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reduced graphene oxide assembled film as a promising NO2 gas sensing material
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Polyelectrolytes/reduced graphene oxide assembled film as a promising NO2 gas sensing material Zhi Yang∗, Yushuang Chen, Jiayuan Deng School of Materials Science and Engineering, Xihua University, Chengdu, 610039, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Polyelectrolyte Reduced graphene oxide Layer-by-layer assembly technique NO2 gas sensor
The nitrogen dioxide (NO2) is a typical hazardous gas and there is an urgent demand to develop effective, sensitive and reproducible sensing materials for NO2 gas detecting. In this paper, [poly-(sodium4-styrenesulfonate) stabilized reduced graphene oxide/poly-(allylamine hydrochloride)]n nanocomposite film are prepared via layer-by-layer assembly technique. The assembled process was monitored by UV–Vis spectroscopy in the range from 200 nm to 800 nm. Absorption peaks located at 233 nm and 271 nm, and the optical energy band-gap was estimated as 4.57 eV. The film thickness was tested about 95 nm by AFM with 0.95 nm for each assembled circle, and it was featured 975 ± 10 MPa, 28 ± 2 GPa, 105–106 Ω/sq. in modulus, hardness and sheet resistance respectively. The assembled films were extremely sensitive to NO2 with response (ΔR/Ra) above 66% with a linear relationship ranged from 2 to 8 ppm and independent on thickness. The impact of relative humidity was also considered for the film towards 5 ppm of NO2 gas at a room temperature, presenting a decreasing response from 1.8 to 0.89 when the relative humidity reaches 80%. The polyelectrolytes/reduced graphene oxide composite films are promising for application as gas sensing materials.
1. Introduction The nitrogen dioxide (NO2), mainly derived from fossil fuel combustion and vehicle exhaust emissions is a typical hazardous gas that can cause photochemical smog and acid rain, which severely pollute the air and show adverse effects to human beings and other living organisms [1]. It is consensus that, brief exposure to 10–20 ppm of NO2 gas would cause mild irritation of the nose and throat [2], whereas more than 1 h of exposure to NO2 gas cause inflammation in the lungs and edema [3]. Up to now, many semiconductors are explored as a gas sensor, such as metal oxide and metal sulfide [4–9], n-type phthalocyanine, 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-petacene) and other emerging materials [10–12]. However, most of these sensors are complicated to prepare and expensive. Thus, there is an urgent demand to develop and fabricate efficient, highly effective, sensitive and reproducible sensing materials for NO2 gas detecting. Graphene, as one of the most cited materials, has broadened its applications in many areas such as large-area electronics [13], high strength and conducting fillers in composites [14,15], and high-surfacearea electrodes for energy storage [16] owning to its unique properties [17–19]. As a typical two-dimensional (2D) material, each atom is located on the surface. Therefore, each atom can interact with gaseous molecules, which provides ultrahigh detection sensitivity and ultralow ∗
detection limit. Besides, the low electrical noise and exceptional transport properties make graphene attractive as a gas sensing material [20]. NO2 has a high electron affinity (2.3 eV), and there is full electron transfer from graphene to adsorbed NO2 molecules at low NO2 pressures. Scientists intend to take full advantage of the strong electron transfer to investigate graphene's feasibility as a chemical sensor [21]. Iezhokin has measured the electrical response to NO2 for epitaxial graphene and quasi freestanding epitaxial graphene on 6H–SiC substrates, and found that quasi freestanding epitaxial graphene shows a 6fold increase in NO2 sensitivity compared to epitaxial graphene owing to a strongly surface doping dependent sample resistance [22]. However, annealed sensor after each gas exposure to NO2 was tenfold higher than that of an as-fabricated graphene sensor, and the detection limit was estimated to be 0.6 parts per trillion (ppt) [23]. However, as we know, the growth conditions of epitaxial graphite are harsh, the perfect flat structure is not optimal for the applications in some specific realms and it is usually difficult to be transferred onto other substrate. Low cost preparation processes, for example, chemical method makes the reduced graphene oxide (rGO) rich in defects, and there is a tendency of irreversibly stacking and aggregation because of the strong π-π interactions, resulting in a significant reduction in surface area compared with theoretical value. In this work, we developed a kind of polyelectrolyte/PSS-rGO assembly films by conventional layer
Corresponding author. E-mail address: [email protected] (Z. Yang).
https://doi.org/10.1016/j.ceramint.2019.10.255 Received 4 July 2019; Received in revised form 9 October 2019; Accepted 26 October 2019 0272-8842/ © 2019 Published by Elsevier Ltd.
Please cite this article as: Zhi Yang, Yushuang Chen and Jiayuan Deng, Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.255
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by layer assembly technique. Compared with the traditional material composite method, layer by layer assembly technique has been widely used in various fields as a new and very important nano-processing method [24]. It is simple and easy to construct novel multi-layered materials in nano-scale, and to gain an improved comprehensive performance of the physics, chemistry and mechanics properties. By adsorbing electrolytes and graphene with different charges in solution layer by layer, functional film coatings with nano-scale can be prepared on arbitrary shaped materials. Although layer by layer assembly technique has been used for a long time, there are relatively few studies on the preparation of conductive films by layer by layer assembly of graphene and graphene derivatives. Our basic strategy involved the complete exfoliation of graphite oxide into individual PSS stabilized GO sheets followed by them in situ reduction to produce individual PSS stabilized graphene sheets. Here PSS was selected as a stabilizer profited from (1) π-π interactions between graphene and benzene rings and (2) electrostatic interactions [25,26] to prevent conglomerates appearing during reduction because of its lower hydrophilicity, which was deleterious to assemble uniformly. Then positive charged PAH and previously reported negative charged PSS stabilized graphene were assembled alternatively to growth the hybridized functional film. The structures and properties were characterized, which showed great promise in applications in the NO2 gas detections.
Fig. 1. Schematic diagram of layer-by-layer assembly technique.
2. Experimental section
graphite oxide) was added. A homogeneous RGO aqueous dispersion was obtained after filtration with a membrane and filtrate was used for next step directly. Finally, hydrazine (5 μl) was added to reduce graphene oxide at 95 °C for 24 h. The polymer solution is prepared as shown in the table.
2.1. Materials Natural graphene flake (−300mu), were purchased from SigmaAldrich and used as received. poly-(ethylenimine) (PEI, 50%in water, average Mn = 60 000 by GPC, average Mw = 750 000 by LS), poly(sodium-4-styrene sulfonate) (PSS, Mw = 70 000), poly-(allylamine hydrochloride) (PAH, Mw = 50 000–65 000) were all purchased from Sigma-Aldrich. Other regents all were obtained from Sinopharm Chemical Reagent CO., Ltd and used directly for next step without further purification. All aqueous solutions were prepared with ultrapure water (> 18 MΩ) from a Milli-Q Plus system (Millipore).
2.3.2Prepare of (PSS-graphene/PAH)n multilayer film Polyelectrolyte/graphene assembled films were prepared on single crystal silicon and quartz substrates via layer by layer assembly. The schematic diagram is shown in Fig. 1. Prior to deposition, all substrates were ultrasonically cleaned in absolute ethyl alcohol, propone, isopropyl alcohol in sequence, then followed by cleaning with Piranha solution [28] (H2SO4/H2O2, V/V, 3/1) at 80 °C for 1 h. The PSS-graphene and PAH solutions were layer by layer assembled onto silicon or quartz supporter. The preparation of polyelectrolyte solutions is shown in Table 1. These PSS-graphene/PAH multi-layers were alternatively deposited from PAH aqueous solutions and the prepared PSS-stabilized graphene solution, using an immersion time of 5 min, followed by rinsing with water and drying under N2 flow after each layer deposition. For each cycle, a bilayer of PAH/PSS-graphene was formed, and the UV–Visible spectra of the growing layers were recorded in air after each five assembly cycle. This cycle procedure was repeated until a free-standing film was obtained.
2.2. Instruments UV–Vis spectra of films on quartz slides were collected on a Hitachi U-4100 UV–Vis near-FTIR spectrometer. The scanning range was 200–800 nm, and the scanning speed was 300 nm/min. Raman spectra were obtained with a Renishaw micro-Raman 1000 spectrometers. A laser wavelength of 532 nm and an intensity of 1.5 mW were selected as the test conditions. X-ray diffraction (XRD) analysis was carried out using a D/Max 2500 V/PC diffractometer (Rigaku Corporation, Japan) with CuKα targets (λ = 0.154 nm) at a scanning rate of 2θ = 2°/min under a voltage of 40 kV and a current of 100 mA. FTIR Spectra were collected using Fourier Transform Infrared Spectrometer 3100. Atomic force microscope (AFM) images were obtained on a Veeco Icon with taping mode. Scanning electron microscopy (SEM) analysis was operated on HITACHI S4300 system. Nano-indentation experiments were carried out on Nano Indenter XP system after a standard calibration. Evaluation of the sensing capability of each material was carried out in a homemade setup, using Keithley S4200 with two probe configurations.
2.3.3Detecting NO2 gas Evaluation of the sensing capability of each material was carried out in a homemade setup, using Keithley S4200 with two probe configurations. A conventional photolithographic method was used to fabricate the dot electrode with a diameter of 160 μm and a spacing of 600 μm. The electrodes (30 nm Cr and 50 nm Au) were thermally evaporated on the film. The sensor was placed inside a stainless chamber of Table 1 Preparation of polyelectrolyte solutions.
2.3. Experiments 2.3.1. Prepare of PSS stabilized graphene The graphite oxide was synthesized from natural graphite powder based on a modified Hummers' method [27]. Then, exfoliation of graphite oxide to GO was achieved by ultra-sonication of the dispersion for 30 min (500 W, 40% amplitude) after PSS (ten times the weight of 2
Solutions
Cons. (mg/ml)
NaCl (mg/ml)
pH
PSS RGO-PSS PEI PAH
1.00 0.50 1.00 1.00
0.50 – 0.50 0.50
6.15 – 7.25 7.26
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60 mL volume, which was kept under continuous flowing of testing gas mixtures or air at a constant flow rate of 100 mL min−1. NO2 gas was prepared by reaction of nitric acid and copper, then dried by concentrated sulfuric acid. NO2 gas with concentrations from 2 ppm (0.6 μl) to 10 ppm (3 μl) was extracted with a miniature syringe utilized to evaluate the sensing performance. Prior to the measurement, the interdigitated electrode was blown with dry N2 flow and NO2 was injected into a gas chamber. The resistance was monitored continuously over time. The resistance changes of assembled film with different thickness based sensors were monitored with a computer controlled source meter (Keithley S4200). The sensitivity of the sensor was monitored by applying a constant bias voltage of 1.4 V on the sensor and recording the conductance change. All the gas sensing measurements were performed at room temperature (30 °C).
vibrations from COOH and H2O. The broaden band located at 1621 and 1728 cm−1 was related to asymmetric stretching of C]O. At lower wave number region, 1391, 1182, and 628/1042 cm−1 were assigned to the vibrations of C]C, C–O–C, C–S, respectively. The peak at 800 cm−1 was a characteristic vibration peak originated from Paradisubstituted benzene rings [30–33]. Comparing GO-PSS with RGOPSS, we could easily conclude that after reduction, the broad peak stretching of hydrogen-bonded alcohol groups with vibration frequencies above 3000 cm−1, such as 3376 cm−1, 3417 cm−1 and 3583 cm−1. Fig. 3(b) shows Raman spectra before and after reduction of PSS stabilized graphene. The peak located at 1580 cm−1 (named, G band) is the characteristic peak of crystallized graphite. D band, located at 1340–1350 cm−1, originated from sp3 hybridized carbon atoms isolated sp2 domains, which is a sign of imperfections in graphene sp2 planar structure [34–37]. After reduction, two apparent changes occurred: (a) intensity of D band enhanced, ID/IG ∼ 1.1, (b) about 10 units red shift of D band; which revealed a structural transform from sp3 to sp2 and an area decrease of the sp2 structure domains.
3. Results and discussion 3.1. Fabrication of the multilayer films via layer by layer assembly technique
XRD analysis The XRD curves in Fig. 4 indicate the successful preparation of PSSgraphene and (PSS-graphene/PAH)100 film. As can be seen, graphite oxide shows a sharp peak located at 2θ = 10.5°, corresponding to an interlayer spacing of 0.84 nm. The interlayer distance is increased due to the presence of oxygen-containing functional groups in GO sheets compared to the value for graphite (0.337 nm). Furthermore, the small peak located at 2θ = 22.9° (0.388 nm) is due to the presence of unoxidized graphite in graphite oxide. Alternatively, instead of the sharp peak at 2θ = 10.5°, PSS-graphene and film exhibit broad peaks at 22.7° (0.391 nm) and 2θ = 19.8° (0.448 nm), respectively. Therefore, interlayer spacing decreased in composites as compared to GO due to the reduction and restacking of the graphene sheets. It is notable that the interlayer spacing of film is larger than that of PSS-graphene, indicating that bulkier size of branched PAH as well as greater amount of PAH introduced.
In order to monitor the assembly process in real time and control the film growth process, ultraviolet–visible spectroscopy (UV–Vis) was used to analyze the film assembled on quartz. The film was tested every five cycle until the one hundred bilayers reached. The results were as shown in Fig. 2(a). With the increase of the number of bilayers, the transmittance of the assembled films to ultraviolet and visible light gradually decreases. After 100 bilayers are assembled, the minimum transmittance of the films in the ultraviolet–visible region decreases to about 20%. The above results confirm that the growth of the film conforms to the theoretical model, and the polyelectrolyte in the solution is gradually enriched on the substrate. At the same time, the film still has certain transmittance to ultraviolet and visible light. In order to study the variation of the transmittance qualitatively, we plotted the number of bilayers with the transmittance at 271 nm and 550 nm. The illustration in Fig. 2(b) shows that the transmittance has a good linear correlation with the number of layers. That is to say, the thickness of the film increases uniformly in the process of layer by layer assembly, which provides a feasible scheme for controlling the thickness of the film at nanometer level. The band gap energy of the 100-bilayer film was 4.57 eV of using the subsequent equations, Eg = 1240/λ (eV), in which λ equals to 271 nm [4,29].
AFM and SEM characterization A typical AFM tapping-mode image of functional film deposited onto a SiO2/Si substrate via layer-by-layer assembly (Fig. 5) with superimposed cross-section measurements taken along the line indicating a film thickness of 95 nm. That's about 0.95 nm for each assembled circle. Three-dimensional representations of the AFM topographic data show some graphene aggregates on the surface. The color ribbon shows the height ranged from −130 nm to 150 nm. Fig. 6(a)-(c) are SEM images of the 100-bilayer film with magnifications of × 18 k, × 70 k and × 150 k, respectively. It indicates a smooth surface and polymer featured wormy texture. Fig. 6(d) is the SEM image of the cross section and well organized stacking structure is presented.
3.2. Characterization of the multilayer films: (PAH/graphene-PSS)100 Raman and IR spectrum Fig. 3(a) shows the transmittance spectra (KBr) of GO-PSS, RGOPSS, FILM. The FTIR spectrum showed a broad band between 3000 cm−1 and 3700 cm−1 that was attributed to hydroxyl with all
Fig. 2. (a) UV–vis spectra following the LBL assembly of a (PAH/Graphene-PSS)100 multilayer film on a quartz slide. (b) Transmittance at 271 nm and 550 nm versus the number of bilayers assembled. 3
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Fig. 3. FT-IR spectra (a) and Raman spectra (b) and of PSS stabilized graphene oxide (GO-PSS), PSS stabilized graphene (RGO-PSS) and (PEI/PSS)2(PAH/GraphenePSS)10+0 film (FILM).
reveal that assembly films with a higher content of rGO flakes are more conductive, and their resistance is more easily reduced by thermal annealing, making them suitable as transparent conducting films. Mechanical properties Elastic modulus, hardness and other mechanical properties can be measured by means of nano-indentation tests. Here, no special sample preparation is required and indentation load and projected contact area: tests can be performed quickly and inexpensively. Nano-indentation is comparable to conventional hardness tests with better performance on a much smaller scale using pre-projected special equipment [38,39]. Elastic modulus can be calculated from indentation force versus penetration depth curve for 100-bilayer film; and after a transformation, hardness was obtained. Fig. 8(a) depicts the hardness and modulus versus distances displacing into surface of the nanocomposite film. We can easily read out an average hardness and modulus about 975 ± 10 MPa, 28 ± 2 GPa under the film thickness of 100-bilayer measured by AFM in previous section. The excellent mechanic properties make the assembled film easy to peel off the substrate and transfer to the target object. Fig. 8(b) and (c) show the self-supporting 50 bilayers thick (light colored) and 100 bilayers thick (darker colored) assembled films respectively.
Fig. 4. XRD curves of graphene oxide, PSS-graphene, and (PSS-graphene/ PAH)100.
Sheet resistances of the assembled multilayer films The sheet resistance every 10 bilayers were measured by a fourprobe method under the vacuum of 10−6 Pa. With the sheet resistance as the ordinate and the bilayer numbers as the abscissa, the relationship curve of the sheet resistances to the assembled bilayer numbers was drawn in Fig. 7(a). The sheet resistance decreases slightly from 1.63 × 106 Ω/sq. at 10 bilayers to 4.12 × 105 Ω/sq. at 100 bilayers, which attributes to an increased amount of assembled PSS-rGO. Furthermore, the variations of the sheet resistance of 10 bilayers, 50 bilayers and 100 bilayer thick films with the temperature were discussed in Fig. 7(b). It's found that the sheet resistance reduces very slowly at the temperature ranged from −50 °C to 200 °C. In brief, our studies
3.3. Gas sensing measurement As shown in Fig. 9, the current versus voltage (I–V) relationship was linear between −4 V and 4 V, exhibiting good ohmic contact between the layer by layer assembled films and the sensor electrodes. In other words, the Schottky barriers were absent and ensures the accuracy of the gas-sensing measurement in our work. The gas-sensing system was employed to record the resistance variation of fabricated sensors, and the response is calculated by the ratio of resistance which captured in an atmosphere of air (Ra) and NO2 (Rg) respectively, i.e., response = Rg/Ra. The detailed gas-sensing measurements revealed that
Fig. 5. AFM images of the Sub-(PEI/PSS)2(PAH/G-PSS)100 film. 4
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Fig. 6. SEM images of the Sub-(PEI/PSS)2(PAH/G-PSS)100 film. (a)–(c) are those with three different magnifications ( × 18 k, × 70 k and × 150 k), and (d) is the cross section.
the 100-bilayer films sensors exhibited the highest response (Rg/ Ra = 8.1) toward 10 ppm of NO2 gas compared to other layer by layer assembled films, i.e., 50-bilayer films (Rg/Ra = 4.1), and 10-bilayer films (Rg/Ra = 2.9) as shown in Fig. 9(a). The evaluation of response was converted as response = ΔR/Ra = | Ra-Rg |/Ra ( × 100%). The assembled films sensor exhibited the high response to NO2, including 10 bilayers (ΔR/Ra = 66%), 50 bilayers (ΔR/Ra = 76%) and 100 bilayers (ΔR/Ra = 66%). The results indicated that polyelectrolyte/graphene assembly film sensors were extremely sensitive to NO2. It can be attributed to: (1) the sulfonic groups (−SO3−) of PSS possess strong absorbability specially toward NO2 molecules [40]. (2) the strong electron transfers from graphene to NO2 [21]. The above two reasons should together be responsible for the eventual high NO2 selectivity of polyelectrolyte/graphene assembly film sensors. Fig. 9(b) shows a successive response and recovery curve toward different NO2 concentrations. As we expected, the response was gradually increasing along with the rising of NO2 concentration, and the response and recovery time were uniform under different concentrations. The polyelectrolyte/PSS-rGO sensors revealed an excellent linear detection ranged from 2 to 8 ppm with corresponding response (Rg/Ra) measured from 2.9 to 8.1 as shown inlet. The impact of relative humidity on reaction has been considered for the 100-bilayer film towards
5 ppm of NO2 gas at a room temperature of 30 °C and represented in Fig. 9(d). The sensor responses were obtained to be decreasing from 1.8 to 0.89 for the film at a higher humidity of 80%. Above results demonstrate that the polyelectrolyte/PSS-rGO multilayer films are promising for application as gas sensing materials. 4. Conclusions In the present work, we report a kind of polyelectrolyte/PSS-rGO nano-composited film prepared via layer by layer assembly technique. The growth of the multilayer film can be carefully manipulated and monitored by UV–Vis spectra. With the increase of the number of bilayers, the transmittance of the assembled films to ultraviolet and visible light gradually decreases. After 100 bilayers are assembled, the minimum transmittance of the films in the ultraviolet–visible region decreases to about 20%. Two absorption peaks located at 233 nm and 271 nm, and the optical energy band-gap was estimated as 4.57 eV. The morphologies are characterized by AFM and SEM, and the 100-bilayer films are tested about 95 nm thick with 0.95 nm for each assembled circle. The modulus and hardness are 975 ± 10 MPa, 28 ± 2 GPa. The sheet resistance obtained via four-probe method is about 105–106 orders of magnitude. The assembled films sensor exhibited the high
Fig. 7. Sheet resistance variations with bilayer number (a) and temperature (b). 5
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Fig. 8. (a) Nano-indentation tests of the assembled 100-bilayer film, (b) and (c) are the pictures of the free assembled 50-bilayer and 100-bilayer films pealed from the substrates.
Declaration of competing interests
response to NO2, including 10 bilayers (ΔR/Ra = 66%), 50 bilayers (ΔR/Ra = 76%) and 100 bilayers (ΔR/Ra = 66%). The impact of relative humidity was also considered for the film towards 5 ppm of NO2 gas at a room temperature, presenting a decreasing response from 1.8 to 0.89 when the relative humidity reaches 80%. These results indicated that polyelectrolyte/PSS-rGO assembly films sensors were extremely sensitive to NO2 and revealed a linear detection ranged from 2 to 8 ppm.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements This research was generously supported by “Youth Scholars Reserve Talents” Program of Xihua University (No. DC190000788).
Fig. 9. (a) I–V characteristics with different film thickness in the tests; (b) Response comparison of 10-bilayer, 50-bilayer and 100-layer films sensors toward 10 ppm of NO2; (c) Successive response and recovery curve of 100-bilayer films sensors with NO2 concentrations ranging from 2 to 8 ppm and corresponding linear fitting curve (inset), error bars for the data points lie within the symbols themselves; (d) Response to 5 ppm NO2 gas with different relative humidity (%RH) at room temperature (30 °C). 6
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Polyelectrolytes/reduced graphene oxide assembled film as a promising NO2 gas sensing material Zhi Yang∗, Yushuang Chen, Jiayuan Deng School of Materials Science and Engineering, Xihua University, Chengdu, 610039, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Polyelectrolyte Reduced graphene oxide Layer-by-layer assembly technique NO2 gas sensor
The nitrogen dioxide (NO2) is a typical hazardous gas and there is an urgent demand to develop effective, sensitive and reproducible sensing materials for NO2 gas detecting. In this paper, [poly-(sodium4-styrenesulfonate) stabilized reduced graphene oxide/poly-(allylamine hydrochloride)]n nanocomposite film are prepared via layer-by-layer assembly technique. The assembled process was monitored by UV–Vis spectroscopy in the range from 200 nm to 800 nm. Absorption peaks located at 233 nm and 271 nm, and the optical energy band-gap was estimated as 4.57 eV. The film thickness was tested about 95 nm by AFM with 0.95 nm for each assembled circle, and it was featured 975 ± 10 MPa, 28 ± 2 GPa, 105–106 Ω/sq. in modulus, hardness and sheet resistance respectively. The assembled films were extremely sensitive to NO2 with response (ΔR/Ra) above 66% with a linear relationship ranged from 2 to 8 ppm and independent on thickness. The impact of relative humidity was also considered for the film towards 5 ppm of NO2 gas at a room temperature, presenting a decreasing response from 1.8 to 0.89 when the relative humidity reaches 80%. The polyelectrolytes/reduced graphene oxide composite films are promising for application as gas sensing materials.
1. Introduction The nitrogen dioxide (NO2), mainly derived from fossil fuel combustion and vehicle exhaust emissions is a typical hazardous gas that can cause photochemical smog and acid rain, which severely pollute the air and show adverse effects to human beings and other living organisms [1]. It is consensus that, brief exposure to 10–20 ppm of NO2 gas would cause mild irritation of the nose and throat [2], whereas more than 1 h of exposure to NO2 gas cause inflammation in the lungs and edema [3]. Up to now, many semiconductors are explored as a gas sensor, such as metal oxide and metal sulfide [4–9], n-type phthalocyanine, 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-petacene) and other emerging materials [10–12]. However, most of these sensors are complicated to prepare and expensive. Thus, there is an urgent demand to develop and fabricate efficient, highly effective, sensitive and reproducible sensing materials for NO2 gas detecting. Graphene, as one of the most cited materials, has broadened its applications in many areas such as large-area electronics [13], high strength and conducting fillers in composites [14,15], and high-surfacearea electrodes for energy storage [16] owning to its unique properties [17–19]. As a typical two-dimensional (2D) material, each atom is located on the surface. Therefore, each atom can interact with gaseous molecules, which provides ultrahigh detection sensitivity and ultralow ∗
detection limit. Besides, the low electrical noise and exceptional transport properties make graphene attractive as a gas sensing material [20]. NO2 has a high electron affinity (2.3 eV), and there is full electron transfer from graphene to adsorbed NO2 molecules at low NO2 pressures. Scientists intend to take full advantage of the strong electron transfer to investigate graphene's feasibility as a chemical sensor [21]. Iezhokin has measured the electrical response to NO2 for epitaxial graphene and quasi freestanding epitaxial graphene on 6H–SiC substrates, and found that quasi freestanding epitaxial graphene shows a 6fold increase in NO2 sensitivity compared to epitaxial graphene owing to a strongly surface doping dependent sample resistance [22]. However, annealed sensor after each gas exposure to NO2 was tenfold higher than that of an as-fabricated graphene sensor, and the detection limit was estimated to be 0.6 parts per trillion (ppt) [23]. However, as we know, the growth conditions of epitaxial graphite are harsh, the perfect flat structure is not optimal for the applications in some specific realms and it is usually difficult to be transferred onto other substrate. Low cost preparation processes, for example, chemical method makes the reduced graphene oxide (rGO) rich in defects, and there is a tendency of irreversibly stacking and aggregation because of the strong π-π interactions, resulting in a significant reduction in surface area compared with theoretical value. In this work, we developed a kind of polyelectrolyte/PSS-rGO assembly films by conventional layer
Corresponding author. E-mail address: [email protected] (Z. Yang).
https://doi.org/10.1016/j.ceramint.2019.10.255 Received 4 July 2019; Received in revised form 9 October 2019; Accepted 26 October 2019 0272-8842/ © 2019 Published by Elsevier Ltd.
Please cite this article as: Zhi Yang, Yushuang Chen and Jiayuan Deng, Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.255
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by layer assembly technique. Compared with the traditional material composite method, layer by layer assembly technique has been widely used in various fields as a new and very important nano-processing method [24]. It is simple and easy to construct novel multi-layered materials in nano-scale, and to gain an improved comprehensive performance of the physics, chemistry and mechanics properties. By adsorbing electrolytes and graphene with different charges in solution layer by layer, functional film coatings with nano-scale can be prepared on arbitrary shaped materials. Although layer by layer assembly technique has been used for a long time, there are relatively few studies on the preparation of conductive films by layer by layer assembly of graphene and graphene derivatives. Our basic strategy involved the complete exfoliation of graphite oxide into individual PSS stabilized GO sheets followed by them in situ reduction to produce individual PSS stabilized graphene sheets. Here PSS was selected as a stabilizer profited from (1) π-π interactions between graphene and benzene rings and (2) electrostatic interactions [25,26] to prevent conglomerates appearing during reduction because of its lower hydrophilicity, which was deleterious to assemble uniformly. Then positive charged PAH and previously reported negative charged PSS stabilized graphene were assembled alternatively to growth the hybridized functional film. The structures and properties were characterized, which showed great promise in applications in the NO2 gas detections.
Fig. 1. Schematic diagram of layer-by-layer assembly technique.
2. Experimental section
graphite oxide) was added. A homogeneous RGO aqueous dispersion was obtained after filtration with a membrane and filtrate was used for next step directly. Finally, hydrazine (5 μl) was added to reduce graphene oxide at 95 °C for 24 h. The polymer solution is prepared as shown in the table.
2.1. Materials Natural graphene flake (−300mu), were purchased from SigmaAldrich and used as received. poly-(ethylenimine) (PEI, 50%in water, average Mn = 60 000 by GPC, average Mw = 750 000 by LS), poly(sodium-4-styrene sulfonate) (PSS, Mw = 70 000), poly-(allylamine hydrochloride) (PAH, Mw = 50 000–65 000) were all purchased from Sigma-Aldrich. Other regents all were obtained from Sinopharm Chemical Reagent CO., Ltd and used directly for next step without further purification. All aqueous solutions were prepared with ultrapure water (> 18 MΩ) from a Milli-Q Plus system (Millipore).
2.3.2Prepare of (PSS-graphene/PAH)n multilayer film Polyelectrolyte/graphene assembled films were prepared on single crystal silicon and quartz substrates via layer by layer assembly. The schematic diagram is shown in Fig. 1. Prior to deposition, all substrates were ultrasonically cleaned in absolute ethyl alcohol, propone, isopropyl alcohol in sequence, then followed by cleaning with Piranha solution [28] (H2SO4/H2O2, V/V, 3/1) at 80 °C for 1 h. The PSS-graphene and PAH solutions were layer by layer assembled onto silicon or quartz supporter. The preparation of polyelectrolyte solutions is shown in Table 1. These PSS-graphene/PAH multi-layers were alternatively deposited from PAH aqueous solutions and the prepared PSS-stabilized graphene solution, using an immersion time of 5 min, followed by rinsing with water and drying under N2 flow after each layer deposition. For each cycle, a bilayer of PAH/PSS-graphene was formed, and the UV–Visible spectra of the growing layers were recorded in air after each five assembly cycle. This cycle procedure was repeated until a free-standing film was obtained.
2.2. Instruments UV–Vis spectra of films on quartz slides were collected on a Hitachi U-4100 UV–Vis near-FTIR spectrometer. The scanning range was 200–800 nm, and the scanning speed was 300 nm/min. Raman spectra were obtained with a Renishaw micro-Raman 1000 spectrometers. A laser wavelength of 532 nm and an intensity of 1.5 mW were selected as the test conditions. X-ray diffraction (XRD) analysis was carried out using a D/Max 2500 V/PC diffractometer (Rigaku Corporation, Japan) with CuKα targets (λ = 0.154 nm) at a scanning rate of 2θ = 2°/min under a voltage of 40 kV and a current of 100 mA. FTIR Spectra were collected using Fourier Transform Infrared Spectrometer 3100. Atomic force microscope (AFM) images were obtained on a Veeco Icon with taping mode. Scanning electron microscopy (SEM) analysis was operated on HITACHI S4300 system. Nano-indentation experiments were carried out on Nano Indenter XP system after a standard calibration. Evaluation of the sensing capability of each material was carried out in a homemade setup, using Keithley S4200 with two probe configurations.
2.3.3Detecting NO2 gas Evaluation of the sensing capability of each material was carried out in a homemade setup, using Keithley S4200 with two probe configurations. A conventional photolithographic method was used to fabricate the dot electrode with a diameter of 160 μm and a spacing of 600 μm. The electrodes (30 nm Cr and 50 nm Au) were thermally evaporated on the film. The sensor was placed inside a stainless chamber of Table 1 Preparation of polyelectrolyte solutions.
2.3. Experiments 2.3.1. Prepare of PSS stabilized graphene The graphite oxide was synthesized from natural graphite powder based on a modified Hummers' method [27]. Then, exfoliation of graphite oxide to GO was achieved by ultra-sonication of the dispersion for 30 min (500 W, 40% amplitude) after PSS (ten times the weight of 2
Solutions
Cons. (mg/ml)
NaCl (mg/ml)
pH
PSS RGO-PSS PEI PAH
1.00 0.50 1.00 1.00
0.50 – 0.50 0.50
6.15 – 7.25 7.26
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60 mL volume, which was kept under continuous flowing of testing gas mixtures or air at a constant flow rate of 100 mL min−1. NO2 gas was prepared by reaction of nitric acid and copper, then dried by concentrated sulfuric acid. NO2 gas with concentrations from 2 ppm (0.6 μl) to 10 ppm (3 μl) was extracted with a miniature syringe utilized to evaluate the sensing performance. Prior to the measurement, the interdigitated electrode was blown with dry N2 flow and NO2 was injected into a gas chamber. The resistance was monitored continuously over time. The resistance changes of assembled film with different thickness based sensors were monitored with a computer controlled source meter (Keithley S4200). The sensitivity of the sensor was monitored by applying a constant bias voltage of 1.4 V on the sensor and recording the conductance change. All the gas sensing measurements were performed at room temperature (30 °C).
vibrations from COOH and H2O. The broaden band located at 1621 and 1728 cm−1 was related to asymmetric stretching of C]O. At lower wave number region, 1391, 1182, and 628/1042 cm−1 were assigned to the vibrations of C]C, C–O–C, C–S, respectively. The peak at 800 cm−1 was a characteristic vibration peak originated from Paradisubstituted benzene rings [30–33]. Comparing GO-PSS with RGOPSS, we could easily conclude that after reduction, the broad peak stretching of hydrogen-bonded alcohol groups with vibration frequencies above 3000 cm−1, such as 3376 cm−1, 3417 cm−1 and 3583 cm−1. Fig. 3(b) shows Raman spectra before and after reduction of PSS stabilized graphene. The peak located at 1580 cm−1 (named, G band) is the characteristic peak of crystallized graphite. D band, located at 1340–1350 cm−1, originated from sp3 hybridized carbon atoms isolated sp2 domains, which is a sign of imperfections in graphene sp2 planar structure [34–37]. After reduction, two apparent changes occurred: (a) intensity of D band enhanced, ID/IG ∼ 1.1, (b) about 10 units red shift of D band; which revealed a structural transform from sp3 to sp2 and an area decrease of the sp2 structure domains.
3. Results and discussion 3.1. Fabrication of the multilayer films via layer by layer assembly technique
XRD analysis The XRD curves in Fig. 4 indicate the successful preparation of PSSgraphene and (PSS-graphene/PAH)100 film. As can be seen, graphite oxide shows a sharp peak located at 2θ = 10.5°, corresponding to an interlayer spacing of 0.84 nm. The interlayer distance is increased due to the presence of oxygen-containing functional groups in GO sheets compared to the value for graphite (0.337 nm). Furthermore, the small peak located at 2θ = 22.9° (0.388 nm) is due to the presence of unoxidized graphite in graphite oxide. Alternatively, instead of the sharp peak at 2θ = 10.5°, PSS-graphene and film exhibit broad peaks at 22.7° (0.391 nm) and 2θ = 19.8° (0.448 nm), respectively. Therefore, interlayer spacing decreased in composites as compared to GO due to the reduction and restacking of the graphene sheets. It is notable that the interlayer spacing of film is larger than that of PSS-graphene, indicating that bulkier size of branched PAH as well as greater amount of PAH introduced.
In order to monitor the assembly process in real time and control the film growth process, ultraviolet–visible spectroscopy (UV–Vis) was used to analyze the film assembled on quartz. The film was tested every five cycle until the one hundred bilayers reached. The results were as shown in Fig. 2(a). With the increase of the number of bilayers, the transmittance of the assembled films to ultraviolet and visible light gradually decreases. After 100 bilayers are assembled, the minimum transmittance of the films in the ultraviolet–visible region decreases to about 20%. The above results confirm that the growth of the film conforms to the theoretical model, and the polyelectrolyte in the solution is gradually enriched on the substrate. At the same time, the film still has certain transmittance to ultraviolet and visible light. In order to study the variation of the transmittance qualitatively, we plotted the number of bilayers with the transmittance at 271 nm and 550 nm. The illustration in Fig. 2(b) shows that the transmittance has a good linear correlation with the number of layers. That is to say, the thickness of the film increases uniformly in the process of layer by layer assembly, which provides a feasible scheme for controlling the thickness of the film at nanometer level. The band gap energy of the 100-bilayer film was 4.57 eV of using the subsequent equations, Eg = 1240/λ (eV), in which λ equals to 271 nm [4,29].
AFM and SEM characterization A typical AFM tapping-mode image of functional film deposited onto a SiO2/Si substrate via layer-by-layer assembly (Fig. 5) with superimposed cross-section measurements taken along the line indicating a film thickness of 95 nm. That's about 0.95 nm for each assembled circle. Three-dimensional representations of the AFM topographic data show some graphene aggregates on the surface. The color ribbon shows the height ranged from −130 nm to 150 nm. Fig. 6(a)-(c) are SEM images of the 100-bilayer film with magnifications of × 18 k, × 70 k and × 150 k, respectively. It indicates a smooth surface and polymer featured wormy texture. Fig. 6(d) is the SEM image of the cross section and well organized stacking structure is presented.
3.2. Characterization of the multilayer films: (PAH/graphene-PSS)100 Raman and IR spectrum Fig. 3(a) shows the transmittance spectra (KBr) of GO-PSS, RGOPSS, FILM. The FTIR spectrum showed a broad band between 3000 cm−1 and 3700 cm−1 that was attributed to hydroxyl with all
Fig. 2. (a) UV–vis spectra following the LBL assembly of a (PAH/Graphene-PSS)100 multilayer film on a quartz slide. (b) Transmittance at 271 nm and 550 nm versus the number of bilayers assembled. 3
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Fig. 3. FT-IR spectra (a) and Raman spectra (b) and of PSS stabilized graphene oxide (GO-PSS), PSS stabilized graphene (RGO-PSS) and (PEI/PSS)2(PAH/GraphenePSS)10+0 film (FILM).
reveal that assembly films with a higher content of rGO flakes are more conductive, and their resistance is more easily reduced by thermal annealing, making them suitable as transparent conducting films. Mechanical properties Elastic modulus, hardness and other mechanical properties can be measured by means of nano-indentation tests. Here, no special sample preparation is required and indentation load and projected contact area: tests can be performed quickly and inexpensively. Nano-indentation is comparable to conventional hardness tests with better performance on a much smaller scale using pre-projected special equipment [38,39]. Elastic modulus can be calculated from indentation force versus penetration depth curve for 100-bilayer film; and after a transformation, hardness was obtained. Fig. 8(a) depicts the hardness and modulus versus distances displacing into surface of the nanocomposite film. We can easily read out an average hardness and modulus about 975 ± 10 MPa, 28 ± 2 GPa under the film thickness of 100-bilayer measured by AFM in previous section. The excellent mechanic properties make the assembled film easy to peel off the substrate and transfer to the target object. Fig. 8(b) and (c) show the self-supporting 50 bilayers thick (light colored) and 100 bilayers thick (darker colored) assembled films respectively.
Fig. 4. XRD curves of graphene oxide, PSS-graphene, and (PSS-graphene/ PAH)100.
Sheet resistances of the assembled multilayer films The sheet resistance every 10 bilayers were measured by a fourprobe method under the vacuum of 10−6 Pa. With the sheet resistance as the ordinate and the bilayer numbers as the abscissa, the relationship curve of the sheet resistances to the assembled bilayer numbers was drawn in Fig. 7(a). The sheet resistance decreases slightly from 1.63 × 106 Ω/sq. at 10 bilayers to 4.12 × 105 Ω/sq. at 100 bilayers, which attributes to an increased amount of assembled PSS-rGO. Furthermore, the variations of the sheet resistance of 10 bilayers, 50 bilayers and 100 bilayer thick films with the temperature were discussed in Fig. 7(b). It's found that the sheet resistance reduces very slowly at the temperature ranged from −50 °C to 200 °C. In brief, our studies
3.3. Gas sensing measurement As shown in Fig. 9, the current versus voltage (I–V) relationship was linear between −4 V and 4 V, exhibiting good ohmic contact between the layer by layer assembled films and the sensor electrodes. In other words, the Schottky barriers were absent and ensures the accuracy of the gas-sensing measurement in our work. The gas-sensing system was employed to record the resistance variation of fabricated sensors, and the response is calculated by the ratio of resistance which captured in an atmosphere of air (Ra) and NO2 (Rg) respectively, i.e., response = Rg/Ra. The detailed gas-sensing measurements revealed that
Fig. 5. AFM images of the Sub-(PEI/PSS)2(PAH/G-PSS)100 film. 4
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Fig. 6. SEM images of the Sub-(PEI/PSS)2(PAH/G-PSS)100 film. (a)–(c) are those with three different magnifications ( × 18 k, × 70 k and × 150 k), and (d) is the cross section.
the 100-bilayer films sensors exhibited the highest response (Rg/ Ra = 8.1) toward 10 ppm of NO2 gas compared to other layer by layer assembled films, i.e., 50-bilayer films (Rg/Ra = 4.1), and 10-bilayer films (Rg/Ra = 2.9) as shown in Fig. 9(a). The evaluation of response was converted as response = ΔR/Ra = | Ra-Rg |/Ra ( × 100%). The assembled films sensor exhibited the high response to NO2, including 10 bilayers (ΔR/Ra = 66%), 50 bilayers (ΔR/Ra = 76%) and 100 bilayers (ΔR/Ra = 66%). The results indicated that polyelectrolyte/graphene assembly film sensors were extremely sensitive to NO2. It can be attributed to: (1) the sulfonic groups (−SO3−) of PSS possess strong absorbability specially toward NO2 molecules [40]. (2) the strong electron transfers from graphene to NO2 [21]. The above two reasons should together be responsible for the eventual high NO2 selectivity of polyelectrolyte/graphene assembly film sensors. Fig. 9(b) shows a successive response and recovery curve toward different NO2 concentrations. As we expected, the response was gradually increasing along with the rising of NO2 concentration, and the response and recovery time were uniform under different concentrations. The polyelectrolyte/PSS-rGO sensors revealed an excellent linear detection ranged from 2 to 8 ppm with corresponding response (Rg/Ra) measured from 2.9 to 8.1 as shown inlet. The impact of relative humidity on reaction has been considered for the 100-bilayer film towards
5 ppm of NO2 gas at a room temperature of 30 °C and represented in Fig. 9(d). The sensor responses were obtained to be decreasing from 1.8 to 0.89 for the film at a higher humidity of 80%. Above results demonstrate that the polyelectrolyte/PSS-rGO multilayer films are promising for application as gas sensing materials. 4. Conclusions In the present work, we report a kind of polyelectrolyte/PSS-rGO nano-composited film prepared via layer by layer assembly technique. The growth of the multilayer film can be carefully manipulated and monitored by UV–Vis spectra. With the increase of the number of bilayers, the transmittance of the assembled films to ultraviolet and visible light gradually decreases. After 100 bilayers are assembled, the minimum transmittance of the films in the ultraviolet–visible region decreases to about 20%. Two absorption peaks located at 233 nm and 271 nm, and the optical energy band-gap was estimated as 4.57 eV. The morphologies are characterized by AFM and SEM, and the 100-bilayer films are tested about 95 nm thick with 0.95 nm for each assembled circle. The modulus and hardness are 975 ± 10 MPa, 28 ± 2 GPa. The sheet resistance obtained via four-probe method is about 105–106 orders of magnitude. The assembled films sensor exhibited the high
Fig. 7. Sheet resistance variations with bilayer number (a) and temperature (b). 5
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Fig. 8. (a) Nano-indentation tests of the assembled 100-bilayer film, (b) and (c) are the pictures of the free assembled 50-bilayer and 100-bilayer films pealed from the substrates.
Declaration of competing interests
response to NO2, including 10 bilayers (ΔR/Ra = 66%), 50 bilayers (ΔR/Ra = 76%) and 100 bilayers (ΔR/Ra = 66%). The impact of relative humidity was also considered for the film towards 5 ppm of NO2 gas at a room temperature, presenting a decreasing response from 1.8 to 0.89 when the relative humidity reaches 80%. These results indicated that polyelectrolyte/PSS-rGO assembly films sensors were extremely sensitive to NO2 and revealed a linear detection ranged from 2 to 8 ppm.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements This research was generously supported by “Youth Scholars Reserve Talents” Program of Xihua University (No. DC190000788).
Fig. 9. (a) I–V characteristics with different film thickness in the tests; (b) Response comparison of 10-bilayer, 50-bilayer and 100-layer films sensors toward 10 ppm of NO2; (c) Successive response and recovery curve of 100-bilayer films sensors with NO2 concentrations ranging from 2 to 8 ppm and corresponding linear fitting curve (inset), error bars for the data points lie within the symbols themselves; (d) Response to 5 ppm NO2 gas with different relative humidity (%RH) at room temperature (30 °C). 6
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References
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