zinc oxide nanoparticles layered thin film QCM sensor

zinc oxide nanoparticles layered thin film QCM sensor

Author’s Accepted Manuscript Enhanced Humidity-sensing properties of novel graphene oxide/zinc oxide nanoparticles layered thin film QCM sensor Zhen Y...

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Author’s Accepted Manuscript Enhanced Humidity-sensing properties of novel graphene oxide/zinc oxide nanoparticles layered thin film QCM sensor Zhen Yuan, Huiling Tai, Xiaohua Bao, Chunhua Liu, Zongbiao Ye, Yadong Jiang www.elsevier.com

PII: DOI: Reference:

S0167-577X(16)30122-7 http://dx.doi.org/10.1016/j.matlet.2016.01.122 MLBLUE20250

To appear in: Materials Letters Received date: 6 January 2016 Revised date: 22 January 2016 Accepted date: 24 January 2016 Cite this article as: Zhen Yuan, Huiling Tai, Xiaohua Bao, Chunhua Liu, Zongbiao Ye and Yadong Jiang, Enhanced Humidity-sensing properties of novel graphene oxide/zinc oxide nanoparticles layered thin film QCM sensor, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2016.01.122 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Enhanced humidity-sensing properties of novel graphene oxide/zinc oxide nanoparticles layered thin film QCM sensor Zhen Yuan, Huiling Tai*, Xiaohua Bao , Chunhua Liu, Zongbiao Ye, Yadong Jiang* State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Optoelectronic Information, University of Electronic Science and Technology of China (UESTC), Chengdu 610054, P. R. China Abstract In this letter, a novel graphene oxide (GO, top layer)/zinc oxide nanoparticles (ZnO, bottom layer) layered thin film was sprayed on quartz crystal microbalance (QCM) for humidity detection within the range of 11.3% RH and 97.3% RH. A remarkable enhancement in sensitivity (41.10 Hz/%RH) of GO/ZnO sensor was achieved, compared with that of ZnO/GO sensor (25.11 Hz/%RH) and single film sensors (8.85 Hz/%RH for ZnO and 16.67 Hz/%RH for GO). Meanwhile, the resultant GO/ZnO sensor also exhibited rapid response/recovery rates (less than 9 s/5 s @63.2%) and stable repeatability. Based on morphological and structural analyses, it was speculated that the superior humidity-sensing properties of GO/ZnO sensor might be ascribed to the supporting role of ZnO layer, the smallest nanoparticles aggregation size and the higher mechanical modules. Keywords: Graphene oxide (GO); Zinc oxide (ZnO); Nanoparticles; Layered thin film; Quartz crystal microbalance (QCM); Sensors. 1. Introduction In recent years, many efforts have been focused on developing novel humidity sensing materials based on different types of sensing devices[1, 2]. Among them, zinc oxide (ZnO) is one of the most promising materials that owns perfect chemical and mechanical durability with thermal stabilities[3], and graphene oxide (GO) has been employed as an effective novel humidity-sensing material due to its abundant hydrophilic groups[4]. On the other hand, quartz crystal microbalance (QCM) has been proven to be a sensitive technique for humidity detection according to Sauerbrey’s equation[5]. However, to our knowledge, GO/ZnO layered film based QCM humidity sensor has not been explored yet. In this letter, a novel GO/ZnO layered thin film QCM humidity sensor was proposed and fabricated by a facile spray process to detect the relative humidity (RH) from 11.3% RH to 97.3% RH at room temperature, and morphology, structure and impedance analyses were performed to investigate the characteristics of sensing films. 2.Experimental 2.1 Fabrication of layered film QCM sensors All the chemicals used in the experiment were analytical reagents. Firstly, ZnO colloid (Sigma-Aldrich Co. LLC.) and GO solution (Sinocarbon Materials Technology Co., Ltd) were sonicated prior to use, and then 0.5 ml ZnO colloid (1 mg/ml) and 0.5 ml GO solution (1 mg/ml) were deposited on both sides of rinsed QCM devices (AT-cut, 10MHz, Wuhan Hitrusty Electronics Co. Ltd. China) in sequence by spray method as reported before [6], marked as GO/ZnO film. For comparison, *Corresponding author:Huiling Tai, E-mail: [email protected] Tel: 86-28-83206505 Fax: 86-28-83206123; Yadong Jiang, E-mail: [email protected]

ZnO (top layer)/GO, pure GO and ZnO films QCM sensors were prepared through the same process. After that, QCM sensors were dried by heat treatment at 60°C for 48 h in vacuum. 2.2 Measurement and characterization The surface morphologies of films were observed by field emission scanning electron microscope (FESEM) (Inspect F50, FEI Co. Ltd). Ultraviolet-Visible (UV-Vis) and Fourier transform infrared (FTIR) spectra were measured with UV-1700 pharmaspec (Japan, Shimadzu) and Spectrum 400 (American, PerkinElmer) to investigate the interaction between GO and ZnO, respectively. QCM were excited by QCM-5 Oscillator (Shenyang Vacuum Technology Institute, China), and humidity-sensing properties were evaluated by collecting the resonant frequency of QCM by SS7200 Frequency Counter (SUIN INSTRUMENTS Co. Ltd., Shijiazhuang, China), in which various RH was generated by the saturated solutios of LiCl (11.3% RH), MgCl2 (32.8% RH), NaBr (57.6% RH), NaCl (75.3% RH) and K2SO4 (97.3% RH), respectively. Impendence analysis was executed by Agilent 4294A precision impedance analyzer (Agilent Tech. Inc., America). 3. Results and discussion High magnification FESEM images of pure GO (a), pure ZnO (b), ZnO/GO (c) and GO/ZnO (d) films are shown in Fig. 1. The obvious wrinkle surface morphology could be observed for pure GO and ZnO/GO films, whereas GO/ZnO film displays relatively smooth morphology feature, which might be due to the supporting effect of ZnO nanoparticles or their agglomerates. One interesting phenomenon here is the decreased size of ZnO nanoparticles agglomerates for GO/ZnO film based on statistics results. The average Feret's diameter of ZnO agglomerates in pure ZnO film is 690 nm, whereas it becomes smaller in ZnO/GO (590 nm) and GO/ZnO films (521 nm), which effectively enlarges the interphase boundaries and grain boundaries. It has been demonstrated that the presence of defects like interphase boundaries and grain boundaries would effectively enhance the physical properties of ZnO[7, 8]. The morphological difference was further supported by the structure analyses. FTIR spectra of GO and GO/ZnO films are demonstrated in Fig. 2(a). Compared with FTIR spectrum of pure GO film, the peak at 411cm-1 in the spectrum of GO/ZnO film is attributed to the Zn-O vibration of ZnO which confirms the existence of ZnO. Meanwhile, a slight decrease of the side peak at 3586 cm-1(stretch vibration of hydroxyl) could be observed and peaks at 1730 cm-1(stretch vibration of C=O in carboxyl )[9], 1257 cm-1(stretch vibration of epoxy group) and 1060 cm-1(stretch vibration of C-O)[10] almost disappeared. These changes imply the decrease of GO's hydroxyl groups. It is presumed that hydroxyl groups on GO nanosheets and ZnO surface might form C-O-Zn bond, therefore, combined with the redissolution effect at the interface of ZnO nanoparticles and GO sheets, the aggregation of ZnO nanoparticles is weakened and the dispersity of ZnO nanoparticles is further improved, leading to the decreased size of ZnO agglomerates. Fig. 2(b) shows the UV-Vis adsorption spectra of pure GO, pure ZnO and GO/ZnO films. An obvious peak at 225 nm is observed for GO film, which could be assigned to -* transition of aromatic C-C bonds. A peak at 362 nm with a sharp slope is assigned to the forbidden band transition of ZnO[11] whereas it shifts to 357 nm after the introduction of GO. According to Kubo theory, the excitation energy of ZnO nanoparticles will increase with the decrease of grain diameter[12]. Therefore, the blue shift of absorption peak of ZnO in GO/ZnO layered film indicates that the grain size of ZnO nanoparticles is decreased. Therefore, the spectral analyses are consistent with FESEM results. For humidity-sensing characteristics evaluation of sensors, ZnO, GO, ZnO/GO and GO/ZnO films QCM sensors were exposed to various RH levels with fixed intervals (300 s), and the dynamic

response curves of all sensors are depicted in Fig. 3(a), in which 11.3% RH level is defined as the baseline. The sensitivity is calculated as the ratio of response difference to RH change (f/RH), and the response/recovery time is defined as the sensor achieves 63.2% of frequency shift. From Fig.3(a), a remarkable improvement of sensitivity for GO/ZnO film sensor (41.10Hz/%RH) could be achieved compared with ZnO/GO film sensor (25.11 Hz/%RH) and pure film based sensors (16.67Hz/%RH for GO and 8.85 Hz/%RH for ZnO). The response/recovery time of ZnO/GO sensor is less than 9 s/5 s. Meanwhile, a stable behavior is also obtained at different RH levels (32.8% RH and 75.2% RH) with three repeated cycles, as shown in Fig.3(b) which is inserted in Fig.3(a), indicating the excellent reproducibility. By fitting the impedance spectrum with BVD equivalent circuit model , the quality factors and motional resistances of GO and GO/ZnO film sensors in various RH are calculated and exhibited in Fig. 3(c). According to the research reported by Goyal, the energy losses of QCM could be effectively suppressed by sensing films with high quality factor[2]. As shown in Fig.3(c), GO/ZnO sensor possesses higher quality factor and lower motional resistance than pure GO one, indicating the higher mechanical modules of GO/ZnO layered film and being beneficial to the resonance of QCM sensor, which should be attributed to the high modulus of ZnO nanoparticles (approximately 140 GPa[13]). Therefore, the GO/ZnO layered film sensor should possess superior mechanical stability than pure GO one. Based on the characterization and measurement results, the sensing mechanism model of purposed GO/ZnO sensor is established and shown in Fig. 4. It has been proved that layered film sensors exhibit superior performances than pure film sensors, which should be attributed to the superimposed sensing components (GO and ZnO) and further more water molecules adsorption sites[14]. Furthermore, the distinct behavior between GO/ZnO and ZnO/GO film sensors might be ascribed to different structure features. For GO/ZnO layered film, on one hand, ZnO nanoparticles play a supporting role for upper GO nanosheets and thus water molecules could diffuse to adsorption sites from both surface and gaps between GO and ZnO[15] ; On the other hand, the agglomerates size in the lower ZnO layer is less than that in ZnO/GO film based on FESEM and FTIR results. Therefore, the humidity-sensing area as well as interphase boundaries and grain boundaries of GO/ZnO film could be further improved, leading to the increased sensitivity and short response/recovery time. 4. Conclusion A novel GO/ZnO layered thin film was utilized onto QCM device through spray method for the humidity detection from 11.3% RH to 97.3% RH at room temperature, and its superior performances with high sensitivity (41.10 Hz/%RH), rapid response/recovery (9 s/5 s @63.2%) and excellent repeatability were obtained compared with pure film (ZnO and GO) and ZnO/GO film sensors, which might be ascribed to superimposed absorption sites, supporting role of ZnO layer and decreased ZnO (including grains and agglomerates) size. The impedance analysis indicated that the GO/ZnO layered thin film possesses higher modules and thus better mechanical properties. The proposed GO/ZnO layered thin film QCM sensor might give a promise for high-performance humidity sensing applications. 5. Acknowledgment This work is partially supported by the National Science Funds for Creative Research Groups of China (Grant No. 61421002), Program for New Century Excellent Talents in University (Grant No. NCET-13-0096) and the Open Foundation of State Key Laboratory of Electronic Thin Films and Integrated Devices(KFJJ201413).

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Figure captions: Fig. 1 FESEM image of (a)GO, (b)ZnO, (c)ZnO/GO and (d)GO/ZnO films. Fig. 2 (a)FTIR spectra and (b)UV-Vis spectra of ZnO, GO and GO/ZnO films Fig. 3 (a)Dynamic response curve of QCM sensors (b)repeatability curve of GO/ZnO sensor in n low RH(32.8%RH) and high RH(75.2%RH) and (c) The Quality factor and motional resistance of GO/ZnO(solid symbol) and GO(open symbol). Fig. 4 Schematic diagram of sensing mechanism of (a)GO, (b)ZnO, (c)ZnO/GO and (d)GO/ZnO

Highlights 1. Graphene oxide/zinc oxide layered film was first proposed for humidity sensing.

2. synergistic effect between GO and ZnO nanoparticles was observed and discussed. 3. High sensitivity, rapid response/recovery and stable repeatability were obtained. 4. Humidity-sensing models were built and discussed.