2D Hybrid Nanomaterials for Selective Detection of NO2 and SO2

Sep 14, 2017 - Very recently, the emergence of vdW (van der Waals) heterostructures brought new strategies to open the band gap for graphene because o...
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Cite This: ACS Appl. Mater. Interfaces 2017, 9, 37191-37200

2D Hybrid Nanomaterials for Selective Detection of NO2 and SO2 Using “Light On and Off” Strategy Aimin Chen,*,† Rui Liu,† Xiao Peng,† Qiaofen Chen,‡ and Jianmin Wu*,‡ †

College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, China Institute of Microanalytical System, Department of Chemistry, Zhejiang University, Hangzhou 310058, China

‡

S Supporting Information *

ABSTRACT: In order to distinguish NO2 and SO2 gas with one sensor, we designed a paper chip assembled with a 2D g-C3N4/rGO stacking hybrid fabricated via a layer-by-layer self-assembly approach. The g-C3N4/rGO hybrid exhibited a remarkable photoelectric property due to the construction of a van der Waals heterostructure. For the first time, we have been able to selectively detect NO2 and SO2 gas using a “light on and off” strategy. Under the “light off” condition, the g-C3N4/rGO sensor exhibited a p-type semiconducting behavior with a low detection limit of 100 ppb of NO2, but with no response toward SO2. In contrast, the sensor showed n-type semiconducting behavior which could detect SO2 at concentration as low as 2 ppm under UV light irradiation. The effective electron transfer among the 2D structure of g-C3N4 and rGO nanosheets as well as highly porous structures could play an important role in gas sensing. The different sensing mechanisms at “light on and off” circumstances were also investigated in detail. KEYWORDS: van der Waals heterostructures, g-C3N4/rGO stacking hybrid, layer-by-layer self-assembly, NO2 gas sensing, SO2 gas sensing

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INTRODUCTION The integration of electronic devices with flexible substrates has attracted considerable attention because it enables the development of bendable, wearable, and portable sensors. Particularly, paper has been considered to be an ideal flexible substrate owing to its biodegradable, insulating, cheap, lightweight, recyclable, and flexible (even foldable) properties. Recently, the printing of electronic circuits and assembly of nanomaterials on paper substrates has been realized; these technologies make paper-based chips a promising platform to construct a flexible sensing device.1,2 2D materials have drawn a lot of attention due to their potential application in chemical and biological sensing.3,4 Graphene with atom-thick conjugated structures has been considered as one of the leading candidates to be a material for high-performance gas sensor applications owing to its excellent electronic properties, large specific surface area, and high intrinsic carrier mobility.5−8 Unfortunately, due to the absence of a direct band and few dangling bonds on the surface, graphene-based sensors without proper surface modification always show poor selectivity.9−11 It is essential to develop novel graphene-based composite materials with band gaps suitable for gas sensing. Very recently, the emergence of vdW (van der Waals) heterostructures brought new strategies to open the band gap for graphene because of their unique properties and potential applications in electronic and optoelectronic devices.12−14 So far, vdW heterostructures of graphene and other 2D materials © 2017 American Chemical Society

such as hexagonal boron nitride (hBN), dichalcogenides, and carbon nitride (g-C3N4) have been fabricated, and they allow the emergence of new properties.15−17 Among these 2D layer materials, g-C3N4 is an important metal-free semiconductor that is an analogue of graphite with N substitution. More importantly, unlike graphene, the bulk g-C3N4 possesses superior stability, a medium band gap, and unique photoelectron chemical properties, which have been widely demonstrated to be the most potentially useful material for photocatalysts. Bulk g-C3N4 also possesses energy storage capability as well as environmental sensing and biosensing applications.18−24 In addition, carbon nitride possesses a large of amount of pyridine N, which shows strong affinity for adsorbed molecules.25 However, the low conductivities of nanostructures usually limit its performance in gas sensing. Theoretical studies have predicted that the hybrid graphene/gC3N4-layered material promotes electron-rich and hole-rich regions, i.e., forming a well-defined electron−hole puddle, on the supported graphene layer.26 Blending 2D graphene with gC3N4 sheets to form van der Waals stacked hybrids might show interesting electrical and optical properties distinctly different from their parent layers.27−29 Although graphene/g-C3N4 nanocomposites with tunable band structures have been Received: July 29, 2017 Accepted: September 14, 2017 Published: September 14, 2017 37191

DOI: 10.1021/acsami.7b11244 ACS Appl. Mater. Interfaces 2017, 9, 37191−37200

Research Article

ACS Applied Materials & Interfaces Scheme 1. Schematic Illustration for Synthesis of g-C3N4/rGO Hybrid Using Layer-by-Layer Method

successfully fabricated by different approaches such as thermal treatment,30 sonication exfoliation,31 and an in situ chemical synthetic approach,32 few of these approaches have actually taken full advantage of the 2D nanoarchitecture, which seriously affects the charge transfer between g-C3N4 and graphene. As main atmospheric pollutants, nitric dioxide (NO2) and sulfur dioxide (SO2) are very harmful to mankind and the environment because these acidic gases can cause various respiratory and cardiovascular diseases even at low concentration. The American Conference of Governmental Industrial Hygienists recommended a threshold exposure limit of 200 ppb of NO 2 (https://www.osha.gov/dts/chemicalsampling/ data/CH_257400.html). The long-term and short-term exposure limits for SO2 gas are 2 and 5 ppm, respectively.33,34 Accordingly, portable devices to rapidly and precisely monitor low concentrations of NO2 or SO2 gas emissions are desirable for human health protection. Although, graphene-based gas sensors for NO2 gas sensing have been well developed,35−38 there are still limitations for detecting SO2 gas due to its weak adsorption energies and the relatively small charge-transfer ability.39 Until now, very few articles have reported the selective detection of NO2 and SO2 gas using a single sensor.40 The aim of this paper is to design a sensor to distinguish NO2 and SO2 gas at room temperature. This work created a 2D van der Waals g-C3N4/rGO stacking hybrid with alternating layers supported on a paper chip using a layer-by-layer selfassembly approach, as shown in Scheme 1. For the first time, we have been able to selectively detect NO2 and SO2 gas using a “light on and off” strategy. Under the “light off” condition, the g-C3N4/rGO sensor exhibits a p-type semiconducting behavior to lower concentration of NO2 at room temperature, but no response toward SO2. In contrast, the sensor showed ntype semiconducting behavior, which could detect a low concentration of SO2 under UV light irradiation. The different sensing mechanisms at the “light on and off” circumstances were also investigated in detail.

Figure 1. (a) XRD patterns of GO, protonated g-C3N4, multilayer gC3N4/GO, and multilayer g-C3N4/rGO stacking hybrid; (b) structure model of multilayer g-C3N4/rGO stacking hybrid; (c,d) TEM image of multilayer g-C3N4/rGO stacking hybrid; (e,f) HTEM image of multilayer g-C3N4/rGO stacking hybrid.

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C3N4 nanosheets compared to bulk g-C3N4 (see Figure S2), reflecting the effective exfoliation of bulk g-C3N4.42 For the gC3N4/GO stacking hybrid, it is noteworthy that the highintensity XRD peak of GO at 9.23° is still observed, which confirms that the GO layer structure was not destroyed after the layer-by-layer assembly process. However, further observation indicates that the (002) peak shifts toward a lower diffraction angle from that of g-C3N4 (27.6°, d = 0.325 nm) to that of g-C3N4/GO of (23.0°, d = 0.391 nm), which resulted from an increase of the g-C3N4 interlayer distance (Figure 1b). Furthermore, the peaks at 17.7° and 18.2° corresponding to the (100) and (001) peaks appear to shift, also reflecting that the layer structure reformed after the self-assembly process. Due to GO nanosheets with a carboxyl group having a negative charge

RESULTS AND DISCUSSION Figure 1a shows the XRD patterns of GO nanosheets, protonated g-C3N4 nanosheets, the 2D g-C3N4/GO hybrid, and the g-C3N4/rGO hybrid. The XRD pattern of GO nanosheets presents the diffraction peak at 9.23° corresponding to (001) at an interlay space (d-spacing) of around 0.96 nm, which is about 3 times of the d spacing (0.34 nm) of natural graphite, suggesting GO nanosheets have a three layer structure.41 From the XRD pattern of protonated g-C3N4, the strong XRD peak at 27.6° (d = 0.325 nm) is evidence which corresponds to the typical graphitic interlayer stacking (002) peak of g-C3N4. The peak at 13.3° is remarkably weakened in g37192

DOI: 10.1021/acsami.7b11244 ACS Appl. Mater. Interfaces 2017, 9, 37191−37200

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Figure 2. (a) Raman spectra of g-C3N4, rGO nanosheet, and multilayer g-C3N4/rGO stacking hybrid; XPS spectra of protonated g-C3N4 and multilayer g-C3N4/rGO hybrid: (b) C 1s, (c) N 1s, and (d) O 1s.

types of spacing of d = 0.391 nm correspond to the interlayer structural packing, indexed for the (002) crystallographic plane of g-C3N4, which is consistent with the XRD results. Raman spectra of rGO and the g-C3N4/rGO hybrid are illustrated in Figure 2a. rGO nanosheets exhibit two characteristic peaks, corresponding to the D and G bands. The D band at 1343 cm−1 represents the structural defects, which are associated with the breathing mode of k-point photons of A1g symmetry. The G band near 1605 cm−1 implies sp2-hybridized carbon domains arise due to the E2g vibration mode.45 The ID/ IG for the g-C3N4/rGO hybrid increases in comparison to the bare rGO in the same test conditions (see Table S1). The phenomenon might be attributed to the formation of chargetransfer complexes between g-C3N4 and rGO nanosheets. As it is well known, the band located at ∼2700 cm−1 is an indicator of the number of graphene layers. But, we do not detect the 2D band in Raman results, which indicates that the resulting multilayer g-C3N4/rGO hybrid deposited on paper is far more than a single or a few layers. A second-order peak at ∼2900 cm−1 is found as an S3 band derived from the D and G peak combination, indicating the presence of a small disorder of rGO nanosheets in the hybrid.46,47 This disorder is caused by the deposition of graphene on uneven cellulose surfaces as well as the incorporation with g-C3N4 nanosheets. The g-C3N4/rGO hybrid nanomaterial was further analyzed with X-ray photoelectron spectrometry (XPS), as shown in Figure 2b−d. For the C 1s spectrum (Figure 2b), five fitted peaks located at 284.5, 285.7, 287.5, 288.6, and 289.3 eV can be observed in the gC3N4/rGO stacking hybrid. The lower peak centered at 284.5 eV, which is attributed to sp2-hybridized carbons (C−C), illustrates that the g-C3N4/rGO stacking hybrid has a higher graphitic degree and higher conductivity.30 Compared with pure g-C3N4, the shift of peaks for g-C3N4/rGO to higher binding energies is observed, which indicates a stronger interaction of g-C3N4 and rGO in the g-C3N4/rGO stacking hybrid.32,48 The peak centered at 285.7 eV is attributed to C N. The peak located at 288.6 is attributed to CN bonds. The peak centered at 287.5 eV is attributed to the formation of C− O−C bonding configurations during the layer-by-layer

and protonated g-C3N4 having a positive charge on the surface (see Figure S3), GO nanosheets and g-C3N4 nanosheets were stacked in an alternating manner with intimate interfacial contact, resulting in a multilayer alternating structure. The electrostatic attraction between the GO and protonated g-C3N4 nanosheets is the main reason for the increasing of the interlayer distance of g-C3N4. The unusual results are different from the results reported,43,44 since we fabricated the multilayer g-C3N4/rGO hybrid using a layer-by-layer selfassembly approach instead of an ultrasonic technique. Therefore, the formation process of the multilayer hybrid is different from those reported in other literatures.43,44 If there is no external force, such as grinding, the multilayer alternating structure deposited on the paper can remain relatively stable. After reduction, the peak at 9.23° for GO disappears because GO nanosheets were mainly reduced to rGO nanosheets. A large number of carboxyl groups disappear after reduction, and the electrostatic attraction is greatly weakened, but there is still an existing van der Waals force between g-C3N4 and the rGO heterostructure, so the multilayer alternating structure can remain unchanged. The TEM image of the g-C3N4/rGO stacking hybrid (Figure 1c) shows that the nanosheets were uniformly deposited at the external surface of rGO, which exhibited lamellar-like structures similar to the exfoliated gC3N4 but formed multiple corrugated layers buckled together with no changes in size. The TEM and HRTEM images also illustrate nanosheets overlapping between g-C3N4 and rGO nanosheets (Figure 1d,e), exhibiting the existence of a heterojunction in the alternating layer structure. The obtained interface with sufficient contact would be favorable for the transfer of photoexcited carriers. The multilayer alternating structure was also confirmed by an HRTEM image (shown in Figure 1e). The presence of alternating dark lines indicates the existence of a layered structure with a thickness of ∼8.6 nm, suggesting that the g-C3N4/rGO hybrid was dozens of multilayers thick. The thickness of rGO in the hybrid is about 2.3 nm, indicating about seven layers of rGO nanosheets in the multilayer g-C3N4/rGO hybrid. The lattice fringes for gC3N4 in the multilayer hybrid are shown in Figure 1f. Two 37193

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Figure 3. Current vs voltage curve of (a) multilayer g-C3N4/rGO hybrid, (b) rGO, and protonated g-C3N4 with no light, visible light (≥420 nm), and UV light (365 nm); photoresponse behaviors of g-C3N4/rGO stacking hybrid: (c) under visible light illumination (≥420 nm) and (d) under UV light illumination (365 nm).

Figure 4. (a) Conductance response of multilayer g-C3N4/rGO sensor to NO2 gas (100 ppb−4 ppm) at room temperature; (b) enlarged part of current response vs time for multilayer g-C3N4/rGO to 100 ppb of NO2 gas; (c) response vs NO2 concentration; (d) selectivity histogram of multilayer g-C3N4/rGO sensor for different detection gases.

further confirmation that the reduction for g-C3N4/rGO is not complete, and there are still existing parts of the carboxyl group which can act as the cross-linker for forming C−O−C bonds for the van der Waals stacking g-C3N4/rGO hybrid. I−V characteristics of the g-C3N4/rGO stacking hybrid deposited on filter paper were studied with and without light assistance by applying a voltage between +5 V and −5 V as shown in Figure 3a. The linear behavior of the I−V curve of gC3N4/rGO hybrid nanomaterial bridged the electrodes (IE), indicating good ohmic contact, which reflects that the electrical contact played a negligible role in the sensing process. The absence of Schottky barriers between the g-C3N4/rGO hybrid and IE allows the accurate evaluation of the interactions between the sensing layer and target gas. A dramatic

assembly method. The peak located at 289.3 eV is assigned to HO−CO bonding resulting from GO nanosheets that hybridized with protonated g-C3N4.32 The N 1s spectrum (Figure 2c) was deconvoluted into three peaks. The peak at 398.1 eV is assigned to the sp2-hybridized nitrogen in triazine rings (C−NC), also called pyridine N.49 The peak at 399.5 eV corresponds to pyrrolic-like nitrogen (N−(C)3), and the peak at 400.2 eV may be attributed to amino functional groups (−NH2 and NH), which play a role in the electronic and covalent link between g-C3N4 and rGO.50 The fitting curve of O 1s spectra (Figure 2d) simply indicates two peaks centered at 530.2 and 531.6 eV, which are commonly ascribed to the surface oxygen complexes (H2O and CO2 molecules) and HO−CO bonding.51 The existence of HO−CO bonds is 37194

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Figure 5. (a) Conductance response of multilayer g-C3N4/rGO sensor to SO2 gas (20−800 ppm) under visible light irradiation (≥420 nm) at room temperature; (b) conductance response of multilayer g-C3N4/rGO sensor to SO2 gas (2−100 ppm) under UV light irradiation (365 nm) at room temperature; (c) enlarged part of current response vs time for multilayer g-C3N4/rGO to 2 ppm of SO2 gas; (d) response vs SO2 concentration.

was favorable for spatially separating the photogenerated charge carriers. The dynamic response of g-C3N4/rGO hybrid sensor toward different concentrations of NO2 gas (from 100 ppb to 4 ppm) without light irradiation was measured at room temperature (shown in Figure 4a). Upon exposure to NO2, the conductive response increased with the increasing of NO2 concentration. The g-C3N4/rGO hybrid sensor exhibits higher conductance response to NO2 than the rGO sensor does (see Figure S9). The sensitivity of g-C3N4/rGO (110% at 2 ppm) is roughly three times greater than that of rGO (35%, 2 ppm). Figure 4b shows the sensor response to 100 ppb of NO2 at room temperature. The conductance of the sensor undergoes a drastic increase after exposure to the target gas, and it could also decline nearly to its initial value after blowing N2 for several minutes. The response and recovery time for the NO2 concentration are relatively fast at room temperature. The average time to reach 90% of the stable sensor signal (t90) is 138 s for response and 318 s for recovery, respectively. The sensing response is linearly proportional to the NO 2 concentration in the region of lower than 1 ppm. The detection limit of g-C3N4/rGO sensor is 100 ppb, which is lower than the threshold exposure limit of 200 ppb recommended by the American Conference of Governmental Industrial Hygienists (https://www.osha.gov/ dts/chemicalsampling/data/CH_257400.html). By using a S/ N threshold of 3, we calculate the theoretical detection limit for NO2 to be approximately 11.2 ppb. This ppb level of the detection limit for NO2 suggests its potential usage in various applications such as environmental monitoring and breath analysis, especially for diagnosing asthma. The sensing selectivity of the g-C3N4/rGO stacking hybrid was also studied upon exposure to various gases and vapors, including NO2, SO2, ammonia, chlorine, toluene, ethyl acetate, and hexane. As shown in Figure 4d, among the studied analytes, the sensor showed the strongest response toward NO2, demonstrating the high-sensing selectivity of g-C3N4/rGO for NO2 detection. Compared to other graphene-based NO2 sensors listed in Table

enhancement of conductivity was obtained after exposing the sensor under visible and UV light, which originated mainly from the electron−hole pairs excited by incident light with energy greater than that of the band gap of the g-C3N4. In other words, only illumination with enough energy can induce a crucial increase in photoconductance. The improved photoconductive response observed in gC3N4/rGO stacking hybrids indicates that the van der Waals stacking nanostructures are good candidates for photodetectors, photoassistant gas sensors, and photocatalysts. Compared with g-C3N4/rGO hybrid nanomaterial, protonated g-C3N4 has no electrical conductivity (see Figure 3b). To further examine the optoelectronic properties of the g-C3N4/rGO lateral interface heterostructure, photoresponse characteristics (PC) were measured at a bias voltage of 3 V using visible (≥420 nm) and UV LED (365 nm) light sources. Under illumination, the photocurrent rises to a high value (“on” state) and returns to a low value when the light is off (“off” state) (Figure 3c,d). It is clear that, under UV illumination, in which the photon energy is higher than that of the energy gap of the g-C3N4/rGO hybrid, the photocurrent is much higher than that under visible light illumination and has a relatively quick response. Moreover, under UV light illumination, the photoconductivity is sustained even after the exciting source has been turned off, displaying a pronounced persistent photoconductivity (PPC) effect.52 With the PPC effect, there are obvious distinctions between the UV light and visible light illumination in recovery time. This PPC effect might be caused by deoxidation of rGO nanosheets under UV illumination. Although the graphene has low photoresponsivity due to its intrinsic metallic property, g-C3N4, like TMDs such as MoS2 and MoSe2, is a highly photoresponsive material with strong optical absorption and band gaps in visible and UV spectra. When the van der Waals stacking hybrid was constructed, the electron−hole recombination was severely inhibited due to the possible existence of traps at the interface of the heterojunction between g-C3N4 and rGO nanosheets.53,54 Furthermore, the existence of potential barriers or surface barriers on the intimate interface of the heterojunction 37195

DOI: 10.1021/acsami.7b11244 ACS Appl. Mater. Interfaces 2017, 9, 37191−37200

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ACS Applied Materials & Interfaces S2, the sensor presented higher sensing performance with characteristics such as low detection limit as well as fast response and recovery with higher selectivity. It can be observed that the g-C3N4/rGO stacking hybrid sensor has the highest sensing performance among the graphene-based NO2 sensors with fast response and recovery at room temperature. The conductance response of g-C3N4/rGO sensor to NO2 gas with light assistance was also investigated. After exposure to NO2 under visible light or UV light irradiation, the responding behaviors toward NO2 changed remarkably (see Figures S10 and S11). Under visible light irradiation (≥420 nm), the gC3N4/rGO sensor still exhibited a positive conductance response, even though the conductance response and sensitivity significantly decreased. In contrast, the g-C3N4/rGO showed a negative conductance response under UV light illumination (365 nm). The conductance responses to different concentrations of NO2 on rGO and g-C3N4/rGO sensors with or without light irradiation are compared in Figure S12, which clearly show different sensing performances for different types of materials and conditions. With visible light assistance, the t90 for response and for recovery are 100 and 129 s, respectively, which are faster than those without irradiation (see Table S2). Although SO2 and NO2 molecules are both electron-withdrawing molecules, the intrinsic graphene is not an efficient material to capture SO2 due to weak adsorption energies and the relatively small charge transfers to SO2 molecules. The dynamic response of the rGO sensor toward SO2 gas with and without light illumination at room temperature was measured. As shown in Figure S13, the rGO sensor did not respond to SO2 gas without light assistance, which is inconsistent with the reported results.39 Moreover, even with light assistance, the rGO-based sensor still had no response to SO2 gas. Although the g-C3N4/rGO sensor also showed no response to low concentrations of SO2 without light irradiation, it is interesting to note that the g-C3N4/rGO sensor showed remarkable conductance-responsive behavior toward SO2 gas with light assistance. Figure 5a,b exhibits the dynamic responses of the gC3N4/rGO hybrid to SO2 gas under visible light and UV light illumination at room temperature, respectively. The current of the g-C3 N 4 /rGO stacking hybrid-based sensor rapidly decreased upon exposure to SO2, indicating n-type semiconducting behavior with light assistance. When clean N2 was fed into the chamber, the conductance response completely recovered to its initial value. The g-C3N4/rGO hybrid sensor is able to detect SO2 concentrations of 2 ppm with stable performance under UV light at room temperature. Figure 5c shows the enlarged part of current response vs time for gC3N4/rGO to 2 ppm of SO2 gas under UV light. The t90 for response and for recovery are 207 and 212 s at room temperature, respectively. As shown in Figure 5d, the sensing response is linearly proportional to SO2 concentrations lower than 16 ppm when under UV irradiation. From the S/N ratio, we estimate the theoretical detection limit for SO2 to be 685 ppb. This low detection limits for SO2 also suggests its potential for use in various applications. Compared to the other SO2 sensor listed in (Table S3), the sensor presented higher sensing performance characteristics such as a low detection limit as well as a fast response and recovery. The conductance responses of the g-C3N4/rGO sensor to NO2 gas and SO2 gas under “light off” and “light on” conditions were also compared in Figure 6. The g-C3N4/rGO sensor indicated a different conductance response type to NO2 gas in

Figure 6. Response comparison of multilayer g-C3N4/rGO to 1 ppm of NO2 and 20 ppm of SO2 under “light off and on” (visible light and UV light).

contrast to SO2 gas under “light on” and “light off” conditions. The g-C3N4/rGO senor showed positive response to NO2, while it showed no response to SO2 gas under the “light off” condition. Under the “light on” condition, the g-C3N4/rGO senor showed a positive response to NO2 gas in contrast to the negative response it showed to SO2 gas with visible light assistance, although the g-C3N4/rGO senor showed the negative response to NO2 and SO2 gas with UV light assistance. The results clearly demonstrate that the g-C3N4/ rGO sensor can be used to selectively detect or discriminate NO2 and SO2 using the “light on and off” approach. We also investigated the stability of the gas sensors by measuring their responsivity as a function of testing cycles (shown in Figures S14 and S15). After 10 cycles of testing, the sensor response was almost unchanged, indicating that the g-C3N4/rGO hybrid sensor is very stable and suitable for commercial applications. The sensing mechanism for g-C3N4/rGO hybrid sensor relies on the direct charge transfer between NO2 and the g-C3N4/ rGO stacking hybrid (shown in Scheme 2). In the system of the g-C3N4/rGO hybrid, a hole depletion region on the surface of rGO and a p−n junction were formed at the interface of the gC3N4 and rGO nanosheets. When in contact with the NO2 gas, more electrons were attracted from rGO toward g-C3N4, which shifted the Fermi level of the rGO toward the valence band and enhanced the hole conductivity (shown in Scheme 2a). The improvement in the sensitivity of the g-C3N4/rGO sensor toward NO2 gas can be ascribed to the heterojunction at the interface of g-C3N4 and rGO. The 2D structure of the gC3N4 nanosheets on graphene not only increases the contact area for efficient charge transfer across the interface but also shortens the charge-transport time and distance, thereby improving the device performance. In addition, the resultant 2D van der Waals stacking hybrid exhibits large surface area and a porous structure, which possesses a large amount of pyridine N. The pyridine N is occupied by negative charges, which show strong affinity for adsorbed NO2 molecules.25 Furthermore, the paper substrate may also contribute to the enhanced sensitivity of the g-C3N4/rGO sensor, because the cellulose network provides a highly porous structure to support the lamellar-like g-C3N4/rGO nanostructure. Consequently, the sensing materials have larger surface accessibility and greater sensitivity toward gas analytes. In the g-C3N4/rGO hybrid system, n-type g-C3N4 nanosheets served as the light harvesters, photogenerated electron producers, and the gas-sensing units, while 37196

DOI: 10.1021/acsami.7b11244 ACS Appl. Mater. Interfaces 2017, 9, 37191−37200

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layer decreased, leading to the negative response of conductivity (shown in Scheme S1). Compared with visible light irradiation, more photoelectrons were produced under UV irradiation, resulting in higher sensitivity. Without the assistance of light irradiation, the electrons would not be excited from the valence band of g-C3N4. This would be the reason why the g-C3N4/rGO hybrid sensor did not generate any conductance response to SO2 gas without light assistance. We also tested whether other mildly polar compounds, like acetone, would be photoreactive over the g-C3N4/rGO hybrid sensor with UV assistance (shown in Figure S16). Acetone also produced a conductive response on this type of sensor under UV light; however, the sensitivity to acetone is significantly lower than that to SO2. The responsivity is unstable due to the interference from noise and its low conductive response. The response mechanism of moderately polar molecular compounds to photoelectrons needs to be further studied in detail.

Scheme 2. Schematic Illustration of the NO2 Detection Mechanisma

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CONCLUSIONS In summary, we demonstrated that the paper chip assembled with the 2D g-C3N4/rGO stacking hybrid acts as a promising active material to distinguish NO2 and SO2 gases using the “light on and off” strategy. Under the “light off” condition, the 2D sensor showed remarkable p-type-responsive behavior toward lower concentrations of NO2 gas with higher selectivity as well as fast response and recovery in contrast to SO2 gas at room temperature. With UV light irradiation, the 2D sensor also showed n-type semiconducting behavior which could detect a SO2 concentration as low as 2 ppm at room temperature. Results suggest that effective electron transfer at the heterojunction of the van der Waals g-C3N4/rGO stacking hybrid plays an important role in gas sensing. The layer-bylayer approach provides an affordable approach to assemble various types of functional 2D nanomaterial on a paper substrate that can be potentially used in wearable sensor and flexible electronic device. Furthermore, gas discrimination ability can be enhanced using the “light on and off” strategy, owing to the different sensing mechanism.

a

(a) Multilayer g-C3N4/rGO sensor without light irradiation; (b) multilayer g-C3N4/rGO sensor under visible light irradiation; (c) multilayer g-C3N4/rGO sensor under UV light irradiation.

rGO sheets acted as photogenerated electron acceptors and pathways for electron transfer. When the g-C3N4/rGO sensor was irradiated under light, the photogenerated electrons would be excited from the valence band of n-type g-C3N4 to the conducting band first, as expressed in equation: g-C3N4 + hv → g-C3N4 (h+ + e−). The photoelectrons then migrated to the surface of the rGO layer, which were preferentially attracted by NO2 gas molecules in the interface between the g-C3N4 and rGO nanosheets. Under visible light irradiation, the photoelectrons were not sufficiently attractive to NO2 molecules, resulting in the hole still being the major charge carrier (shown in Scheme 2b). That is the reason that the g-C3N4/rGO sensor still exhibited p-type semiconducting behavior under visible light irradiation, even though the conductance response and sensitivity significantly decreased. Under UV light irradiation, more photoelectrons were excited from the valence band of gC3N4. Although a few of the photoelectrons were absorbed by NO2 molecules, there were still a large number of photoelectrons reaching the surface of the rGO layer, on which photoelectrons became the majority carriers (shown in Scheme 2c). Adsorption of NO2 molecules reduced the photoelectron density, leading to the decrease of conductivity of the g-C3N4/ rGO material under UV irradiation. That is the reason why the g-C3N4/rGO sensor showed decreasing sensitivity and a positive response under visible light and a negative response toward NO2 gas, respectively. As mentioned above, the g-C3N4/rGO sensor showed no response to SO2 without light assistance due to weak charge transfer between SO2 and rGO sheets. However, under light irradiation, large amounts of photoelectrons were produced from the valence band of g-C3N4 and transferred to the rGO nanosheets, which constructed a negative charge layer over the interface between g-C3N4 and rGO. After exposure to SO2 gas, the photoelectrons would be extracted by SO2, as expressed in equation: SO2 + e−→ SO2− (ads).48,55 As a result, the photoelectron density on the interface of the g-C3N4/rGO

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EXPERIMENTAL SECTION

Reagents. Dicyandiamide (98%) and hydrazine hydrate (98%) were purchased from Aladdin Co. (China). Graphene oxide (GO) solution (10 mg mL−1) was purchased from the Chengdu Institute of Organic Chemistry, Chinese Academy of Science. The characterization of the GO sample was conducted by CAS (http://www.timesnano. com). The filter paper was purchased from Sinopharm Chemical Reagent Co. (China). The standard gas samples including NO2 (20 ppm), SO2 (900 ppm), chlorine (20 ppm), ammonia (300 ppm), toluene (300 ppm), ethyl acetate (300 ppm), and hexane (300 ppm) were purchased from Hangzhou Xinshiji Gas Co. (China). The pure N2 (99.999%) was purchased from Hangzhou jingong Gas Co. (China). Material Synthesis. g-C3N4 was prepared by heating dicyandiamide to 550 °C at a heating rate of 2 °C min−1 and maintaining this temperature for another 4 h under air. A typical protonation treatment was undertaken by stirring g-C3N4 (3.0 g) with hydrochloric acid (ROTH, 37%, 30 mL) at 80 °C for 3 h, centrifugal washing with water until neutral conditions are achieved, and drying at 105 °C under air overnight.56 The resulting protonated g-C3N4 powder was dispersed in 30 mL of ethanol and sonicated for 2 h; the protonated g-C3N4 colloid was obtained after centrifugation of the initial formed suspension at 4000 rpm. As shown in Scheme 1, blank filter paper was immersed in GO nanosheets solution (2 mg mL−1, ethanol as solvent) for 3 min, which acted as the precursor film with a negatively charged surface, and then the filter paper was washed with ethanol to remove the excess 37197

DOI: 10.1021/acsami.7b11244 ACS Appl. Mater. Interfaces 2017, 9, 37191−37200

Research Article

ACS Applied Materials & Interfaces ORCID

GO nanosheets without adsorption, followed by drying with a gentle stream of N2. Subsequently, the resultant filter paper was immersed in a protonated g-C3N4 solution (2 mg mL−1, ethanol as solvent) for 3 min to construct multilayered g-C3N4/GO films via electrostatic and π−π stacking interactions. Then, the resultant substrate was washed with ethanol and dried with N2 again. By repeating the above operation for one cycle, we got g-C3N4/GO layer-by-layer selfassembly 2D stacked hybrid. It should be emphasized that in such hybrid films, GO nanosheets and g-C3N4 nanosheets were stacked in an alternating manner with intimate interfacial contact, which would be beneficial for shuttling photogenerated electrons from g-C3N4 to GO nanosheets. Afterward, the obtained 2D stacked hybrid g-C3N4/ GO films deposited on the cellulose network were reduced by hydrazine hydrate steam at 90 °C for 3 h, and the color of the multilayer material changed from dark brown to dark gray. The resulting complex was named the g-C3N4/rGO stacking hybrid. Meanwhile, to highlight the advantage of the g-C3N4/rGO stacking hybrid, counterparts of pure rGO and g-C3N4 films were also fabricated via an analogous layer-by-layer self-assembly strategy. Material Characterization. The surface morphologies and microstructures of rGO nanosheets, protonated g-C3N4 nanosheets, and the g-C3N4/rGO stacking hybrid film deposited on the filter paper were investigated using a S-4800 scanning electron microscope (Hitachi, Japan) and Tecnai G2 F30 S-Twin transmission electron microscope (Philips-FEI, Holland). XRD patterns were recorded on an X′ Pert PRO X-ray diffractometer produced by Dutch PANalytical company (Cu Kα radiation λ = 0.1541 nm, 40 kV, 40 mA) with a scan speed of 10° min−1 and a scan range of 5−80°. The Raman spectra were obtained using Lab RAM HR UV800 (JDB in Yvon, France) and X-ray photoelectron spectra (XPS), and characterization was carried out using a photoelectron spectrometer (Kratos AXIN Ultra DLD) with a monochromatic X-ray source of Al Kα (1487 eV). The multilayer g-C3N4/rGO hybrid supported on the filter paper was directly characterized using XRD, SEM, Raman, XPS, and I−V technology without any pretreatment. Before the characterization of the g-C3N4/rGO hybrid using TEM, the g-C3N4/rGO hybrid was eluted from the filter paper by ultrasonication for 2−3 h in ethanol solvent. The I−V curves and photocurrent were recorded using a Keithley 6487 picoammeter, and the photocurrent was measured at 3.0 V bias voltage. The light sources for the testing process are visible (≥420 nm) and UV (365 nm) LED. Gas Sensor Detection. The sensors were fabricated by a layer-bylayer approach supported on the paper, which was then admitted to a PMAA chamber. Evaluation of the sensing capability of each material was carried out in a homemade setup, see Figure S5. The target gas with different concentrations was achieved by a dynamic dilution system (National Institute of Metrology, China). The voltage applied on the sample was set at 3.0 V, and the current charge was measured using a Keithley 6487 picoammeter. The light sources for the testing process are visible (≥420 nm) and UV (365 nm) LED. A Labview computer program was developed to continuously monitor the current of the circuit. The current was normalized and plotted against the time. Sensitivity is defined as R = [(I − I0)/I0] × 100%, where I is the current detected under target gas exposure, and I0 is the current detected under N2 exposure.

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Aimin Chen: 0000-0002-9189-3726 Jianmin Wu: 0000-0002-0999-9194 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China (No. 21575127) and the Natural Science Foundation of Zhejiang province (No. Z15B050001).

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b11244. Additional plots, tables, schematics describing various aspects of these experiments (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*(A.C.): E-mail: [email protected] *(J.W.): E-mail: [email protected] 37198

DOI: 10.1021/acsami.7b11244 ACS Appl. Mater. Interfaces 2017, 9, 37191−37200

Research Article

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