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Sensors and Actuators B 197 (2014) 66–72

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

jo u r nal homep age: www.elsev ier .com/ locate /snb

umidity-sensing properties of chemically reduced graphenexide/polymer nanocomposite film sensor based on layer-by-layerano self-assembly

ongzhi Zhang ∗, Jun Tong, Bokai Xiaollege of Information and Control Engineering, China University of Petroleum (East China), Qingdao 266580, China

r t i c l e i n f o

rticle history:eceived 26 November 2013eceived in revised form 19 February 2014ccepted 21 February 2014vailable online 2 March 2014

eywords:raphene oxideayer-by-layer

a b s t r a c t

Chemically reduced graphene oxide (RGO)/poly(diallylimethyammonium chloride) (PDDA) nanocom-posite film sensor with high-performance humidity properties was reported in this paper. The film sensorwas fabricated on flexible polyimide substrate with interdigital microelectrodes structure. By the layer-by-layer nano self-assembly approach, graphene oxide and PDDA were exploited to form hierarchicalnanostructure, and then was partially reduced via solution-based chemically reduction for obtainingboth conductivity and chemically active defect sites. The effect of hydrobromic acid treatment on theconductivity properties of PDDA/GO film was examined, further verifying the advantage of hydrobromicacid reduction. The humidity sensing properties of the presented nanocomposite film sensor, such as

elf-assemblylexible deviceumidity sensor

repeatability, hysteresis, stability, response–recovery characteristics, were investigated by exposing tothe wide relative humidity range of 11–97% at room temperature. As a result, the sensor exhibited notonly excellent sensing behavior to humidity, but also fast response–recovery time and good repeatability,highlighting the unique advantages of layer-by-layer nano self-assembly for film sensors fabrication. Aslast, the possible humidity sensing mechanism of the proposed sensor was discussed in detail.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

Portable, reliable and low-cost humidity sensors play an impor-ant role in many measurement and control applications, includingndustry, agriculture and environmental fields [1,2]. So far, manyfforts have been made to develop high-performance humidityensors using various transduction techniques, such as capacitance3], resistance [4], optical fiber [5], field effect transistor (FET) [6],urface acoustic wave (SAW) [7] and quartz crystal microbalanceQCM) [8]. Furthermore, several kinds of sensing materials haveeen employed in humidity sensors, such as polymers [9], metalxide [6], carbon nanotubes [3] and composites [4,7], but theyave their own advantages and specific conditions of application.ecently, graphene has aroused tremendous interest for variousensing applications mainly due to its large specific surface areaor molecular adsorption and outstanding electrical properties such

s low noise level and high carrier mobility [10–12]. The applica-ion of graphene-based electronic sensors is still in its infancy untilow, but their promising performances are remarkable [13–16]. For

∗ Corresponding author. Tel.: +86 532 86981813x426; fax: +86 532 86981335.E-mail address: dzzhang@upc.edu.cn (D. Zhang).

ttp://dx.doi.org/10.1016/j.snb.2014.02.078925-4005/© 2014 Elsevier B.V. All rights reserved.

instance, mechanically exfoliated graphene has demonstrated aneffective detection of gaseous species down to the single molecularlevel [17].

Graphene can be obtained through various physical and chem-ical routes. Micromechanical cleavage of graphite was the initialapproach to produce single-layer graphene, but is not suitable forlarge-scale production due to its inefficient process and no con-trol over the number of layers [17]. Epitaxial grown of grapheneand chemical vapor deposition (CVD) had been reported to fab-ricate large-area graphene film, however, the two methods arelimited for specific conditions of application, such as high tem-perature, ultrahigh vacuum, and capital equipment dependence[14,18]. An alternative approach to cost-effectively large-scale pro-duce graphene-based devices is to first produce graphene oxide(GO) and then reduce it to obtain graphene for device applications.The basal planes and edges of GO are decorated with many oxy-gen functional groups, such hydroxyl, epoxy and carboxylic acidgroups, which make GO facilitate to form film by solution-basedfabrication process [19]. Nevertheless, chemical groups make GO

electrically insulating, which enables the inconvenient incorpora-tion of graphene oxide into resistive sensors. Recently, chemicallyreduction in GO and convert it into conductive state has beenreported [20,21].

D. Zhang et al. / Sensors and Actuators B 197 (2014) 66–72 67

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maintaining their good integrity. The reduction was carried out byimmersing the nanocomposite film devices into HBr acid solution

Fig. 1. Optical image of 4 × 6 sensors array on a flexible PI substrate.

In this present work, we fabricated a resistive-typeumidity sensor with chemically reduced graphenexide/poly(diallylimethyammonium chloride) (PDDA) nanocom-osite film by using layer-by-layer (LbL) nano self-assemblyethod. The film sensor was fabricated on flexible polyimide

ubstrate with interdigital microelectrodes structure. The sensinglm was characterized by using SEM and XRD. The humidityensing properties of the presented film sensor were investigatedy exposing to various relative humidity environments. As a result,he sensor exhibited excellent sensing properties to humidityt room temperature, demonstrating the unique advantages ofbL nano self-assembly for film sensors fabrication. The possibleumidity sensing mechanism of the sensors was also discussed.

. Experiment

.1. Materials

The commercially available high-purity graphene oxide (GO)anosheets (>99%) supplied by Chengdu Organic Chemicals Co.td (Chengdu, China) with thickness of 0.55–1.2 nm and diam-ter of 0.5–3 �m were employed in our experiment. The GOsed was a graphene nanosheet negatively decorated with oxy-en functional groups and carboxylic groups located at the sheeturface, facilitating the uniformly dispersion of GO into deion-zed (DI) water. The GO suspension was 0.25 wt.% in concentrationt pH 4.5. Polyelectrolytes used for LbL assembly were 1.5 wt.%oly(diallylimethyammonium chloride) [PDDA (Sigma–Aldrich

nc.), molecular weight (MW) of 200–350 K, polycation] at pH 7.5nd 0.3 wt.% poly(sodium 4-styrenesulfonate) [PSS (Sigma–Aldrichnc.), MW of 70 K, polyanion] at pH 6.5 with 5 M NaCl in both foretter surface coverage. The 40 vol.% hydrobromic (HBr) acid wasurchased from Sigma–Aldrich Inc., and used as received.

.2. Fabrication

The humidity sensor was fabricated on a flexible polyimidePI) substrate through microfabrication technology, includingpin coating of photoresist, exposure, development, and lift-offechnique. Ni/Cu interdigital electrode (IDE, 20 �m thick) wasputter-deposited on the PI substrate (75 �m thick). The IDE pat-ern window on the PI substrate provided an outline dimension of

mm × 5 mm, the IDE finger thickness was 20 �m, and the width

nd gap both was 75 �m. Fig. 1 shows the optical image of 4 × 6ensors array on the PI substrate, a good flexibility for the sensor isbserved. Fig. 2 illustrates the layer-by-layer (LbL) self-assemblyrocess of graphene oxide film as sensing materials. First, two

Fig. 2. Layer-by-layer fabrication process of PDDA/GO nanocomposite film.

bi-layers of PDDA/PSS were self-assembled as precursor layers forcharge enhancement, followed by the alternative sequence of theimmersion into PDDA and GO suspensions for five repetitive cycles.The immersing time here used was 10 min for polyelectrolytes and15 min for GO, and intermediate rinsing with DI water and dryingwith N2 were required after each monolayer assembly to reinforcethe interconnection between layers. Finally, the device was placedin the oven at 50 ◦C for 5–8 h. Thus, five bi-layer PDDA/GO film wasperformed by using LbL self-assembly technology. Fig. 3 depictsthe schematic diagram of the LbL-assembled sensor along withhierarchical structure of sensing films. Subsequently, a simple buthighly effective hydrohalic acid reducing method was developedto reduce PDDA/GO films into highly conductive films while

Fig. 3. Schematic diagram of the LbL-assembled sensor along with hierarchicalstructure of sensing films.

68 D. Zhang et al. / Sensors and Actuators B 197 (2014) 66–72

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n a sealed cuvette for 5–60 s at room temperature. After that, 100%ure ethanol rinse and air dry was performed.

.3. Instrument and analysis

The surface microscope of graphene oxide, chemically reducedraphene oxide, self-assembled (PDDA/PSS)2 and (PDDA/RGO)5anocomposite films were measured with Energy Dispersed Spec-roscopy (EDS, Oxford INCA PentaFETx3). The significant structuralhanges occurring during the chemical processing from pristineraphite to GO, and then to RGO and PDDA/RGO nanocomposite, areeflected in the X-ray diffraction (XRD) spectrum. XRD measure-ents are performed by the X-ray diffractometer (Rigaku D/Max

500PC, Japan) using Cu K� radiation with a wavelength of 1.5418 At temperature 25 ◦C and a relative humidity of approximately5%. The electrical properties of chemically reduced PDDA/GOanocomposite film with 40 vol.% HBr acid are measured by using

source measurement unit (Keithley 2400, USA).A schematic diagram of the experimental setup for exposure

f relative humidity (RH) is shown in Fig. 4. The humidity sensingroperties were investigated by exposing the PDDA/RGO film sen-or to various RH environments, which are achieved by severalaturated aqueous solutions and desiccant phosphorus pentox-de (P2O5) powder. Saturated solutions of LiCl, CH3COOK, MgCl2,2CO3, Mg(NO3)2, CuCl2, NaCl, KCl and K2SO4 in a closed vesselere used to yield approximately 11%, 23%, 33%, 43%, 52%, 67%, 75%,

5% and 97% RH levels, respectively. The experiments were per-ormed at an ambient temperature of 25 ◦C. The electrical resistancef presented film sensor was measured in DC mode using a data log-ing/switch unit (Agilent 34970A), which was connected to a PChrough RS-232 interface. The response of the sensor as a functionf RH was achieved by exposing the sensor inside the closed vessels

ith different RH environment for the uptake of water molecules.

he sensor was exposed to dry air conditioned by P2O5 powder (RH%) for the release of water molecules. The figure of merit used forhe evaluation of sensor performance is the normalized response

ty sensing experimental setup.

(S), determined by S = �R/R0 = (RRH − R0)/R0 × 100%, where RRH andR0 are the electrical resistance of the sensor in the given RH and dryair, respectively.

3. Results and discussion

3.1. SEM and XRD characterization

Fig. 5 shows the observed SEM images of graphene oxide, chem-ically reduced graphene oxide, self-assembled (PDDA/PSS)2 and(PDDA/RGO)5 nanocomposite film. The results in Fig. 5(a) and (b)indicate that the graphene oxide and chemically reduced grapheneoxide consist of randomly aggregated, thin, crumpled sheets closelyassociated with each other and forming a disordered structure.Fig. 5(c) illustrates the robust wormlike or vermiculate patternfor the morphology of polyelectrolyte multilayer film (PDDA/PSS)2deposited, and the rough ridge structures facilitated the sequentself-assembly of GO/polymer. Fig. 5(d) shows the observed contin-uous and crumpled structures of PDDA/RGO film.

Fig. 6 plots the XRD spectrum for pristine graphite, GO, RGO andPDDA/RGO film. The measured XRD results indicate the interlayerspacing and stacking behavior of the RGO film via the reductionwith HBr acid. It shows that the interlayer distance of the RGO filmshrinks to 3.60 A (2� = 24.7◦) from 8.27 A (2� = 10.7◦) for the initialGO film before the reduction and demonstrates that the oxygen-containing groups are eliminated from the graphene sheets due tothe treatment of HBr acid [20]. We also find that the value of inter-layer distance of the PDDA/RGO film is 3.67 A (2� = 24.2◦), which islarger than that of RGO film (3.60 A) due to the surrounding polymerchains and difficulties to form obvious tight lamellas.

3.2. Reduction results and mechanism

Fig. 7 plots the experimental current–voltage (I–V) curve ofthe PDDA/RGO films treated with 40 vol.% HBr acid under differ-ent reduction duration. The I–V characteristic is investigated by

D. Zhang et al. / Sensors and Actuators B 197 (2014) 66–72 69

hene

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Fig. 5. SEM characterization of (a) graphene oxide, (b) chemically reduced grap

pplying a sweeping voltage of −1 to +1 V. The linearly I–V curveseasured for the PDDA/RGO film indicates that well-ohmic con-

act formed between the electrode and the PDDA/RGO films. Theeciprocal of the slope of the I–V curve represents the resistance of

he PDDA/RGO films. The HBr acid treatment results in a decreasen the resistance of PDDA/RGO films, and the treatment duration isontributed to tune the electrical properties of PDDA/RGO films. Fornstance, the film resistance is changed from electrically insulating

ig. 6. XRD spectrum for pristine graphite, GO, RGO film and PDDA/RGO nanocom-osite.

oxide, (c) self-assembled (PDDA/PSS)2 and (d) PDDA/RGO nanocomposite film.

to 381 � after HBr acid reduction for 5 s, and keeps decreasing to127 � after reduction for 60 s. Thereby, PDDA/RGO films with resis-tance on the order of hundreds of ohm were obtained after HBrtreatment by decades of seconds.

The effectively removal of epoxy and hydroxyl groups via HBracid reduction was the key for creating conductive film from insu-

lating GO [21]. During the reduction of GO in the presence of HBracid, the ring-opening reaction of epoxy groups attached aboveand below the carbon sheet can be catalyzed by acidic agents andconverted them into hydroxyl groups [22], and then the hydroxyl

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Fig. 7. Electrical characterization of chemically reduced PDDA/GO film under dif-ferent reduction time by HBr acid.

70 D. Zhang et al. / Sensors and Actuators B 197 (2014) 66–72

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tive humidity (RH) X can be represented as Y = 0.32X + 4.66, and thelinear regression coefficient, R2, is 0.9960. The error bars repre-sent standard deviation (SD) from the mean based on six sensors

ig. 8. Resistance measurement of the PDDA/RGO film sensor under switching RH.

roups were substituted by bromine atoms without destroying thearbon lattice [23], at last the bromine atoms were easily removedrom the carbon lattice than hydrogen because the binding energyf the C Br bond was lower than that of the C H bond. There-ore, a highly effective reduction of GO into conductive RGO byydrobromic acid is expected.

.3. Humidity-sensing properties and mechanism

Fig. 8 shows the real-time resistance measurement in practicalpplication level of the PDDA/RGO film sensor exposed to vary-ng relative humidity. The switching RH test is performed throughxposure/recovery cycles for different RH environments between% and 11%, 23%, 33%, 43%, 52%, 67%, 75%, 85%, 97% RH, and thenonversely from high RH to low RH. Each exposure/recovery cycles carried out by an exposure interval of 100 s followed by a recov-ry interval of 100 s at dry air. Each cycle is indicated by therea between two closely adjacent dotted lines marking start andnd. A clear increase in the sensor resistance is observed with thencreasing of RH in a large range of 11–97%, and the correspond-ng normalized response values are calculated to be 8.69%, 13%,4.99%, 18.23% 20.91%, 25.32%, 28.14%, 31.19%, 37.41% when theensor exposed to 11%, 23%, 33%, 43%, 52%, 67%, 75%, 85%, 97% RH.

e also measured the hysteresis of the fabricated sensor throughhe comparison of RH-increasing for water molecules absorptionnd RH-decreasing for water molecules desorption with RH rangingrom 11% to 97%, the maximum hysteresis was about 2% occurredt 33% RH.

Fig. 9 shows the repeatability of PDDA/RGO film sensor per-ormed under the same experimental conditions. The repeatabilityharacteristics is measured for five exposure/recovery cyclesepeatedly for 43%, 75% and 97% RH. The film humidity sen-or exhibited a clear response–recovery behavior and acceptableepeatability for humidity sensing. Here, we define the repeatablerror as the ratio of maximum deviation to full-scale measurementnder the same conditions, and the repeatable error is less than.5%.

In the previous measurements, the RH exposure interval washosen to be 100 s in order to demonstrate the potential of theabricated sensor for being employed in real-time RH monitoring

pplications, where an immediate response to varying RH is nec-ssary. However, it is equally important to consider the responsend recovery times of our sensors to given RH environment. Fig. 10emonstrates the time-dependent response and recovery curves

Fig. 9. Repeatability of PDDA/RGO film sensor for (a) 43%, (b) 75% and (c) 97% RH atroom temperature.

of the sensor to a relative humidity pulse between 0% and 43%,75%, 97% RH, respectively. Longer exposure interval of 300 s is per-formed for reaching saturation stage. The time taken by a sensor toachieve 90% of the total resistance change is defined as the responseor recovery time [24]. Response time and recovery time of around108–147 s and 94–133 s are observed, respectively, better than thatof conventional humidity sensors [25–27].

The normalized responses of six sensors prepared under thesame procedure are investigated in a wide RH range. The averagenormalized response for the sensors exhibited a linear relation-ship toward the RH range from 11% to 97%, which is shown inFig. 11. The fitting equation for normalized response Y and rela-

Fig. 10. Typical response and recovery curves of the PDDA/RGO film sensor to arelative humidity pulse between 0% and 43%, 97% RH, respectively.

D. Zhang et al. / Sensors and Actuators B 197 (2014) 66–72 71

Table 1Performance of the presented sensor in this work compared with previous work.

Sensor type Sensing material Fabrication method Measurement range Response Reference

Resistive-type PDDA/RGO LbL self-assembly 11–97% RH 8.69–37.43% This paperCapacitive-type Graphene oxide Solution dripping 25–65% RH −9.5 fF/%RH [28]Quartz crystal

microbalance(QCM)-type

Graphene oxide Spin-coating 6.4–93.5% RH 22.1 Hz/% RH [29]

Stress-type Graphene oxide-silicon bi-layer Spin-coating method 10–98% RH 28.02 �V/%RH [30]Surface acoustic wave

(SAW)-typeGraphene oxide Droplet-by-dropl

Conductive-type Defect graphene Solvothermal me

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ig. 11. Normailized response of PDDA/RGO film sensors as a function of RH.

xposed to given RH. A SD within 2.6% was obtained, indicating aood sensor-to-sensor reproducibility.

Fig. 12 presents the long-term stability of the PDDA/RGO filmoated sensor. The stability of the sensor was measured over dif-erent days. The response of the sensor did not significantly changemong the three testing RH environment of 23%, 52% and 75% RH ateast 60 days. Table 1 presents the humidity sensing characteristics

f the proposed humidity sensor in comparison with previous work28–32]. The response and measurement range for the preparedensor are comparable to those of the sensor made from graphene

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ig. 12. Long-term stability of PDDA/RGO film sensor exposed to 23%, 52% and 75%H.

et atomization 8–18% RH 1.54 kHz/%RH [31]

thod 3–30% RH 0.27–3.33% [32]

oxide or defect graphene film by solution dripping, spin-coating,droplet-by-droplet atomization and solvothermal methods.

PDDA/RGO thin film is very sensitive to water molecules absorp-tion, thus making it a very promising platform for highly sensitivehumidity sensor. RGO films not only have high surface-to-volumeratios, but also possess a large density of surface vacancies andhydrophilic functional groups, such as carboxylic groups, thus RGOcan facilely capture water molecules from external environment.At low RH, the electron transport through RGO exhibits p-typesemiconducting behavior and is dominated by positive charge car-riers (holes), whereas the adsorbed water molecules on the sensingfilm surface serves as electron donors [33,34]. The chemisorbedwater molecules cause a reduction of hole concentration in the p-type RGO sheets, resulting in the increase of film resistance. Thisp-type semiconducting behavior agrees with the results reportedfor RGO prepared by chemically or thermally reduction [35,36]. Athigh RH, the multilayer adsorbed water molecules may be inoizedto produce hydronium ions (H3O+) as charge carriers. The protonstransport via ionic conductivity that was presented by Grotthuss[37], H2O + H3O+ → H3O+ + H2O, which cause a decrease of sen-sor resistance. Meanwhile, the adsorbed water molecules enterinto the multi-layer film can lead to an interlayer swelling effectoccurs. The swelling of the PDDA/RGO increases their interlayerdistance largely and deteriorates the degree of connectivity in theLbL-assembled nanocomposite film, resulting in an increase of sen-sor resistance [38,39]. Consequently, there is a trade-off betweenhumidity-induced swelling effect and ionic conduction occurs inthe humidity sensor. In our systems, an increase in sensor resistanceis observed at high RH. This can be interpreted by the contributionof ionic conductivity is relatively less compared with the swellingeffect of the nanocomposite films. Based on the discussion above,we can consider that the major sensing mechanism is attributedto the p-type semiconducting properties of RGO at low RH, andthe interlayer swelling of PDDA/RGO film at high RH rather thanthe ionic conductivity. The observed fast response and recoveryproperty of the sensor is due to hydrophilic and multi-layer thinfilm structure with high spatial dispersion in PDDA/RGO. On onehand, the hydrophilic property makes the sensing film capturewater molecules easily, leading to a fast response. On the otherhand, the multi-layer thin film structure facilitates the release ofwater molecules from the interlayer for reaching dynamic equilib-rium between external humidity level and internal water content.Hence, a clear and swift recovery of the sensor can be exhibited.The humidity dependent electrical characteristics of PDDA/RGOnanocomposite render it as an ideal candidate material for buildingMEMS/NEMS humidity sensors.

4. Conclusions

In this study, a humidity sensor with reduced grapheneoxide/polymer film coated was fabricated by using layer-by-layer(LbL) nano self-assembly and chemical reducing method. Grapheneoxide and PDDA were alternatively deposited on flexible polyimide

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ubstrate to form hierarchical nanostructure, and then was partiallyeduced via solution-based chemically reduction. Hydrobromiccid reducing method realized the conversion of insulating GO intoighly conductive RGO while maintaining their good integrity atoom temperature. The humidity sensing properties of PDDA/RGOlm sensors were investigated with varying RH at room temper-ture, exhibiting a swift response–recovery characteristic whenxposed to different relative humidity levels, with an outstand-ng stability and linearity. The possible mechanisms for graphenexide reduction and humidity sensing of PDDA/RGO film werelso investigated in this study. It is suggested that the humidityesponse of the presented sensor is ascribed to the p-type semicon-uctor behavior and humidity-induced interlayer swelling effectf PDDA/RGO film. This work highlights the unique advantages ofayer-by-layer nano self-assembly for film sensors fabrication.

cknowledgements

This work was supported by the National Natural Science Foun-ation of China (Grant No. 51205414), the Promotive Researchoundation for the Excellent Middle-Aged and Youth Scientists ofhandong Province of China (Grant No. BS2012DX044), the Sci-nce and Technology Development Plan Project of Qingdao (Granto. 13-1-4-179-jch), and the Fundamental Research Funds for theentral Universities of China (Grant No. 12CX04065A).

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Biographies

Dongzhi Zhang received his BS degree from Shandong University of Technology in2004, MS degree from China University of Petroleum in 2007, and obtained PhD.degree from South China University of Technology in 2011. From 2009 to 2011, heworked as a visiting scholar of Mechanical Engineering at the University of Min-nesota, USA. He is currently an assistant professor at China University of Petroleum(East China), Qingdao, China. His fields of interests are gas and humidity sensingmaterials, nanotechnology, and polymer electronics.

Jun Tong received his BS degree in electrical engineering and automation fromChina University of Mining and Technology in 2011. Currently, he is graduate stu-dent at China University of Petroleum (East China), Qingdao, China. His fields ofinterests include carbon nanomaterials-based gas sensors, precision measurementtechnology and instruments.

Bokai Xia received his PhD degree in chemical processing from China University ofPetroleum in 2001. From 2007 to 2008, he worked as a visiting scholar at Loughbor-ough University. Currently, he is professor at China University of Petroleum (EastChina), Qingdao, China. His areas of interest are precision measurement technologyand instruments.