Highly Sensitive, Graphene Oxide Supported Zinc stannate (Zn2SnO4) Nanocubes and their Room Temperature NO2 Gas Sensor Properties V. P. Dinesh, P. Biji Nanosensor Laboratory, PSG Institute of Advanced Studies Coimbatore 641004, INDIA *
[email protected] Abstract— Graphene based hetero-nanostructured sensor systems have gained much attention for improving sensing behavior towards harmful gases, especially NO2. In this report, enhanced room temperature sensing response towards NO2 gas using rGO@ Zn2SnO4 nanocubes. A facile two step synthesis method is adopted involving hydrothermal synthesis for the preparation of highly monodispersed zinc stannate (Zn2SnO4) nanocubes having clear edges and length of 80 ± 0.5 nm were found to be anchored on the exfoliated rGO nanosheet supports. X-ray diffraction (XRD) studies divulge the clear existence of crystalline rhombohedral Zn2SnO4 and rGO sheets. Gas sensing studies revealed that the Zn2SnO4 nanocubes supported on rGO can detect NO2 even at 30°C and showed enhanced sensing response compared to pristine Zn2SnO4 nanocubes based on the spill-over effect and unwrapped rGO sheets.
I.
INTRODUCTION
Heterostructured nanomaterials have gained much attention in sensor industry due to their unique catalytic, optical, electrical, and stability features [1]. Multifunctional ternary oxides have drawn extensive attention of gas sensing researchers owing to their ease in preparation, unique structure, superior sensor response, stability and significantly enhanced selectivity [2]. Among ternary oxides, morphologically controlled zinc stannate (Zn2SnO4) nanostructured materials have addressed some of the serious technical issues existing in the current sensor field, such as, high operating temperature, stability, selectivity, robustness and integration with field effect transistors (FET) devices [3]. Apart from heterostructures, in the recent past years Graphene, a two-dimensional (2D) sheet consisting of a single layer of carbon atoms arranged in honeycomb lattice, has drawn great interest because of its remarkable properties, such as high surface area and high electron mobility [4]. Hence, the applications of graphene have been widely explored in varied fields starting from electronics, sensors, catalysis and energy systems [4]. To the best of our knowledge zinc stannate hetero-nanostructures based gas sensors can detect NO2 only around 350°C and no reports on Zn2SnO4@ rGO for NO2 detection. Hence, attempt is made to synthesis Zn2SnO4 nanocubes decorated on rGO for an enhanced room temperature NO2 gas sensor.
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II. A.
EXPERIMENTAL
Synthesis of Zn2SnO4@rGO heterostructures A facile two step synthesis method is adopted involving hydrothermal synthesis for the preparation of highly monodispersed Zn2SnO4 nanocubes with a clear edges followed by in-situ hydrothermal reaction of rGO resulting in the formation of Zn2SnO4@ rGO heteronanocubes. Zn2SnO4 nanocubes were grown by simple solvothermal method using Zinc acetate (ZnCl2. 5H2O) (99.9 %, Merck), Tin (II) chloride (SnCl2.2H2O) (99.9 %, Merck), Potassium hydroxide (KOH) (99.9 %, Merck) and CTAB (99.9 %, Merck), graphene oxide (GO), (Aldrich) and poly vinyl pyrrolidone (PVP) (98.9%, Merck). A 0.0018 M of zinc acetate and 0.0018 M of tin (II) chloride was taken in round bottom flask and kept for stirring at room temperature. 0.0112 M of KOH solution was added dropwise to this solution which resulted in the formation of white floccules of zinc stannate hydroxide. Finally 0.00182 M of CTAB was added. After 5 hours of stirring, the solution was transferred to Teflon lined autoclave and kept at 220°C for 24 hours. Then autoclaved solution was centrifuged and washed several times using MilliQ water and ethanol. Finally the precipitates were vacuum dried at 80°C overnight to obtain white powders of Zn2SnO4 and used as such without further annealing. rGO was prepared by well known Hummers method [5] and highly dispersed rGO solution in water was prepared by probe sonication process (Sonics, U.S.A.). A mixture of rGO suspension (22.5 mL of water + 2.5 mL of rGO) and 0.5g of PVP was kept for stirring at room temperature for 4 hours and to the solution 0.2g of the synthesised Zn2SnO4 was added and kept for additional stirring (0.5 hours). Before loading into the hydrothermal setup, probe sonication was given to the solution for 30 min and kept at 180°C for 3 hours to complete the hydrothermal reaction. Similar purification process was adopted for obtaining powders of rGO@ Zn2SnO4 heteronanocubes. UVvisible absorption spectra of the samples were acquired using T90 + UV-Visible spectrophotometer (PG Instruments, U.K.) in solution mode. Structure and morphology of the heteronanostructures were examined using XRD (XRD, JSO-
DEBYFLEX 2002, Cu Kα radiation, 1.54Å) and HR-TEM (JEOL JEM-2010, Japan). B. Gas Sensing Studies Gas sensing studies were carried out using a custom built exposure facility system [6]. The facility consists of double walled stainless steel chamber containing a hot stage (PID controlled). The gas flows are monitored using mass flow controllers. The gas sensing properties were investigated by exposing the samples to gas stream entrained with NO2 of different concentrations (0.5 – 5ppm) at different operating temperatures. The sensor response (S) is defined as the ratio of [Rg-Ra]/Ra, where Ra and Rg are the electrical resistance of sensor in air and gas respectively. III.
RESULTS AND DISCUSSIONS
The UV-Visible spectra of Zn2SnO4 and rGO decorated Zn2SnO4 nanocubes were shown in Fig. 1. Zn2SnO4 nanocubes exhibit its characteristic peak centered at 268 nm corresponding to the excitonic peak of zinc stannate. For rGO two characteristic peaks were observed at 230 nm and 300 nm corresponding to C=C bond and C=O bond respectively. Whereas, for rGO decorated with Zn2SnO4 nanocubes infer a characteristic quenching effect for the prominent rGO and interestingly blue shift for the zinc stannate exciton peak due to the electronic conjugation and transfer process with increased electron-hole pair formation at the heterojunctions. This further confirms the formation of rGO@ Zn2SnO4 nanocubes heterojunctions.
Figure 2. XRD studies of Zn2SnO4 and rGO decorated Zn2SnO4 nanocubes.
TEM image (Fig.3a,b) shows the monodispersed nanocubes of Zn2SnO4 sythesised using solvothermal method. Clear well defined edges of nanocubes with highly crystalline nature (inset of Fig.3b) were observed for the pristine ternary oxides as revealed from their lattice spacing (0.34 nm).
Figure 1. UV-Vis spectra of Zn2SnO4, GO and rGO decorated Zn2SnO4 nanocubes.
XRD studies of pristine Zn2SnO4 nanocubes and rGO decorated Zn2SnO4 nanocubes were shown in Fig. 2. The crystal structures and composition of Zn2SnO4 examined by XRD agree well with the rhombohedral (JCPDS card No.040736) phase with no impurity peaks either from ZnO and SnO2, thus proving the purity of synthesis method (inset of Fig.1d). Hybrid heterojunctions, Zn2SnO4@rGO shows peaks corresponding to ternary oxide along with a sharp diffraction peaks at 2θ = 11.4° and 28° corresponding to rGO sheets.
Figure 3. (a) and (b) TEM image of Zn2SnO4 nanocubes (inset showing the d-spacing value of Zn2SnO4) .
Figure 4. (a) and (b) TEM image of Zn2SnO4 nanocubes (inset showing the d-spacing value of Zn2SnO4) .
Fig.4 shows the HR-TEM studies of hybrid heterostructures of rGO@ Zn2SnO4 nanocubes. As evident from the TEM studies (Fig. 4a), the decoration of Zn2SnO4 nanocubes over the rGO sheets are clearly visible. Simillarly in the HR-TEM (Fig.4b) image the nanocubes with clear surface structures are visible which are decorating the rGO nanosheets and thus confirming the formation of hybrid heterostrucutres of rGO@ Zn2SnO4. Synthesised Zn2SnO4 nanocubes and rGO decorated Zn2SnO4 nanocubes were dispersed into solution state using ethanol and spin coated over the inter-digitated gold electrodes and annealed in oxygen atmosphere for 4 hours at 400°C. Then NO2 of varied concentrations (0.5 – 5 ppm) were introduced into the chamber and the corresponding changes in resistance were monitored using Labview software. The current voltage (I-V) characteristics performed at room temperature for the materials (Fig.5a) ensured the contact and adhesion between electrodes and sensing layers. Sensor response (S) towards the exposure to 1 ppm of NO2 gas a function of temperature (250 °C–500 °C) was measured for Zn2SnO4 nanocubes and depicted in Fig. 5b. The maximum response towards NO2 gas was observed at 400 °C, which was fixed as operating temperature of the sensor. Whereas, rGO decorated Zn2SnO4 nanocubes sensor showed response even in room temperature (30°C) owing to the formation of heterostructured nanocubes.
Figure 5. (a) I-V characteristic graph and (b) Temperature vs Concentration graph of Zn2SnO4 nanocubes and Zn2SnO4@rGO.
Dynamic gas sensing studies of Zn2SnO4 nanocubes were done at different concentrations of NO2 gas at the operating temperatures of 400 °C. The sensing performance of Zn2SnO4 nanocubes sensor are shown in Fig.6a. The graph shows an increase in resistance change upon exposure to NO2 gas, the typical behavior observed by n-type semiconductors materials [5]. It was observed that for higher concentrations of gas, response and recovery are fast, where as for lower concentration recovery was slow due to the memory effect. Simillarly Fig. 6b shows the dynamic response and recovery graph of rGO decorated Zn2SnO4 nanocubes towards varied concentrations of NO2. Compared to pristine Zn2SnO4 nanocubes, rGO decorated with Zn2SnO4 hybrid nanocubes showed an enhanced sensing performance (Fig. 7) at room temperature owing to the presence of heterostructures, spill over mechanism and potentiality of ultrafast electron transport along with the best surface to volume ratio from rGO sheets.
Figure 7. Comparitive sensor response graph of Zn2SnO4 nanocubes (blue) and Zn2SnO4@rGO (red).
Figure 6. Dynamic sensing graph of (a) Zn2SnO4 nanocubes at 400°C and Zn2SnO4@rGO at 30°C
Comparative sensor performance analysis (Fig.7a) revealed that hybrid heterostrucutres of rGO@ Zn2SnO4 nanocubes shows improved response than pristine Zn2SnO4 nanocubes and most importantly working temperature had been reduced to a greater extend. For pristine Zn2SnO4 nanocubes the working temperature was found to be higher as material needs more activation energy for the detection of NO2 gas even in sub ppm level. Heteronanostrucutres of zinc stannate shows more affinity towards the oxidizing gas especially NO2 and thus favoring the trace level detection. In hybrid heterostructures of rGO decorated Zn2SnO4 nanocubes further enhanced response were observed as compared to pristine nanocubes. As reported by Wehling et al., graphene and reduced graphene oxide shows an higher sensor response, due to the occurrence of NO2 molecule in the form of a single and open shell configuration favoring more adsorption on the surface [5]. The bulky closed shell species produces lower electron concentration compared to open shell counterpart of graphene and results in the detection of resistivity change even in molecular level and further presence of Zn2SnO4 nanocubes over the rGO sheets and its favored electron transport phenomenon by acting as spill-over zone strengthens the response thereof. Hence in the current study we observe an enhanced response for the hybrid heteronanocubes and opened up a new paradigm to improve the sensitivity and selectivity of semiconductor based gas sensors.
IV Conclusion In summary, we report the simple facile synthesis of hybrid heterostructures of rGO decorated Zn2SnO4 nanocubes by two step hydrothermal method. UV-Visible, XRD and HRTEM studies reveal the successful formation of hybrid heterostructures with uniform nanocubes of Zn2SnO4 decorating rGO sheets. NO2 gas sensing studies performance analysis reveals the hybrid heteronanocubes shows an enhanced sensor response at room temperature than the pristine Zn2SnO4 nanocubes (400°C) owing to the presence of heterostructures, spill over mechanism and potentiality of ultrafast electron transport along with the best surface to volume ratio from rGO sheets. ACKNOWLEDGMENT Authors gratefully acknowledge PSGIAS for providing experimental facilities and Mr. Vijayaraghavan for TEM analysis. REFERENCES [1] A Lianhai Zu, Yao Qin, and Jinhu Yang, “In-situ Synergistic Crystallization-induced Synthesis of Novel Au Nanostar-encrusted ZnO Mesocrystals with High-quality Heterojunctions for High-performance Gas Sensors”, J. Mater Chem. A , DOI: 10.1039/C5TA02182K, 2015. [2] Satyendra Singh, Archana Singh, Meher Wan, R.R. Yadav, Poonam Tandon,S. S. A. Rasool, and B.C. Yadav, “Fabrication of self-assembled hierarchical flowerlike zinc stanate thin film and its application as liquefied petroleum gas sensor”, Sensors and Actuators B, vol 205, p 102–110, 2014. [3] Lei Li, Shuijian He, Minmin Liu, Chunmei Zhang, and Wei Chen, “Three-Dimensional Mesoporous Graphene Aerogel-Supported SnO2 Nanocrystals for High-Performance NO2 Gas Sensing at Low Temperature”, Anal. Chem., vol 87, p 1638−1645, 2015. [4] Xiao Huang, Xiaoying Qi, Freddy Boey, and Hua Zhang, “Graphenebased composites” Chem. Soc. Rev., vol 41, p 666–686, 2012. [5] S. Basua,and P. Bhattacharyya, “Recent developments on graphene and graphene oxide based solid state gas sensors”, Sens. Actuators B, vol 173, pp. 1-21, 2012. [6] A. Sukhananazerin, D. Jayaseelan, V.P. Dinesh and P. Biji, “Controlled Fabrication of Highly Monodispersed, Gold Nanoparticles Grafted Polyaniline (Au@ PANI) Nanospheres and their Efficient Ammonia Gas Sensing Properties”, J Biosens Bioelectron., vol 6, p 165, 2015.
