Microstructure Related Synergic Sensoring Mechanism in Graphene

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Microstructure Related Synergic Sensoring Mechanism in Graphene Oxide Humidity Sensor Neng Wan, Tianmeng Wang, Xinyuan Tan, Shun Lu, Lili Zhou, Jianqiu Huang, Wei Pan, Yuanmin Yang, and Zhiyong Shao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09744 • Publication Date (Web): 08 Dec 2017 Downloaded from http://pubs.acs.org on December 13, 2017

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The Journal of Physical Chemistry

Microstructure Related Synergic Sensoring Mechanism in Graphene Oxide Humidity Sensor

Neng WAN1,*, Tianmeng WANG2, Xin-yuan TAN1, Shun LU1, Li-li ZHOU1, Jian-qiu HUANG1, Wei PAN 2,#, Yuan-min YANG1, Zhi-yong SHAO1

1. SEU-FEI Nano-Pico Center, Key Laboratory of MEMS of Ministry of Education, School of Electrical Science and Engineering, Southeast University, 210096 Nanjing, China 2. Laboratory of Condensed Matter Spectroscopy and Opto-Electronic Physics, and Key Laboratory of Artificial Structures and Quantum Control (Ministry of Education), School of Physics and Astronomy, Shanghai Jiao Tong University, 200240 Shanghai, China

*: [email protected]; #: [email protected]

Abstract: Graphene oxide (GO) is a good candidate for high-performance humidity sensors with high responsitivity and rapid response. The GO sensoring mechanism is highly concerned and still not yet well understood. In this study, a systematic examination of GO capacitive humidity sensors was performed, revealing GO sensors' excellent sensitivity properties with two orders of capacitance change (within a relative humidity range of 12 – 97 RH%) and sub-0.1 seconds ( ) responsive at room temperature being observed. A Fourier transformation infrared technique was used to study the structural characteristics of GO films under both steady and transient humidity states. Interaction of water molecules with the functional groups in GO was detected, which induced evident changes of the GO structure and directly related to the sensoring processes. Free-state liquid water was found to play an important role in the sensoring processes. A synergistic sensoring mechanism is suggested: functional groups work to reserve water and produce larger layer space for better water transportation, while regions without functional groups work as effective channels for water transportation.

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Introduction: Graphene oxide (GO) is a graphene derivative with partially oxidized structures1. It is rich in functional groups such as COOH, C-O-C and C-OH2, 3. Graphene itself is hydrophobic in nature; however, the functional groups are normally hydrophilic. The ability to work as an amphiphilic material with high water solubility makes GO unique4. As a result of its simple preparation, high production, and easy functionalization, GO is a good choice for high-performance sensors. The capacitive humidity sensor is a typical kind of sensor that can be fabricated at low cost and is also easy to integrate. It works due to the different dielectric constant between water and the sensor material5, 6. Water absorption by the sensor material results in a change of the dielectric constant and thus a change in capacitance. Correspondingly, the capacitance is related to the amount of water in the sensor material. The interaction of the host sensor material with water molecules and the transportation/reservation of water molecules in the sensor material thus play central roles in the sensoring process. High-performance capacitive humidity sensors based on GO have been recently reported7 – 10. The GO sensors exhibiting high responsitivity with orders of capacitance change in several tenths of relative humidity (RH%) range were routinely observed. Swift response times down to tenths of micro-seconds at room temperature have been observed. These properties make GO humidity sensors very different from traditional sensors, with probably a rather different sensoring mechanism. However, although previous works have concentrated mainly on the performance of GO sensors, how the high sensoring performance is achieved has not been systematically studied. In the current work, we fabricated a capacitive humidity sensor using GO films. The as-deposited sensor was found to have large responsitivity (capacitance changed by two orders of magnitude in the 12–97 percent RH% range) and fast response ( less than 0.1 second) at room temperature. The effects of post-annealing and working temperature on the sensor's performance were also studied. In order to get deep insights into the sensoring mechanism, we used the Fourier transformation infrared (FTIR) technique to resolve structural information on the GO films under both steady and transient state during GO exposure to different humidities. Proofs were derived from the interaction of water molecules with the functional groups in GO structure. This 2


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interaction was found to be directly related to the sensoring process. Liquid water was also found to play an important role during the sensoring process. The synergistic sensoring mechanism of the GO film will be discussed in detail based on a GO structure model. Our research indicates that the composited hydrophobic and hydrophilic structures may be a good system for creating an efficient humidity sensor.

Experimental Graphene oxide was fabricated by the Hummers method11, 12 using graphite powder as a starting material and KMnO4 as the oxidizing agent. The product was well washed to eliminate metal and salt ions. Sensors were fabricated by spraying GO aqueous solutions (1g/L) onto Si/SiO2 (thickness: 400 micron/200 nm) substrate with prefabricated aluminum interdigital electrodes (IDE, finger width: 5 micron, finger space: 10 micron, finger length: 2500 micron. Number of fingers: 300), giving an active area of 2.5 mm×3 mm. For better dispersion, the GO solution was ultrasonically activated for 10 minutes before use. During spraying, the substrate temperature was maintained at around 80℃ using a hot plate (IKA C-MGA HS-7) in order to promote the solvent evaporation. Annealing at elevated temperatures was performed by using a muffle furnace. Steady state sensoring measurement was performed by using a temperature- and humiditycontrolled chamber (C340, Omega Engineering) with a chamber volume of ~1 m3 and an air mixing system. The sensor was placed at the lower center of the chamber near the reference sensors of the measurement system. In a typical high resolution test cycle, the humidity is ramped up slowly by steps of ~ 5 RH% from a low humidity state to a high humidity state and then ramped back down. To achieve a relative stable humidity distribution in the test chamber, the humidity was allowed to stabilize for at least 10 minutes at each data point. High resolution normally gives an uncertainty of less than 5% for the target RH%. However, due to the very time-assuming test process, a high-resolution test mode was performed only for selected samples. (Normally, we use a quicker test mode with reduced data points but with higher uncertainty (~ 15%) due to the slow chamber response.) A TH2826 (Tonghui Electronic Co., Ltd.) impedance analyzer was used to take the capacitance measurements. Sensor properties at varying temperatures were performed within a 7-L cylinder chamber that was controlled to the desired temperature using ethanol (from the outside wall) with controlled 3



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water vapor injection. The response of the device was tested by shifting the sensors rapidly from a high humidity to a lower level humidity chamber13. The rapid shifting process ensured the measurement of the sub-second response time. Field emission scanning electron microscopy (SEM) was performed using a Zeiss Ultra Plus machine. FTIR was performed by a Bruker vertex 70 in microscopic mode. A gas chamber with BaF2 or ZnSe windows was used. Double-side polished silicon wafer (undoped, thickness: 400 microns) was used as substrates, which were washed with a mixture of NH3·H2O, H2O2 and deionized water (18 MΩ, Millipore water system) before use. Two to four drops of GO aqueous solution were dropped onto the silicon substrate and allowed to spread and dry naturally at room temperature. A blank silicon wafer washed by the same procedures was used as the reference for FTIR measurement. The baseline drifting was verified and checked before and after each measurement sequence by measuring the same blank silicon region; drifting was found to be insignificant. The chamber absorption at different RH% was found to be negligible after being checked at the blank silicon region at the designated RH% values. Steady-state measurements were integrated as usual by 128 lines. Time-resolved measurements were performed by single scans with line intervals of 0.85 seconds. During the time-resolved measurements, the humidity in the chamber was switched rapidly from 97 RH% to 12 RH% (or 12 RH% to 97 RH%). A typical chamber response time of ~ 1 second was estimated by measuring the capacitance response of the GO sensor in the chamber.

Results and Discussion Fig. 1 (a) shows a schematic of the device structure used for sensor measurement. The SEM image (Fig. 1 (b)) of the sensor surface show evenly distributed IDE structures. The GO film was uniformly deposited on the surface. The magnified SEM image (Fig. 1 (c)) obtained between the IDEs shows the GO films with fringe structures as indicated by the yellow arrows. The GO structure was also observable at the edge of the electrode as shown in Fig. 1 (d), covering and contacting the electrode. The GO sensors processed at different temperatures show different sensoring properties as displayed in Figs. 1 (e) – (j): 1) sensors annealed at higher temperatures (see Fig. 1 (e)), for longer 4


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time (see Figs. 1 (h) – (j)) showed larger capacitance. For example, for a low-humidity region (typically, less than 50 RH %), the well annealed (that is, annealed at higher temperatures, for a longer time) sensors showed typical capacitances of several nF, while their counterparts showed capacitances typically below 1 nF; 2) both high temperature and longtime annealing reduced the sensors’ sensitivity (Figs. 1 (e) – (j)). The as-deposited sensors showed a change in capacitance of two orders of magnitude (similar to previous report [10]), from hundreds of pF to several nF in the measured RH % range, while the well annealed sensors showed a capacitance change within 50%. Also, the former normally showed a capacitance change larger than 4 nF, while the latter normally showed a capacitance change of less than 3 nF; 3) annealing changed the sensor response from S-type (Fig. 1 (f)) to a linear dependence (Fig. 1 (g)). The changing of the sensor properties may have been caused by the change of the GO structure upon annealing. The loss of hydrophilic functional groups (typically C-OH and COOH) during annealing14 also made the GO film less hydrophilic. Correspondingly, less amount of water could interact and be absorbed in the GO structure, which resulted in a smaller extent of capacitance change. As a result, in order to achieve better humidity sensor properties, the concentration of hydrophilic functional groups should be maximized. Although calculations indicated that too many functional groups may impede water diffusion in GO structure15, it seems that functional groups in actual GO may not achieve that high level. Thus increasing the concentration of functional groups is still an effective way to get better sensor properties. From another aspect, annealing also reduced the GO gradually to a reduced GO (rGO) structure14. Electrically, rGO is much more conductive than GO due to the existence of a larger portion of conductive phase (that is, the graphene structure). This is likely the reason that the annealed sensors showed much larger capacitance. The S-type response is usually observed in a typical IDE structure using high resistive material, which becomes linear while the sensor material becomes conductive5, 6. It is suggested that residual functional groups that are distributed among the conductive regions may still work as active sensoring sites. Such a structure becomes similar to a normal capacitance structure by placing two opposite electrodes separated by active sensoring material. As we know, such a prototype capacitor normally shows linear capacitance dependent on the dielectric constant. This could be the reason why the annealed GO film showed different sensoring responses from the as-deposited ones. 5



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Figure 1. Structure and electrical properties of the GO humidity sensor. (a) Sample structure model with the GO film deposited on substrate and electrodes. (b) SEM image of the fabricated device. (c) Magnified SEM image of the GO film on substrate. Yellow arrows indicate the fringe structures. (d) Magnified SEM image of the GO film on electrode. (e) Dependence of capacitance on RH% for samples annealed at different temperatures. High resolution humidity sensor properties of (f) as deposited and (g) 250℃ annealed GO film. (g) High resolution humidity sensor properties of GO film. Dependence of capacitance on RH% for samples annealed at (h) 150℃, 200℃ and 250℃ for different duration.

We also compared the transient response properties of sensors annealed at different temperatures. As shown in Figs. 2(a) – (c), the as-deposited sensor showed a response time ( ) in the sub-0.1 second level (comparable to previous reports given in ref. 8), while the well annealed sample showed a value of more than 10 seconds. The responsitivity also decreased gradually upon annealing, which accords well with the steady-state measurements showed in Fig. 1. The rapid response of the as-deposited sensor enabled the real-time monitoring of the humidity fluctuation during normal speaking (Fig. 2(d)). Here the change of transient response properties is attributed to the change of GO structures under annealing. The elimination of hydrophilic 6


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functional groups both on the GO film surface and in the film matrix dramatically slowed down the water absorption and/or diffusion processes. Water molecules absorb and/or diffuses much faster in GO structure with relatively more functional groups. Thus the density of the functional groups is also directly related to the dynamic sensoring response.

Figure 2. (a) Transient electrical response of the GO sensor annealed at different temperatures for 24 minutes. CN indicates normalized capacitance. (b) Close-up of the rapid response region. (c) Dependence of the characteristic response time ( ) on annealing temperature. (d) Real-time response of the GO sensor upon normal speaking, with the sensor placed ~ 5 cm apart from the mouth.

As the as-deposited GO shows much better sensoring properties than the annealed ones, we further studied its temperature-dependent sensoring properties at temperatures ranging from -30℃ to 30℃. We found strong temperature-dependent sensoring properties in the GO sensor, as shown in Fig. 3(a). In a temperature range of higher than 0℃, lower temperatures seemed to give a higher responsitivity. For example, the 10℃ condition showed much higher responsitivity than the 30℃ condition, especially in the concentration range of 0.5 – 1 ppm. However, it had a much lower responsitivity than the 0℃ condition, typically in the concentration range of 0.3 – 0.5 ppm. This is somewhat in contradiction to normal sensors. As is known, most sensors show higher responsitivity at high temperature16. The unusual temperature-dependent responsitivity at 7



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temperatures above 0℃ also indicates that the sensor performance is affected by the relative humidity, but not the absolute vapor concentration. Although the capacitance is not linearly dependent on the amount of water in GO film in the IDE sensor structure, the magnitude of capacitance at dedicate vapor concentration can be compared and related with the amount of water absorbed in the GO film. The vapor – liquid condensation mechanism could be used to study this behavior. The driving force (difference of chemical potential, Δμ) for vapor condensation is: Δμ RT ln

(1)

where P is the vapor pressure, Psat is the saturation vapor pressure (note RH = P/Psat), R is the ideal gas constant, and T is temperature. It can be seen that the lower Psat at lower temperatures resulted in a higher Δμ that drove more water condensation in GO films. This can be used to interpret the sensoring behavior observed above 0℃.

Figure 3. (a) Sensoring of the as-deposited GO film at different temperatures. The RH values at different temperatures are represented by absolute concentration of ppm. (b) The dependence of the responsitivity on temperature calculated in the whole measured range.

A major difference we found was that in higher temperature conditions (> 0 ℃), the responsitivity at a lower temperature was severely depressed at temperatures below 0 ℃. The -10℃ case gave just comparable responsitivity with the 0 ℃ case in the measured humidity range (Fig. 3(a)), and the -30℃case gave nearly zero responsitivity in the whole measured humidity range. The averaged dependences of the responsitivity on temperature are shown in Fig. 3(b) and exhibit clearly a steep degradation at temperature around 0℃. Such a transition implies a relationship 8


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between the sensoring mechanisms with possible phase transition of water (crystallization, vapor – solid transformation, V – S process indicated in Fig. 3(b)). It is supposed that water adsorption in the GO film was crystallized on GO surface below 0℃, which eliminated further water intake and resulted in low capacitance change. Aware of the importance of functional groups on the sensoring properties of GO films, we studied the sensoring mechanism by using in situ FTIR measurements. We were especially concerned with the water absorption mechanism in relation to the functional groups. Steady state FTIR measurements were firstly performed at selected RHs, as shown in Fig. 4(a). The FTIR spectra show three featured regions: 1) a low wavenumber fingerprint region shaded with bands “a” – “d”: band “a” corresponds to the C-OH stretching, centered at 1095 cm-1; band “b” corresponds to the C-O-C stretching, centered at 1240 cm-1; band “c” corresponds to the C-OH bend, centered at 1437 cm-1; band “d” corresponds to the double peak of COOH (overlapping with –OH bending), with peaks centered at 1625 cm-1 (C-OH stretching) and 1725 cm-1 (C=O stretching), respectively (see Fig. 4(b)); 2) a wide, strong band ranging 2700 – 3800 cm-1, overlapped by several sub-peaks: sharp double peaked C-H absorption at 2853 and 2922 cm-1; overlapped –OH absorption bands centered around 3110, 3250, 3400, and 3600 cm-1, which may come from COOH, C-OH or water (see Fig. 4(b)) ; 3) wavy structures in the range of 4000 – 7000 cm-1 caused by thickness interference. The optical thickness of the GO films was calculated by the wavy structures in the FTIR spectra (see Fig. 4(c))17, 18. The layered GO film structure observed in the SEM image (inset of Fig. 4(c)) indicates a water transfer process taking place among adjacent GO flakes, which contributed to the change in film thickness. A linear increase of film thickness upon elevating RH was observed, which gave a slope of 2.6 nm/RH%, corresponding to 4.6×10-3 nm/RH%/layer for single layer GO (assuming a single layer GO thickness of 0.7 nm, total film thickness 400 nm). This roughly coincides with previous observations using a scanning probe microscope (SPM) technique (~ 1.5 ×10-3 nm/RH%/layer19). The relative larger value measured here may be due to having GO structures with more functional groups. The subtracted spectra help us to better interpret the water dynamics at different RHs. As shown in Fig. 4(b), clear absorption bands overlapping on the wavy background (due to thickness interference) can be observed in all the subtracted spectra (noted as s97 to s29, by subtracting the 9



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initial spectra to the 12 RH% one) in regions relating to –OH absorptions. By comparing to the unsubtracted spectrum (97 RH%, replotted in the figure), we find that the gradual water intake (under increasing RH) induced increasing absorption at band “a”, “c” and “d”. Careful comparison of the subtracted spectra indicate FTIR absorption occurred at the high wavenumber side of the C-OH absorption upon increasing RH: the subtracted spectra show absorption peaks at 1106, 1453, and 1644 cm-1, respectively, instead of the original absorption peaks at 1095, 1437, and 1625 cm-1. Notably, the C-O-C and C=O absorption bands were not affected (not even the C=O bond in the COOH group). Such characteristics gave us information on directly water-related dynamics during the GO sensoring process. By comparing with the liquid water infrared spectra20 – 22, the increasing absorption at 1644 cm-1 in the subtracted spectra was attributed to the bending mode of the –OH bond in liquid water. However, liquid water shows no absorption at positions of 1106 and 1453 cm-1, even in solid-state ice structures20 – 22. Here we tentatively attribute these absorptions to the formation of hydrogen bonds between water molecules and the functional groups during water intake. As we know, formation of hydrogen bonds normally induces a blue shift of the absorption peaks. Furthermore, all the absorption bands of “a”, “c” and “d” show linear dependence upon RH as presented in Fig. 4(d). Band “d” (1644 cm-1) shows a larger slope in the linear dependence than that of “a” and “c” (1106 and 1453 cm-1), while similar slopes are observed for “a” and “c.” The different slopes also suggest different origination of 1644 cm-1 with 1106 and 1453 cm-1. A smaller slope for the latter two peaks could be due to the fact that not all water molecules interact directly with functional groups in the GO structure. However, the interaction probability increases upon increasing water amount, mimicking a similar linear dependence. The linear dependence of the absorption at 1644 cm-1 on the RH can be interpreted based on Eq. (1) considering a dependent of water in-taking amount (w) on the chemical potential Δμ: w ~ exp (Δμ), which gives a linear relationship of w ~ . Thus the FTIR absorption intensity is thought to be a faithful representation of the amount of water in the GO film. Due to the strong overlapping of the wide-band –OH absorption and the thickness interference in the region of 2700 – 3800 cm-1, it was hard to get accurate peak position for different –OH contributions in the subtracted spectra. However, in Fig. 3(b), we could see that the peak positions are roughly coincident with the original ones with probably small variation. Also, the absence of 10


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the sharp absorption from the C-H bonds indicates that they are not much related with the humidity sensoring mechanism, like the C-O-C and C=O bonds. The pronounced absorption in this region, especially 3250, 3400, and 3600 cm-1, suggests again that liquid type water should exist in the GO film20 – 22. Water molecules may condense around the functional groups and form liquid cluster structures, according to previous observations19.

Figure 4. (a) FTIR spectra of the GO films at different humidities. Band “a” corresponds to the C-OH absorption, centered at 1095 cm-1; band “b” corresponds to the C-O-C absorption, centered at 1240 cm-1; band “c” corresponds to the C-OH absorption, centered at 1437 cm-1; Band “d” corresponds to the double peak absorption of COOH, with peaks centered at 1625 cm-1 (C-OH) and 1725 cm-1 (C=O), respectively. The wide, strong band ranging from 2700 – 3800 cm-1 is overlapped by several sub-peaks: a sharp double peak C-H absorption at 2853 and 2922 cm-1; overlapping –OH absorption bands centered around 2885, 3110, 3250, 3400, and 3600 cm-1, which may come from COOH, C-OH or water (note that the 3250 cm-1 peak is less evident due to the thickness interference). The wavy structures in the range of 4000 – 7000 cm-1 are caused by thickness interference. (b) Subtracted FTIR spectra from s97 (= 97 RH% – 12 RH%) to s29. The 97 RH% spectrum is also shown for comparison. (c) Dependence of optical thickness on RH as determined from the thickness interference region. The results from two separate measurements 11



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are shown by squares and circles, respectively. The inset shows a typical SEM image of the cross-sectional multilayer structure of GO films. (d) Dependence of the absorption intensity upon RH at selected absorption peaks.

The FTIR spectra, measured under dry conditions such as in nitrogen gas or in a pumped vacuum, give structural information on the “dry” state GO. In such measurements, GO film with a reduced thickness was used in order to reduce the thickness interference effect. As shown in Fig. 5 (a), (b), nitrogen gas or vacuum conditions gave similar spectra, indicating that both routes had similar GO structures. The subtracted spectra of the “97 RH % - pumped” give clear absorption at 1106 and 1644 cm-1. A relative weak absorption at 1453 cm-1 (due to small film thickness) could be also observed. These features are in accordance to the steady state measurements (Fig. 4). Moreover, the –OH absorption region (2700 – 3800 cm-1) shows clear peaks in positions around 2885, 3110, 3250, 3400, and 3600 cm-1. In particular, strong absorptions at 3400 and 3600 cm-1 are observed in the subtracted spectra while evidently depressed absorption in this band could be observed in the dry-state condition, resulting in profound line shape alteration between the dry state and the high RH condition. The pumped and nitrogen-subtracted GO showed nearly zero intensity (comparing with relative strong absorption in the 97 RH % condition) around 3600 cm-1 (mainly liquid water absorption), which verifies that liquid water was subtracted from the GO structure. Moreover, considering that liquid water absorbs at only 3250, 3400, and 3600 cm-1 with a characteristic bell shape20 – 22, the additional absorptions edges around 2885, and 3110 cm-1 contributed to the absorption due to water intake inside the GO structure. These edges can be attributed to the formation of hydrogen bonds with much stronger strength than those in liquid water23 – 25. Such strong hydrogen bonds likely come from the interaction of water with functional groups in GO structure. Besides the structured absorption in the range of 2700 – 3800 cm-1 in the subtracted spectrum, multi-peaked absorption was also observable in the dry state spectra in the same region (Fig. 3(b)). Broad absorption peaks at positions around 2885, 3110, 3250 and 3400 cm-1 can be attributed to the functional groups in the GO structure (typically, COOH and C-OH). Formation of intramolecule hydrogen bonds in the GO structure may be also possible, judging from the line shape23

– 25

. Rich functional groups in GO open the way for effective water intake and 12


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transportation during the sensoring process through effective hydrogen bond interactions.

Figure 5. (a) FTIR spectra of the GO films at high (97 RH%) and low (pumped to ~ 2 Pa for ~ 80 minutes, or placed in nitrogen gas environment for ~ 1 hour) humidity conditions. (b) Close-up of the typical regions of the FTIR spectra.

The transient FTIR measurements give direct verification of the rapid response dynamics of the GO film (Figs. 6 (a) – (f)) (reduced film thickness was used in order to eliminate possible thickness interference effect). The original time-dependent FTIR spectra show systematic absorption increasing/decreasing upon RH increasing/decreasing, especially in the –OH absorption region as presented in Figs. 6 (a) and (f). The subtracted spectra show featured absorption centered around 1644 and 3400 cm-1, in good accordance with the steady-state conditions (Figs. 4 (c)). Absence of other absorption may be due to relatively smaller signal/noise ratio, as a single-scan mode was used for better time resolution. Close-up of the –OH region (Figs. 6 (d) and (e)) found mainly the characteristic bell-shaped liquid water absorption, with 3400 cm-1 central peak and shoulders at 3250 and 3600 cm-1. Weak intensity distribution could be also observed at the lower wavenumber side from 2800 cm-1, in accordance with the above FTIR spectra. The dependence of the 3400 cm-1 absorption on time (Figs. 6 (f)) shows a characteristic response time well below the spectra interval (0.85 second). Indeed, the spectra interval is much larger than the value measured in the electrical measurement. The equilibrium state should be already set up during a single FTIR scan, giving a sharp transition in Figs. 6 (f). The appearance of a thickness interference that occurred along with the –OH absorption peaks in the substrated 13



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spectra (Figs. 6 (c) – (e)) also verifies the thickness alteration induced by water absorption in the GO structure. The transient FTIR measurements give direct evidence of the rapid response of the GO film by the mechanism of water diffusion. This shows that the sensoring response of the GO sensor is directly related with the water intake and outflow process.

Figure 6. Time-resolved FTIR spectra of the GO films while RH changed from (a) 12 RH% to 97 RH% and (b) 97 RH% to 12 RH%. Thirty-one lines (lines 0 – 30) with time interval of 0.85 seconds are shown. (c) Typical subtracted spectra (for example, “3-0” indicates that spectra line 3 is subtracted by line 0) with absorption features, during the RH switches from 12 RH% to 97 RH%. (d) Subtracted spectra in the –OH band range for 12 RH% to 97 RH%. (e) Subtracted spectra in the –OH band range for 97 RH% to 12 RH%. (f) Dependence of absorption intensity upon time during time-resolved measurements.

Based on the above studies, the key points related to the sensoring mechanism of GO films may be summarized as follows: firstly, a layered structure with large layer spacing in the GO film is crucial for the sensoring process. Water uptake and release is mediated by the GO layer structure15, 26 – 28

. Comparing this with the typical van de Waals graphite layer structure (0.33 nm layer

spacing), the interaction between GO flakes should be much weaker (~ 0.7 nm layer spacing15, 26 – 28

). The weak interaction between adjacent GO flakes insures tunable layer distances under

different RH. There are typically three kinds of layer structures existing in GO film: 1) overlapped 14


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regions without functional groups or defects (Type I as shown in Fig. 7 (a)). This type of structure is similar to that in pristine graphite. However, larger layer spacing is anticipated due to the existence of the surrounding functional groups. Besides these regions, there are two other possible kinds of overlapping layer structures related with the defective (or functionalized) GO structure: 2) the semi-functionalized overlapping structure (Type II, see Fig. 7 (a)), and 3) the double-functionalized overlapping structure (Type III, see Fig. 7 (a)). Both structures tend to have large layer spacing. The Type II structure consists of typically functional groups interacting with the chemically inert graphene surface. The Type III structure consists of interactive functional groups. The multiple hydrogen bond formation (in the –OH stretching region) observed in dry state FTIR spectra mainly come from the Type III structure. The relative large interlayer spacing induces the formation of network channels between adjacent GO layers, which ensures a rapid and effective water intake process. Previous calculations also indicated enhanced water diffusion in GO structures with larger layer spacing. Moreover, the Type III structure also contributes to the formation of vertical channels along directions perpendicular to the GO plane (Fig. 7 (b)). These channels could work for the water transportation to adjacent layers and to the entire film thickness. In well annealed GO structures, the density of functional groups was greatly reduced, making the channels closed. This resulted in a slower water molecular transfer process, thus a lower sensor response. Less hydrophilic groups also resulted in low levels of water intake, and thus low responsitivity.

Figure 7. (a) Atomic model indicating different overlapping layer structures in GO film. Type I is the region with graphite-like overlappings as shaded by light purple, Type II is the region with semi-functionalized overlapping as shaded by light green, Type III is the region with 15



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double-functionalized overlapping as shaded by light red. (b) Atomic model illustrates the humidity sensoring mechanism in GO thin films. Dashed lines indicate the diffusion path of water molecule. Grey sphere: carbon, red sphere: oxygen, white sphere: hydrogen.

Secondly, the functional groups in the GO structure, both along the GO flake edges and in the plane, play important roles for humidity sensoring. Although the pristine graphene surface enabled the rapid water transportation, the hydrophobic and chemically inert nature of its surface structure resulted in low water intake during sensoring. Sensoring of pristine graphene is mainly a balanced surface physical adsorption/desorption mechanism. Increased sensitivity could be achievable by functionalization of the graphene structure with different functional groups, typically –COOH, –OH, that are rich in GO structure. The existence of functional groups adds chemical adsorption routes during the sensoring processes and results in high water intake, thus responsitivity. The multiple hydrogen bond structure observed in Fig. 4(b) verifies the interaction of water molecules with the surrounding functional groups. Previous SPM observations also found that water molecules tend to be absorbed onto the hydrophilic functional groups and induce an evident increase in surface roughness19. The liquid water absorption observed in the FTIR spectra should come from the condensed state water cluster around the functional groups. The hydrophilic functional groups work as both acceptors and transmitters during sensoring. They could also work as a reservoir for effective water storage. The reduced sensor responsitivity of the annealed GO films is related directly to the reduced amount of functional groups. Thirdly, there is a synergistic mechanism for sensoring in GO structures. It was known that the complete absence of functional groups resulted in just graphite structures, which are structurally waterproof due to their small layer spacing, giving a smaller responsitivity. However, at the other extreme, for pure hydrophilic alcohol or carboxylic acid material, although they interact well with water molecules, they lack effective and fast channels for water transportation. Much slower sensor responses were observed in such kinds of material32,33. Previous studies also indicated that the existence of too many functional groups might be not preferable for rapid water transportation due to their relative strong interaction with water molecules, which tends to impede water traveling29 – 31. As a result, the coexistence of rich functional groups and diffusion channels in GO ensures the effective water intake, reservation and the water transportation process: surface 16


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functional groups (Types II and III structures) work for effective water absorption due to their hydrophilic properties; bulk functional groups work as active sites for water reservation; water molecules can diffuse rapidly through channels among adjacent functional groups. It is this synergic mechanism that ensures the high responsitivity and rapid response humidity sensoring properties in GO films.

Summary To summarize, we studied a GO-based humidity sensor and found the enhanced sensoring properties in the as-deposited GO films. The as-deposited sensor was found to have a large responsiveness (capacitance changed by two orders of magnitude in 12 – 97 RH%) and fast responsitivity ( less than 0.1 second) at room temperature. High temperature annealing reduced the sensoring responsitivity and resulted in a longer response time. This is attributed to the reduced concentration of functional groups in the GO structure. The sensoring mechanism of the as-deposited GO films was studied by FTIR. The water uptake and release during the RH change was observed and related directly to the sensoring process, which also induced evident thickness alteration of the GO thin film (4.6×10-3 nm/RH%/layer). Water intake in the GO structure was accompanied by the formation of hydrogen bonds between the GO structures and water. Liquid water was also found to play an important role during the sensoring process. The sensoring mechanism in GO films was discussed based on the different structural regions according to the synergistic process described: functional groups work to reserve water and produce larger layer spacing for better water transportation; regions without functional groups work as effective channels for water transportation. It is also suggested that such a mechanism may also relate directly to the high efficient water permeation34 and desalination35 observed in GO structures. The coexistence of hydrophilic functional groups in the hydrophobic layer structure makes a prototype structure model that enables such a synergistic process and results in high material performance.

Acknowledgements This work is supported by the National Basic Research Program of China (Grant No. 2015CB352106), the National Natural Science Foundation of China (Grant Nos. 61370042, 17



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11674053, 11304197), and Specialized Research Fund for the Doctoral Program of Higher Education (Grant No. 20100092110014). The authors thank Em Turner Chitty in ELI of UTK for review of the manuscript.

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Microstructure Related Synergic Sensoring Mechanism in Graphene

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