor deposition (CVD),²⁹ chemical or thermal reduction of graphene oxide (GO),^30,31 and other special approaches have been employed for preparation of graphene. Among them, facile and efficient reduction of GO and novel applications of the reduced graphene oxide (rGO) based materials are of current interest.³² Recent advances demonstrated that the decoration of graphene or rGO with metal oxide nanoparticles is a candidate for constructing high-performance gas sensors due to its high sensitivity, short response and recovery times and low cost.²¹ Liang et al. prepared α-Fe₂O₃ nanoparticles decorated on graphene as ethanol gas sensor material, and the result indicate a large enhancement in gas sensing performances in comparison with pure metal oxide or graphene counterpart.³³

SnO₂ is an n-type semiconducting material widely used as candidate in gas sensing applications due to its wide band gap of 3.6 eV and excellent chemical properties. However, the SnO₂ exhibited too low conductivity and always worked at high temperature above 300 °C, and room temperature ethanol sensing is difficult to realize.^34,35 The incorporation of n-type SnO₂ nanoparticles on the p-type graphene sheets creates a hybrid nanostructure and form p–n junction, which could improve the gas-sensing performance in terms of low detection limit, high sensitivity, short response and recovery time, especially work at room temperature. In this paper, we demonstrated a ppm-level ethanol gas sensor based on SnO₂ nanocrystals–reduced graphene oxide (SnO₂–rGO) nanocomposites via a facile route of hydrothermal method. The sensor was fabricated on a PCB substrate with coil-like interdigital microelectrodes. The surface microscopy and microstructure of SnO₂–rGO hybrid film were characterized by SEM, TEM, XRD and BET. The sensing properties of the SnO₂–rGO hybrid film sensor were investigated by exposing to various concentration of ethanol gas varying from 1 ppm to 100 ppm at room temperature. It was found that the presented sensor exhibited a low detection limit of 1 ppm, as well fast response–recovery time and good repeatability, which had outstripped that of pure rGO film sensor. Finally, the possible sensing mechanism for the above presented sensor was discussed.

2. Experiment

2.1 Experimental materials

Graphite powder, NaNO₃ and KMnO₄ were supplied by Aladdin Industrial Corp. (Shanghai, China). SnCl₄·5H₂O (99%) was offered by Shanghai Hansi Chemical Industry Co. Ltd (Shanghai, China). The 98% sulfuric acid was purchased from Sigma-Aldrich Inc., and used as received. All the above chemicals were used as received without further treatment.

2.2 Sensor fabrication

The sensor was fabricated on a printed circuit board (PCB) substrate by using microfabrication technology. PCB as an electrical insulating material, such as plastic, is initially used for a sensor substrate owing to its lower cost than other existing substrates. Two coil-like Ni/Cu interdigital electrodes (IDEs) were deposited on a PCB substrate via photolithographic technology. The IDEs pattern window on the PCB substrate has an outline dimension of 1 × 1 cm, the electrode thickness is 50 μm and the width and gap both is 200 μm.

Graphene oxide (GO) was synthesized from natural graphite powder using a modified Hummers method. 1 g of graphite powder and 0.8 g of NaNO₃ were added into 60 mL of 98% H₂SO₄ solution with stirring in an ice bath for overnight. After that, 5 g of KMnO₄ was added into the mixture slowly, followed by a stir for another 12 h. Subsequently, the resulting solution was added to 250 mL of deionized (DI) water and heated at 90 °C until it turned golden yellow. Finally, the obtained solution was filtered and washed with DI water several times, and then the as-synthesized GO was dispersed in DI water at a concentration of 0.5 mg mL⁻¹ with the aid of ultrasound for further use.

The sensing film of SnO₂–rGO hybrid was prepared by using a facile and low-cost method of hydrothermal treatment of SnCl₄ solution in the presence of rGO. Fig. 1 shows the preparation of SnO₂–rGO hybrid via hydrothermal synthesis. In the typical synthesis, 24 mg of SnCl₄·5H₂O and 1.5 mL of GO (0.5 mg mL⁻¹) were firstly dissolved into 20 mL of DI water with stirring for 1 h. Followed by transferring the resulting solution into a 40 mL Teflon-lined, stainless-steel autoclave and then heated for 12 h at 180 °C. Afterward, when the autoclave cooled down to room temperature, the products were obtained by centrifugation at 3000 rpm for 10 min, and subsequent washing with DI water for several times to remove excess chloride ions. GO was converted into conductive rGO under hydrothermal reduction route. Finally, the resulting SnO₂–rGO dispersion was dropped onto the horizontal-posed PCB substrate through a pipette, followed by drying in an oven at 50 °C for 2 h. The structure illustration of SnO₂–rGO hybrid film sensor is shown in Fig. 2. For making comparison, the rGO film sensor was prepared by drop-casting of rGO solution as received without further treatment.

Fig. 1 Hydrothermal preparation of SnO₂–rGO hybrid as sensing film on sensor.

Fig. 2 Structure illustration of SnO₂–rGO hybrid film sensor.

2.3 Instrument and analysis

The surface morphologies of rGO and SnO₂–rGO composite were inspected with field emission scanning electron microscopy (FESEM, Hitachi S-4800). The microstructure for SnO₂–rGO composite is examined by a transmission electron microscope (FEI Tecnai G2 F20) at an accelerating voltage of 200 kV. The XRD spectrum for rGO and SnO₂–rGO were characterized with X-ray diffractometer (Rigaku D/Max 2500PC). The BET surface area and pore size of the SnO₂–rGO composite was characterized by using NOVA 1000e from Quantachrome Instruments.

The gas-sensing experiments are performed by exposing the SnO₂–rGO hybrid film sensor to various concentrations of ethanol gas. The ethanol gas sensing properties of the presented film sensor are automatically measured by using a data logger (Agilent 34970A), which is connected to a computer via RS-232 interface. The desired gas concentration is obtained by injecting a required quantity of anhydrous liquid ethanol into a sealed glass container by using a syringe. The concentration of injected ethanol gas in the chamber was calculated in ppm by

where C is the concentration of gaseous ethanol (ppm), ρ is the density of liquid ethanol (g mL⁻¹), T is the testing temperature (K), V_s is the volume of liquid ethanol (μL), M is the molecular weight of ethanol (g mol⁻¹), and V is the volume of the chamber (L).

The adsorption of ethanol gas produced a measurable change in the sensor resistance. The sensor recovery was achieved by switching the sensor to dry air for the release of ethanol gas molecules. The figure of merit used to characterize the sensor performance is the normalized response (S), determined by S = abs(ΔR)/R₀ = abs(R_g − R₀)/R₀ × 100%, where R_g and R₀ are the electrical resistance of the sensor in a given concentration of ethanol gas and dry air, respectively. The dry air environment was achieved in a closed vessel by using phosphorus pentoxide (P₂O₅) desiccant powder for getting dry air.

3. Results and discussion

3.1 SEM and XRD characterization

Fig. 3(a) and (b) show the observed SEM images of rGO and SnO₂–rGO films. As shown in Fig. 3(a), it is clearly seen that the rGO has wrinkles with overlap at the edges, and demonstrates randomly aggregated rGO sheets. The result in Fig. 3(b) indicates that SnO₂ nanocrystals (NCs) are attached on the surface of rGO sheets, in which the presence of SnO₂ NCs reveals that hydrothermal treatment of rGO and SnCl₄ solution is an effective method to synthesize SnO₂–rGO hybrid. The crystallinity and morphology of SnO₂–rGO composite was further investigated by TEM imaging. Fig. 3(c) illustrates that plenty of SnO₂ NCs ranging from 3–6 nm were uniformly distributed on the surface of rGO nanosheets. Fig. 3(d) shows the highly resolution TEM (HRTEM) image of the SnO₂ nanocrystals with neighboring lattice spacing of 0.33 nm, corresponding to that of rutile SnO₂ from the reflection of (110) plane.

Fig. 3 (a) SEM image of rGO. (b) SEM image of SnO₂–rGO composite. (c) TEM image of SnO₂–rGO composite. (d) High resolution TEM image of SnO₂–rGO composite.

Fig. 4 illustrates the XRD characterization for rGO, SnO₂ and SnO₂–rGO film. The XRD measurement is performed by using Cu Kα (λ = 1.5418 Å) radiation with a 2θ scanning range of 10–80°. Fig. 4(a) shows an obvious peak at 2θ angle of 24.7° for rGO. According to the Bragg formula, the interlayer spacing of the rGO is 3.6043 Å (2θ = 24.7°) was determined. Fig. 4(b) exhibits the XRD spectrum of SnO₂. Several observed strong peaks at 2θ of 26.41, 33.82, 37.60, 51.73 and 65.68° are attributed to the (1 1 0), (1 0 1), (2 0 0), (2 1 1) and (3 0 1) planes of rutile SnO₂ (JCPDS Card no. 41-1445), indicating the successfully formation of SnO₂ nanocrystals. It is observed that the broad peak from rGO is not obvious in the XRD pattern of SnO₂–rGO film shown in Fig. 4(c), probably due to that the rGO is covered or wrapped by SnO₂ crystals under the hydrothermal treatment, or the weak peak of rGO is swamped by the presence of the high intensity peak of the SnO₂ at (1 1 0) plane. The underlying reasons need further investigation.

Fig. 4 XRD spectra for rGO, SnO₂ and SnO₂–rGO nanocomposite.

To further investigate the surface microstructure of the SnO₂–rGO hybrid composite, Brunauer–Emmett–Teller (BET) measurement was performed and the results were shown in Fig. 5. The pore diameter distribution of SnO₂–rGO composite indicates that a primary pore diameter is observed in the range of 2–5 nm, which is measured by using the Barrett–Joyner–Halenda (BJH) method. The insert in Fig. 5 shows the nitrogen adsorption–desorption isotherms of SnO₂–rGO composite. It is clearly shown that a distinct hysteresis loop at the relative pressure varying from 0.4 to 1.0 and a relatively high surface area of the composite is obtained as 153.40 m² g⁻¹. The above results indicate that the SnO₂–rGO hybrid composite has considerable larger surface area and small pore size, which significantly boost the interactions between ethanol molecules and sensing materials, and thus lead to an enhancement in sensor response.

Fig. 5 Pore diameter distribution of SnO₂–rGO composite. The insert shows nitrogen adsorption–desorption isotherm of SnO₂–rGO composite.

3.2 Ethanol-sensing properties

Fig. 6(a) shows the normalized response of SnO₂–rGO hybrid film sensor exposed to varying concentration of ethanol gas from 1 ppm to 100 ppm at room temperature. The normalized response of the SnO₂–rGO film sensor increases along with the increasing of ethanol gas concentration. Each exposure/recovery cycle is carried out by an exposure interval of 200 s followed by a recovery interval of 200 s at dry air. Each cycle is indicated by the area between two closely adjacent dotted lines marking start and end. The corresponding normalized response values are about 1.47%, 2.10%, 2.71%, 3.41%, 3.89% when the sensor is exposed to the concentration of 1, 5, 10, 50 and 100 ppm, respectively. It is obvious that the sensor has a good sensing performance and a low detection limit of 1 ppm at room temperature. Fig. 6(b) illustrates the repeatability of SnO₂–rGO hybrid film sensor under the same conditions at room temperature. The repeatability characteristics are performed by exposing the sensor to ethanol gas concentration of 1 ppm, 5 ppm, 10 ppm and dry air for three exposure/recovery cycles. The presented sensor manifested an excellent response and recovery behavior and acceptable repeatability for low concentration of ethanol gas sensing.

Fig. 6 (a) Normalized response of SnO₂–rGO hybrid film sensor exposed to varying concentration of ethanol gas at room temperature. (b) The repeatability of SnO₂–rGO hybrid film sensor for ethanol gas of 1 ppm, 5 ppm and 10 ppm at room temperature.

Fig. 7(a) demonstrates the time-dependent response and recovery curves of the sensor to different concentration of ethanol gas varying from 1 ppm to 100 ppm. The time taken by a sensor to achieve 90% of the total resistance change is defined as the response or recovery time. Response time and recovery time of around 78–97 s and 86–104 s are observed, respectively. Fig. 7(b) shows the normalized response of the SnO₂–rGO hybrid film sensor as a function of ethanol gas concentration. The fitting equation for normalized response Y as a function of ethanol gas concentration X can be represented as Y = 1.159X^0.192, and the fitting correlation coefficient, R², is 0.9705. This fitting equation is built based on empirical observations, and the fitting parameters are obtained from the particular experiment and are only accurate for this type of sensor.

Fig. 7 (a) The time-dependent response–recovery curves of SnO₂–rGO hybrid film sensor exposed to various concentrations of ethanol gas. (b) Normalized response of the SnO₂–rGO hybrid film sensor as a function of ethanol gas concentration. (c) Normalized response of the SnO₂–rGO hybrid film sensor versus 100 ppm of various gases, respectively. (d) Performance comparison of ethanol gas sensing between SnO₂–rGO hybrid film sensor and pure rGO film sensor.

Selectivity to target gas is always one of the important factors to gas sensors. Fig. 7(c) shows the measured selectivity of the SnO₂–rGO hybrid film sensor to 100 ppm various gas species at room temperature, including methane, acetone, hydrogen, carbon dioxide, ethanol and sulfur dioxide. The response to 100 ppm ethanol is about 4%, while the response to 100 ppm of CH₄, CH₃COCH₃, H₂, CO₂ and SO₂ were 2.2, 2.89, 2, 1.1 and 1.8, respectively. The sensor response to ethanol was much higher than those of other tested vapors at the same concentrations. Generally, the sensor selectivity is a ticklish issue associating with the intricate film nanostructure and complicated sensing mechanism.³⁶ The sensor selectivity may need further investigation toward critical applications.

Fig. 7(d) illustrates the performance comparison of ethanol gas sensing between SnO₂–rGO hybrid and pure rGO film sensors. The normalized response curves for SnO₂–rGO hybrid film sensor exposed to 1 ppm and 5 ppm of ethanol gas is compared with pure rGO film sensor exposed to 200 ppm and 1000 ppm ethanol gas, respectively. Even though the gas concentrations of SnO₂–rGO hybrid film sensor measured were 1/200 times of that pure rGO film sensor determined, the comparison results indicated the SnO₂–rGO hybrid film sensor yielded much higher responses than that of pure rGO film sensor. We can naturally draw a conclusion that the SnO₂–rGO hybrid film sensor is superior to the pure rGO film sensor in the ethanol gas sensing.

3.3 Gas-sensing mechanism

The above experimental results reveal that SnO₂–rGO hybrid film sensor achieves an excellent response to ethanol gas at room temperature, which makes it a promising potential application. Generally, this is attributed to the incorporation of rGO sheets and SnO₂ NCs provide outstanding physicochemical properties, high surface-to-volume and unique electrical conductivity. However, the exact sensing mechanism of the hybrid towards ethanol gas need to be further investigated. The decoration of SnO₂ NCs on rGO sheets result in a hole depletion region of the rGO sheet near the interface with n-type SnO₂ NCs, which leads to a decrease in the rGO electrical conductivity, suggesting that the SnO₂–rGO hybrid film sensor was a p-type semiconductor.^37,38 Therefore, the decoration of SnO₂ NCs does not change the semiconducting type of the rGO, due to the amount of deposited SnO₂ NCs is limited and the band gap of the rGO is small. The working principle of SnO₂–rGO composite film sensor is based on the charge/electron transfer between the adsorbed gas molecules and the p-type sensing film.³⁹

Fig. 8 shows the adsorption illustration of ethanol gas molecules adsorbed onto the sensing film. One of the gas-sensing mechanisms associated to the adsorbed oxygen molecular is widely employed in previous works.^40–44 Oxygen molecules in the atmosphere are adsorbed on the interface of n-type SnO₂ conduction band and ionized to oxygen ions with a singly negative charge through capturing electrons from the SnO₂ NCs. As a result, depletion layers are formed and an increase in resistance of the sensor is observed. Once the sensor is exposed to the reducing gas such as ethanol, the gas molecules react with the oxygen ions on the surfaces of sensing film. Ethanol gases are firstly oxidized into acetaldehyde, followed by further oxidized into carbon dioxide and water. Such reactions result in retrieving the trapped electrons back to the conduction band of the sensing film and lead to a decrease in resistance for the gas sensor.

Fig. 8 The adsorption of ethanol gas molecules on the SnO₂–rGO film.

The hybrid of SnO₂–rGO nanocomposite exhibited enhanced sensing properties which is potentially superior to pure rGO. This is mainly attributed to the high surface area, 3D nanostructure and special interactions between rGO and SnO₂. Firstly, the loading of SnO₂ NCs onto rGO sheets leads to more active sites, such as vacancies, defects, oxygen functional groups as well as the sp²-bonded carbon, for the adsorption of gas molecules.^45,46 Secondly, the incorporation of SnO₂ NCs has an effect on preventing the aggregation of rGO sheets and form 3D porous nanostructure with much higher surface accessibility, which greatly facilitated the adsorption and diffusion of ethanol gas molecules. Thirdly, the improvement of the sensing performance of the SnO₂–rGO nanostructures may be attributed to the contribution of the heterojunction formed at the interface between SnO₂ NCs and rGO. Once target gas molecules pass through the interface of these contacts and lead to the changes in electrical properties at the heterojunctions. Therefore, SnO₂ NCs play a crucial role and act as a support to rGO sheets. The incorporation of SnO₂ NCs and rGO is beneficial for sensing enhancement to ethanol gas, demonstrating the as-prepared SnO₂–rGO hybrid film sensor was essential for practical application with the outstanding feature.

4. Conclusions

This paper demonstrated a room-temperature ppm-level ethanol gas sensor based on SnO₂–rGO hybrid film, which was constructed via a facile and low-cost method of hydrothermal synthesis. The sensor was fabricated on PCB substrate with coil-like interdigital microelectrodes. The presence of small-sized SnO₂ nanocrystals on rGO sheets were characterized by SEM, TEM, XRD and BET. The sensing properties of the SnO₂–rGO hybrid film sensor, including the sensitivity, response–recovery times and repeatability were investigated by exposing to various concentration of ethanol gas varying from 1 ppm to 100 ppm at room temperature. As a result, the presented sensor exhibited a lower detection limit of 1 ppm at room temperature, as well fast response–recovery time and good repeatability, which had outstripped that of pure rGO film sensor. Finally, the possible mechanism of ethanol gas-sensing for the above presented sensor was summarized and discussed in detail. This work highlighted the SnO₂–rGO hybrid is a candidate nanomaterial for constructing ethanol gas sensors.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant no. 51407200), the Science and Technology Development Plan Project of Shandong Province of China (Grant no. 2014GSF117035), the Promotive Research Foundation for the Excellent Middle-Aged and Youth Scientists of Shandong Province of China (Grant no. BS2012DX044), the Science and Technology Development Plan Project of Qingdao (Grant no. 13-1-4-179-jch), the Open Fund of National Engineering Laboratory for Ultra High Voltage Engineering Technology (Kunming, Guangzhou) (Grant no. NEL201518), and the Science and Technology Project of Huangdao Zone, Qingdao, China (Grant no. 2014-1-51).

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