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Cu2O quantum dots modified by RGO nanosheets for ultrasensitive and selective NO2 gas detection
Ceramics International 43 (2017) 8372–8377
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Cu2O quantum dots modified by RGO nanosheets for ultrasensitive and selective NO2 gas detection
MARK
⁎
Yong Zhoua, , Guoqing Liua,b, Xiangyi Zhua, Yongcai Guoa a Key Laboratory of Optoelectronic Technology and System of Ministry of Education, College of Optoelectronic Engineering, Chongqing University, Chongqing 400044, PR China b Chongqing Research Institute CO., Ltd. Of China Coal Technology & Engineering Group, Chongqing 400037, PR China
A R T I C L E I N F O
A BS T RAC T
Keywords: Gas sensor Thin film Cuprous oxide quantum dots Reduced graphene oxide Nitrogen dioxide
Real-time monitoring of trace NO2 emission has been an emerging challenge in environment and health sectors lately. Aiming to overcome this challenge, NO2 gas sensors based on cuprous oxide quantum dots (Cu2O QDs) anchored onto reduced graphene oxide (RGO) nanosheets serving as a sensitive layer were prepared in this report. Apart from a series of purposive measurements, various characterization techniques such as XRD, Raman, XPS and TEM were employed as well to assist the exploration of sensors performance to NO2 gas. The experimental results revealed a 580% response enhancement for prepared RGO/Cu2O sensors compared with pure RGO counterparts, as well as an excellent selectivity. In a specific experiment, the sensing response attained 4.8% and 29.3% toward 20 ppb and 100 ppb NO2 respectively at 60 °C, which was larger than most Cu2O based resistive gas sensors. Moreover, further subtle modulation of this RGO/Cu2O nanocomposites led to a preferable room-temperature response of 37.8% toward 100 ppb NO2, which also offered a favorable stability of 98.1% response retention after four exposures within ten days. The obtained results imply that the prepared RGO/Cu2O QDs sensors possess a competitive capability of trace NO2 detection.
1. Introduction Nitrogen dioxide (NO2), mainly coming from fossil fuels and motor vehicles, is a hazardous gas that poses great threat to environmental protection and human health. According to World Health Organization, the air quality guideline for NO2 gas is appropriately 410 ppb in an hour period [1]. However, there still exist significant challenges for selective detection of trace NO2 [2], provoking a pressing need to develop highperformance NO2 gas sensors. As the most popular gas-sensing materials, metal oxides have been worldwide applied in the field of chemical sensors. For instance, cuprous oxide (Cu2O) as a typical p-type semiconductor has attracted extensive research in terms of diverse nanostructures including nanospheres [3], nanowires [4], nanorods [5], nanoparticles [6] and nanofilms [7]. However, pure Cu2O always suffers from high operation temperature over 150 °C, thereby bringing about high power-consumption and security issues to hinder its practical applications. To overcome this obstacle, dimensional miniaturization of Cu2O material is a universal approach to realize low-temperature detection. Apart from this approach, elaborate doping with conducting nanofillers can yet be regarded as another favorable means. On the basis of this means,
⁎
carbon nanomaterials including carbon black, carbon nanotubes and graphene have been the subjects of intense research for gas sensing over the past few decades [8–11]. With the inherent superiorities of large specific surface area and high electric conductivity, carbon nanomaterials have exhibited an excellent detection sensitivity especially with interesting transduction properties in a single layer of material [12]. Take two-dimensional (2D) graphene and its derivatives for example. Since the pioneering detection of individual gas molecules [13], graphene materials have been broadly studied from scientific communities of physics, chemistry, materials science and other disciplines. Among these, reduced graphene oxide (RGO) prevails over its counterparts such as pristine graphene and graphene oxide (GO) in chemiresistive gas sensors due to more adsorption sites and preferable conductance, respectively. To facilitate a better sensing performance at low temperature, in this report we leverage the complementary advantages of low-dimensional materials and conductive nanofillers by preparing NO2 sensors featuring a nanocomposite of Cu2O quantum dots (QDs) anchored on reduced graphene oxide (RGO) nanosheets as the sensitive layer. The excellent sensing results make RGO/Cu2O QDs nanocomposites an exciting alternative for trace NO2 detection in the future.
Corresponding author. E-mail address: [email protected] (Y. Zhou).
http://dx.doi.org/10.1016/j.ceramint.2017.03.179 Received 5 January 2017; Received in revised form 23 March 2017; Accepted 28 March 2017 Available online 30 March 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Ceramics International 43 (2017) 8372–8377
Y. Zhou et al.
Fig. 1. Schematic illustrations of (a) fabrication route and (c) IDEs devices, (b) prepared IDEs.
Fig. 2. Characterization features of prepared samples including (a)XRD spectra, (b) Raman analysis, (c) XPS survey spectrum and (d) Cu 2p spectrum.
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Fig. 3. TEM analysis of (a) RGO/Cu2O, (b) further magnification of (a), (c) further enlargement of (b), (d) HRTEM picture of circular zone in (c), (e) SAED pattern of Cu2O and (f) particle distribution.
through the same process without the treatment steps of Cu(Ac)2 or GO aqueous solutions respectively, and dispersed in water/ethanol solution with a volume fraction of 1:4. Regarding film deposition, RGO/Cu2O or RGO solution mentioned earlier was airbrushed onto clean interdigital electrodes (IDEs), which were subsequently placed into a 70 °C vacuum oven overnight for sufficient evaporation of solvent and moisture. The detailed fabrication route of planar IDEs is shown in Fig. 1(a). Lithography and lift-off technologies are employed to pattern Ti and Au layers (100 nm/ 200 nm) onto SiO2/Si substrate. As seen from Fig. 1(b) and (c), the prepared IDEs devices are highly reproduced with the finger and interspacing widths of 50 µm and an active area of 1.1 cm×0.7 cm.
2. Experimental 2.1. Sensor preparation In a typical experiment, a uniform GO solution (0.1 mg/ml, 25 ml, purchased from Sigma-Aldrich) was treated under a mild sonication for an hour, later mixing it with 20 ml 15 mM copper acetate (Cu(Ac)2) aqueous solution. The obtained brown suspension was then stirred magnetically at 25 °C for 12 h, with a subsequent introduction of 20 ml 18 mM sodium hydroxide (NaOH) aqueous solution dropwise. After 10 h stewing without any treatment, the created greenish brown precipitate was then transferred into a 50 ml autoclave containing 30 ml 6 mM glucose aqueous solution for a hydrothermal reaction at 96 °C for 10 h, followed by a swift cooling process with the aid of icewater bath. The resultant products were eventually obtained by centrifugation and washed with water and ethanol several times, and then re-dispersed in ethanol. Pure RGO or Cu2O material was prepared
2.2. Test requirements In this paper, X-ray diffraction spectrum (XRD) was measured by PANalytical X′pert HighScore XRD instrument. Raman spectrum was 8374
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Y. Zhou et al.
Al Kα X-ray source. Transmission electron microscopy (TEM) images were recorded on a JEOL-2010 TEM operated at 200 kV. During the dynamic tests, dry air is utilized as the background gas. Gas concentration is adjusted by a mass flow controller (MT50–4J, Beijing Metron Instruments Co. Ltd., China) with a constant flow rate of 1000 ml/min. The under-test sensors are fixed into a seamless chamber (10 cm×3 cm×0.5 cm), followed by a continuous dry air purification to ensure a minimum effect of ambient humidity on sensing performance. Electric resistance (denoted as R) is measured by Keithley 2700 multimeter/Data Acquisition System, and collected real-timely by PC. In the following part, ΔR/R0=(Rt−R0)/R0 is denoted as the sensing response, where R0 represents the steady-state initial resistance before exposure, and Rt represents the resistance at any time t during test. The adsorption and desorption time-interval are set for 5 min and 10 min respectively, excluding special instructions. 3. Results and discussion Fig. 4. Current-voltage relationships of RGO and RGO/Cu2O sensors for three tests.
3.1. Materials characterization To clarify the structural and componential features of prepared RGO, Cu2O and RGO/Cu2O composites, various characterization techniques such as XRD, Raman, XPS and TEM were adopted. As shown in Fig. 2(a), XRD feature peaks including (110), (111), (200), (220), (311) and (222) could be indexed as cuprous oxide of cuprite phase according to the JCPDS card (No. 78-2076) similar as previous
obtained on a Raman microscope spectrometer (Raman, Renishaw Invia Reflex) in the backscattering configuration using a 514.5 nm laser excitation with 10 mW at the samples. X-ray photoluminescence spectroscopy (XPS) measurements were performed on an Omicron Nanotech operated at 15 kV and 20 mA current using monochromatic
Fig. 5. Sensing performances of prepared sensors to NO2 gas. (a) resistance transients, (b) sensing response, (c) trace NO2 detection, (d) selectivity.
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Fig. 6. Investigation on long-term stability of prepared sensors to NO2 gas. (a) resistance transients, (b) response transients, (c) response variation during ten-day long period.
Pristine GO sheets with epoxy and hydroxyl groups on the basal plane and carboxyl groups at the edges exhibited negative surface charge in water. These defects and open edges on the surface facilitated GO nanosheets to combine with cations. After partial reduction of GO by glucose during the hydrothermal process, the resultant Cu2O were anchored onto RGO sheets, wherein a quick cooling process was employed to eliminate aggregation of Cu2O. Therefore, Cu2O quantum dots with particle diameter smaller than 6 nm were produced by this route (Fig. 3(c)). The crystalline nature of Cu2O was confirmed from the analysis of high resolution transmission electron microscopy (HRTEM) in Fig. 3(d) (obtained from circular zone in Fig. 3(c)), which revealed clear lattice fringes. The lattice spacing of 2.5 Å corresponded to the spacing of (111) lattice plane of Cu2O. In term of the selected area electron diffraction (SAED) pattern, Fig. 3(e) exhibited the character diffraction rings of Cu2O that could be indexed to the planes of (110), (111), (200), (220) and (311). Additionally, particle size frequency was displayed in Fig. 3(f), demonstrating 85% percentage of Cu2O QDs with particle diameter ranging from 3.2 nm to 4.8 nm.
report [14] with no appearance of CuO, indicating a successful preparation of cuprous oxide. RGO material showed a weak diffraction peak (002) at 2θ=22.8° suggesting 3.9 Å of lattice spacing, which was larger than 3.5 Å of intrinsic graphene due to multiple oxygen-containing functional groups. As for RGO/Cu2O composites, all Cu2O diffraction peaks were observed with weaker intensity compared to pure Cu2O, which was ascribed into RGO coating on Cu2O and/or smaller amount of Cu2O. Additionally, Raman spectra were displayed in Fig. 2(b). In the case of RGO and RGO/Cu2O composites, D peak (1348) is a defect peak due to inter-valley scattering, and G peak (1594) refers to the graphene G peak [15–17]. As for Cu2O and RGO/Cu2O composites, feature peaks at 146, 217, 418, 510 and 630 cm−1 appeared corresponding to the formation of Cu2O on the RGO nanosheets [18,19]. The chemical compositions of the RGO/Cu2O nanocomposites were obtained by XPS. As seen from Fig. 2(c), the survey spectrum verified the coexistence of Cu, O and C, which also confirmed the existence of Cu2O in the sample [20]. Cu 2p spectrum in Fig. 2(d) exhibited two peaks at 933.8 eV and 953.6 eV, denoting as Cu2p3/2 and Cu 2p1/2, respectively [21]. To further elucidate the structural properties of prepared materials, TEM analyses were performed. As shown in Fig. 3(a), rippled RGO nanosheets could be observed with no obvious impurities. After further magnification of Fig. 3(a), it was noteworthy that numerous Cu2O quantum dots (QDs) were uniform anchored onto RGO nanosheets (Fig. 3(b)). The synthesis process was discussed as the following steps.
3.2. NO2 sensing performance To distinguish the type of metal-semiconductor contact, current (I)voltage (V) characteristics of both RGO and RGO/Cu2O sensors were tested for three times (−5 V to +5 V, step: +0.1 v), wherein the contact 8376
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4. Conclusions
could be regarded as RGO-Au contact or RGO/Cu2O composites-Au contact, respectively. As shown in Fig. 4, the linear I-V relationship signified the ohmic contact for both sensors, and a negligible effect of contact resistance on overall resistance of prepared sensors. Meanwhile, the slope of I-V lines indicated a better conductivity of pure RGO than that of RGO/Cu2O counterpart. Since Cu2O was nonconductive in our investigated situations, four RGO/Cu2O sensors (denoted as S1, S2, S3 and S4, which are different in Cu2O content, wherein S4 represents RGO sensor with no Cu2O) were tested at room temperature on exposure to NO2 gas with concentration ranging from 2 ppm to 16 ppm, as shown in Fig. 5(b). Combining with resistance transients displayed in Fig. 5(a), we noted that all sensors underwent a resistance decrease after NO2 injection and an increase with air purification, which was ascribed into charge transfer between p-type semiconducting RGO/Cu2O or RGO thin film and electron-withdrawing NO2 molecules [3,22]. When Cu2O QDs or RGO are exposed to air, oxygen molecules adsorb on the surface of Cu2O or RGO materials forming adsorbed oxygen species (O2− is known to be dominant at < 150 °C on the surface of metal oxides [23,24], described in Eqs. (1) and (2)) through trapping electrons from Cu2O or RGO, thereby leading to the formation of hole-accumulation layers (HALs). O2(gas)⇔O2(absorbed)
(1)
O2(absorbed)+e-⇔O2-
(2)
In conclusion, compared with pure RGO sensors, our prepared RGO/Cu2O ones performed a remarkably enhanced sensing response as well as an excellent selectivity and stability to NO2 gas. Based on the investigated results, we believe that after a more elaborate structure modulation of RGO/Cu2O nanocomposite film, these prepared sensors will show a more competitive potential for trace NO2 detection in the future applications. Acknowledgements This work was partially supported by Fundamental Research Funds for the Central Universities (Grant Nos. 106112016CDJXY120006 and 0210005202063). References [1] WHO, WHO Air Quality Guidelines-global Update 2005, World Health Organization, Copenhagen, 2006. [2] L. Giancaterini, C. Cantalini, M. Cittadini, M. Sturaro, M. Guglielmi, A. Martucci, A. Resmini, U. Anselmi-Tamburini, Au and Pt nanoparticles effects on the optical and electrical gas sensing properties of sol-gel-based ZnO thin film sensors, IEEE Sens. J. 15 (2015) 1068–1076. [3] J. Zhang, J. Liu, Q. Peng, X. Wang, Y. Li, Nearly monodisperse Cu2O and CuO nanospheres: preparation and applications for sensitive gas sensors, Chem. Mater. 18 (2006) 867–871. [4] S. Deng, V. Tjoa, H.M. Fan, H.R. Tan, D.C. Sayle, M. Olivo, S. Mhaisalkar, J. Wei, C.H. Sow, Reduced graphene oxide conjugated Cu2O nanowire mesocrystals for highperformance NO2 gas sensor, J. Am. Chem. Soc. 134 (2012) 4905–4917. [5] H. Meng, W. Yang, K. Ding, L. Feng, Y. Guan, Cu2O nanorods modified by reduced graphene oxide for NH3 sensing at room temperature, J. Mater. Chem. A 3 (2015) 1174–1181. [6] L. Wang, R. Zhang, T. Zhou, Z. Lou, J. Deng, T. Zhang, Concave Cu2O octahedral nanoparticles as an advanced sensing material for benzene (C6H6) and nitrogen dioxide (NO2) detection, Sens. Actuators B 223 (2016) 311–317. [7] D. Yan, S. Li, M. Hu, S. Liu, Y. Zhu, M. Cao, Electrochemical synthesis and the gassensing properties of the Cu2O nanofilms/porous silicon hybrid structure, Sens. Actuators B 223 (2016) 626–633. [8] F. Loffredo, A. De. Girolamo Del Mauro, G. Burrasca, V. La Ferrara, L. Quercia, E. Massera, G. Di Francia, D.D. Sala, Ink-jet printing technique in polymer/carbon black sensing device fabrication, Sens. Actuators B 143 (2009) 421–429. [9] Y. Zhou, Y. Jiang, G. Xie, X. Du, H. Tai, Gas sensors based on multiple-walled carbon nanotubes-polyethylene oxide films for toluene vapor detection, Sens. Actuators B 191 (2014) 24–30. [10] Y. Su, G. Xie, J. Chen, H. Du, H. Zhang, Z. Yuan, Z. Ye, X. Du, H. Tai, Y. Jiang, Reduced graphene oxide-polyethylene oxide hybrid films for toluene sensing at room temperature, RSC Adv. 6 (2016) 97840–97847. [11] Y. Zhou, X. Lin, Y. Wang, G. Liu, X. Zhu, Y. Huang, Y. Guo, C. Gao, M. Zhou, Study on gas sensing of reduced graphene oxide/ZnO thin film at room temperature, Sens. Actuators B 240 (2017) 870–880. [12] E. Llobet, Gas sensors using carbon nanomaterials: a review, Sens. Actuators B 179 (2013) 32–45. [13] F. Schedin, A.K. Geim, S.V. Morozov, E.W. Hill, P. Blake, M.I. Katsnelson, K.S. Novoselov, Detection of individual gas molecules adsorbed on graphene, Nat. Mater. (2007) 652–655. [14] X. Yan, X. Tong, Y. Zhang, X. Han, Y. Wang, G. Jin, Y. Qin, X. Guo, Cuprous oxide nanoparticles dispersed on reduced graphene oxide as an efficient electrocatalyst for oxygen reduction reaction, Chem. Commun. 48 (2012) 1892–1894. [15] A.C. Ferrari, Raman spectroscopy of graphene and graphite: disorder, electron-phonon coupling, doping and nonadiabatic effects, Solid State Commun. 143 (2007) 47–57. [16] Y.X. Xu, K.X. Sheng, C. Li, G.Q. Shi, Highly conductive chemically converted graphene prepared from mildly oxidized graphene oxide, J. Mater. Chem. 21 (2011) 7376–7380. [17] H.L. Wang, J.T. Robinson, X.L. Li, H.J. Dai, Solvothermal reduction of chemically exfoliated graphene sheets, J. Am. Chem. Soc. 131 (2009) 9910–9911. [18] N.A.M. Shanid, M.A. Khadar, V.G. Sathe, Frohlich interaction and associated resonance enhancement in nanostructured copper oxide films, J. Raman Spectrosc. 42 (2011) 1769–1773. [19] A. Chakravarty, K. Bhowmik, A. Mukherjee, G. De, Cu2O nanoparticles anchored on amine-functionalized graphite nanosheet: a potential reusable catalyst, Langmuir 31 (2015) 5210–5219. [20] B.X. Li, T.X. Liu, L.Y. Hu, Y.F. Wang, A facile one-pot synthesis of Cu2O/RGO nanocomposite for removal of organic pollutant, J. Phys. Chem. Solids 74 (2013) 635–640. [21] S. Sun, X. Zhang, X. Song, S. Liang, L. Wang, Z. Yang, Bottom-up assembly of hierarchical Cu2O nanospheres: controllable synthesis, formation mechanism and enhanced photochemical activities, CrystEngComm 14 (2012) 3545–3553. [22] G. Lu, L. Ocola, J. Chen, Gas detection using low-temperature reduced graphene oxide sheets, Appl. Phys. Lett. (2009) 083111. [23] M. Takata, D. Tsubone, H. Yanagida, Dependence of electrical conductivity of ZnO on degree of sintering, J. Am. Ceram. Soc. 59 (1976) 4–8. [24] N. Barsan, U. Weimer, Conduction model of metal oxide gas sensors, J. Electroceram. 7 (2001) 143–167.
Considering that the adsorption of NO2 molecules was proceeded in the form of NO2-(ads) displayed in Eqs. (3) and (4), HALs would be extended accompanied with a decreased resistance due to increased density of majority carriers (holes). RGO+NO2 (gas)=RGO+NO2−+h+
(3)
Cu2O+NO2 (gas)=Cu2O+NO2−+h+
(4)
As for RGO/Cu2O composites, nonconductive property of Cu2O QDs meant that RGO predominantly contributed to overall conductivity of the composites by transferring all charge carriers through RGO conducting paths. As also seen from Fig. 5(a), the initial resistance increased from 1.5 to 100 kΩ after Cu2O introduction, which evidenced the poor conductivity of Cu2O QDs as well. Besides, inset in Fig. 5(b) indicated that the sensing response of RGO/Cu2O sensors was at least 580% larger than that of RGO ones. Moreover, this enhancement would become larger with increasing NO2 concentration. We then exposed RGO/Cu2O sensors to trace NO2 gas with concentration ranging from 20 ppb to 300 ppb at 60 °C (Fig. 5(c)). The sensing response attained 4.7% and 29.3% toward 20 ppb and 100 ppb NO2, which was larger than most Cu2O based resistive gas sensors. Further response comparison (Fig. 5(d)) exhibited an excellent selectivity of RGO/Cu2O sensors for NO2 gas against a range of interfering gas species such as NH3, H2O, HCHO and H2S, despite much smaller concentration of NO2 gas than the others. To probe the long-term stability, another three RGO/Cu2O sensors were prepared and then exposed to 100 ppb NO2 gas at room temperature every two days to record the variation tendency of sensing response. In this investigation, each test period was designed with 16 min exposure and 23 min purification intervals to ensure a sufficient gas adsorption and detachment. As shown in Fig. 6(a) and (b), an analogous resistance and response transient for all sensors was displayed as mentioned earlier in Fig. 5. After five exposures within ten days, Sensors 1, 2 and 3 showed a 94.5%, 88.5% and 98.1% response retention respectively, meaning a small susceptibility to environmental factors such as humidity and dusts, and thus a favorable stability of RGO/Cu2O sensors.
8377
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Cu2O quantum dots modified by RGO nanosheets for ultrasensitive and selective NO2 gas detection
MARK
⁎
Yong Zhoua, , Guoqing Liua,b, Xiangyi Zhua, Yongcai Guoa a Key Laboratory of Optoelectronic Technology and System of Ministry of Education, College of Optoelectronic Engineering, Chongqing University, Chongqing 400044, PR China b Chongqing Research Institute CO., Ltd. Of China Coal Technology & Engineering Group, Chongqing 400037, PR China
A R T I C L E I N F O
A BS T RAC T
Keywords: Gas sensor Thin film Cuprous oxide quantum dots Reduced graphene oxide Nitrogen dioxide
Real-time monitoring of trace NO2 emission has been an emerging challenge in environment and health sectors lately. Aiming to overcome this challenge, NO2 gas sensors based on cuprous oxide quantum dots (Cu2O QDs) anchored onto reduced graphene oxide (RGO) nanosheets serving as a sensitive layer were prepared in this report. Apart from a series of purposive measurements, various characterization techniques such as XRD, Raman, XPS and TEM were employed as well to assist the exploration of sensors performance to NO2 gas. The experimental results revealed a 580% response enhancement for prepared RGO/Cu2O sensors compared with pure RGO counterparts, as well as an excellent selectivity. In a specific experiment, the sensing response attained 4.8% and 29.3% toward 20 ppb and 100 ppb NO2 respectively at 60 °C, which was larger than most Cu2O based resistive gas sensors. Moreover, further subtle modulation of this RGO/Cu2O nanocomposites led to a preferable room-temperature response of 37.8% toward 100 ppb NO2, which also offered a favorable stability of 98.1% response retention after four exposures within ten days. The obtained results imply that the prepared RGO/Cu2O QDs sensors possess a competitive capability of trace NO2 detection.
1. Introduction Nitrogen dioxide (NO2), mainly coming from fossil fuels and motor vehicles, is a hazardous gas that poses great threat to environmental protection and human health. According to World Health Organization, the air quality guideline for NO2 gas is appropriately 410 ppb in an hour period [1]. However, there still exist significant challenges for selective detection of trace NO2 [2], provoking a pressing need to develop highperformance NO2 gas sensors. As the most popular gas-sensing materials, metal oxides have been worldwide applied in the field of chemical sensors. For instance, cuprous oxide (Cu2O) as a typical p-type semiconductor has attracted extensive research in terms of diverse nanostructures including nanospheres [3], nanowires [4], nanorods [5], nanoparticles [6] and nanofilms [7]. However, pure Cu2O always suffers from high operation temperature over 150 °C, thereby bringing about high power-consumption and security issues to hinder its practical applications. To overcome this obstacle, dimensional miniaturization of Cu2O material is a universal approach to realize low-temperature detection. Apart from this approach, elaborate doping with conducting nanofillers can yet be regarded as another favorable means. On the basis of this means,
⁎
carbon nanomaterials including carbon black, carbon nanotubes and graphene have been the subjects of intense research for gas sensing over the past few decades [8–11]. With the inherent superiorities of large specific surface area and high electric conductivity, carbon nanomaterials have exhibited an excellent detection sensitivity especially with interesting transduction properties in a single layer of material [12]. Take two-dimensional (2D) graphene and its derivatives for example. Since the pioneering detection of individual gas molecules [13], graphene materials have been broadly studied from scientific communities of physics, chemistry, materials science and other disciplines. Among these, reduced graphene oxide (RGO) prevails over its counterparts such as pristine graphene and graphene oxide (GO) in chemiresistive gas sensors due to more adsorption sites and preferable conductance, respectively. To facilitate a better sensing performance at low temperature, in this report we leverage the complementary advantages of low-dimensional materials and conductive nanofillers by preparing NO2 sensors featuring a nanocomposite of Cu2O quantum dots (QDs) anchored on reduced graphene oxide (RGO) nanosheets as the sensitive layer. The excellent sensing results make RGO/Cu2O QDs nanocomposites an exciting alternative for trace NO2 detection in the future.
Corresponding author. E-mail address: [email protected] (Y. Zhou).
http://dx.doi.org/10.1016/j.ceramint.2017.03.179 Received 5 January 2017; Received in revised form 23 March 2017; Accepted 28 March 2017 Available online 30 March 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Ceramics International 43 (2017) 8372–8377
Y. Zhou et al.
Fig. 1. Schematic illustrations of (a) fabrication route and (c) IDEs devices, (b) prepared IDEs.
Fig. 2. Characterization features of prepared samples including (a)XRD spectra, (b) Raman analysis, (c) XPS survey spectrum and (d) Cu 2p spectrum.
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Fig. 3. TEM analysis of (a) RGO/Cu2O, (b) further magnification of (a), (c) further enlargement of (b), (d) HRTEM picture of circular zone in (c), (e) SAED pattern of Cu2O and (f) particle distribution.
through the same process without the treatment steps of Cu(Ac)2 or GO aqueous solutions respectively, and dispersed in water/ethanol solution with a volume fraction of 1:4. Regarding film deposition, RGO/Cu2O or RGO solution mentioned earlier was airbrushed onto clean interdigital electrodes (IDEs), which were subsequently placed into a 70 °C vacuum oven overnight for sufficient evaporation of solvent and moisture. The detailed fabrication route of planar IDEs is shown in Fig. 1(a). Lithography and lift-off technologies are employed to pattern Ti and Au layers (100 nm/ 200 nm) onto SiO2/Si substrate. As seen from Fig. 1(b) and (c), the prepared IDEs devices are highly reproduced with the finger and interspacing widths of 50 µm and an active area of 1.1 cm×0.7 cm.
2. Experimental 2.1. Sensor preparation In a typical experiment, a uniform GO solution (0.1 mg/ml, 25 ml, purchased from Sigma-Aldrich) was treated under a mild sonication for an hour, later mixing it with 20 ml 15 mM copper acetate (Cu(Ac)2) aqueous solution. The obtained brown suspension was then stirred magnetically at 25 °C for 12 h, with a subsequent introduction of 20 ml 18 mM sodium hydroxide (NaOH) aqueous solution dropwise. After 10 h stewing without any treatment, the created greenish brown precipitate was then transferred into a 50 ml autoclave containing 30 ml 6 mM glucose aqueous solution for a hydrothermal reaction at 96 °C for 10 h, followed by a swift cooling process with the aid of icewater bath. The resultant products were eventually obtained by centrifugation and washed with water and ethanol several times, and then re-dispersed in ethanol. Pure RGO or Cu2O material was prepared
2.2. Test requirements In this paper, X-ray diffraction spectrum (XRD) was measured by PANalytical X′pert HighScore XRD instrument. Raman spectrum was 8374
Ceramics International 43 (2017) 8372–8377
Y. Zhou et al.
Al Kα X-ray source. Transmission electron microscopy (TEM) images were recorded on a JEOL-2010 TEM operated at 200 kV. During the dynamic tests, dry air is utilized as the background gas. Gas concentration is adjusted by a mass flow controller (MT50–4J, Beijing Metron Instruments Co. Ltd., China) with a constant flow rate of 1000 ml/min. The under-test sensors are fixed into a seamless chamber (10 cm×3 cm×0.5 cm), followed by a continuous dry air purification to ensure a minimum effect of ambient humidity on sensing performance. Electric resistance (denoted as R) is measured by Keithley 2700 multimeter/Data Acquisition System, and collected real-timely by PC. In the following part, ΔR/R0=(Rt−R0)/R0 is denoted as the sensing response, where R0 represents the steady-state initial resistance before exposure, and Rt represents the resistance at any time t during test. The adsorption and desorption time-interval are set for 5 min and 10 min respectively, excluding special instructions. 3. Results and discussion Fig. 4. Current-voltage relationships of RGO and RGO/Cu2O sensors for three tests.
3.1. Materials characterization To clarify the structural and componential features of prepared RGO, Cu2O and RGO/Cu2O composites, various characterization techniques such as XRD, Raman, XPS and TEM were adopted. As shown in Fig. 2(a), XRD feature peaks including (110), (111), (200), (220), (311) and (222) could be indexed as cuprous oxide of cuprite phase according to the JCPDS card (No. 78-2076) similar as previous
obtained on a Raman microscope spectrometer (Raman, Renishaw Invia Reflex) in the backscattering configuration using a 514.5 nm laser excitation with 10 mW at the samples. X-ray photoluminescence spectroscopy (XPS) measurements were performed on an Omicron Nanotech operated at 15 kV and 20 mA current using monochromatic
Fig. 5. Sensing performances of prepared sensors to NO2 gas. (a) resistance transients, (b) sensing response, (c) trace NO2 detection, (d) selectivity.
8375
Ceramics International 43 (2017) 8372–8377
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Fig. 6. Investigation on long-term stability of prepared sensors to NO2 gas. (a) resistance transients, (b) response transients, (c) response variation during ten-day long period.
Pristine GO sheets with epoxy and hydroxyl groups on the basal plane and carboxyl groups at the edges exhibited negative surface charge in water. These defects and open edges on the surface facilitated GO nanosheets to combine with cations. After partial reduction of GO by glucose during the hydrothermal process, the resultant Cu2O were anchored onto RGO sheets, wherein a quick cooling process was employed to eliminate aggregation of Cu2O. Therefore, Cu2O quantum dots with particle diameter smaller than 6 nm were produced by this route (Fig. 3(c)). The crystalline nature of Cu2O was confirmed from the analysis of high resolution transmission electron microscopy (HRTEM) in Fig. 3(d) (obtained from circular zone in Fig. 3(c)), which revealed clear lattice fringes. The lattice spacing of 2.5 Å corresponded to the spacing of (111) lattice plane of Cu2O. In term of the selected area electron diffraction (SAED) pattern, Fig. 3(e) exhibited the character diffraction rings of Cu2O that could be indexed to the planes of (110), (111), (200), (220) and (311). Additionally, particle size frequency was displayed in Fig. 3(f), demonstrating 85% percentage of Cu2O QDs with particle diameter ranging from 3.2 nm to 4.8 nm.
report [14] with no appearance of CuO, indicating a successful preparation of cuprous oxide. RGO material showed a weak diffraction peak (002) at 2θ=22.8° suggesting 3.9 Å of lattice spacing, which was larger than 3.5 Å of intrinsic graphene due to multiple oxygen-containing functional groups. As for RGO/Cu2O composites, all Cu2O diffraction peaks were observed with weaker intensity compared to pure Cu2O, which was ascribed into RGO coating on Cu2O and/or smaller amount of Cu2O. Additionally, Raman spectra were displayed in Fig. 2(b). In the case of RGO and RGO/Cu2O composites, D peak (1348) is a defect peak due to inter-valley scattering, and G peak (1594) refers to the graphene G peak [15–17]. As for Cu2O and RGO/Cu2O composites, feature peaks at 146, 217, 418, 510 and 630 cm−1 appeared corresponding to the formation of Cu2O on the RGO nanosheets [18,19]. The chemical compositions of the RGO/Cu2O nanocomposites were obtained by XPS. As seen from Fig. 2(c), the survey spectrum verified the coexistence of Cu, O and C, which also confirmed the existence of Cu2O in the sample [20]. Cu 2p spectrum in Fig. 2(d) exhibited two peaks at 933.8 eV and 953.6 eV, denoting as Cu2p3/2 and Cu 2p1/2, respectively [21]. To further elucidate the structural properties of prepared materials, TEM analyses were performed. As shown in Fig. 3(a), rippled RGO nanosheets could be observed with no obvious impurities. After further magnification of Fig. 3(a), it was noteworthy that numerous Cu2O quantum dots (QDs) were uniform anchored onto RGO nanosheets (Fig. 3(b)). The synthesis process was discussed as the following steps.
3.2. NO2 sensing performance To distinguish the type of metal-semiconductor contact, current (I)voltage (V) characteristics of both RGO and RGO/Cu2O sensors were tested for three times (−5 V to +5 V, step: +0.1 v), wherein the contact 8376
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4. Conclusions
could be regarded as RGO-Au contact or RGO/Cu2O composites-Au contact, respectively. As shown in Fig. 4, the linear I-V relationship signified the ohmic contact for both sensors, and a negligible effect of contact resistance on overall resistance of prepared sensors. Meanwhile, the slope of I-V lines indicated a better conductivity of pure RGO than that of RGO/Cu2O counterpart. Since Cu2O was nonconductive in our investigated situations, four RGO/Cu2O sensors (denoted as S1, S2, S3 and S4, which are different in Cu2O content, wherein S4 represents RGO sensor with no Cu2O) were tested at room temperature on exposure to NO2 gas with concentration ranging from 2 ppm to 16 ppm, as shown in Fig. 5(b). Combining with resistance transients displayed in Fig. 5(a), we noted that all sensors underwent a resistance decrease after NO2 injection and an increase with air purification, which was ascribed into charge transfer between p-type semiconducting RGO/Cu2O or RGO thin film and electron-withdrawing NO2 molecules [3,22]. When Cu2O QDs or RGO are exposed to air, oxygen molecules adsorb on the surface of Cu2O or RGO materials forming adsorbed oxygen species (O2− is known to be dominant at < 150 °C on the surface of metal oxides [23,24], described in Eqs. (1) and (2)) through trapping electrons from Cu2O or RGO, thereby leading to the formation of hole-accumulation layers (HALs). O2(gas)⇔O2(absorbed)
(1)
O2(absorbed)+e-⇔O2-
(2)
In conclusion, compared with pure RGO sensors, our prepared RGO/Cu2O ones performed a remarkably enhanced sensing response as well as an excellent selectivity and stability to NO2 gas. Based on the investigated results, we believe that after a more elaborate structure modulation of RGO/Cu2O nanocomposite film, these prepared sensors will show a more competitive potential for trace NO2 detection in the future applications. Acknowledgements This work was partially supported by Fundamental Research Funds for the Central Universities (Grant Nos. 106112016CDJXY120006 and 0210005202063). References [1] WHO, WHO Air Quality Guidelines-global Update 2005, World Health Organization, Copenhagen, 2006. [2] L. Giancaterini, C. Cantalini, M. Cittadini, M. Sturaro, M. Guglielmi, A. Martucci, A. Resmini, U. Anselmi-Tamburini, Au and Pt nanoparticles effects on the optical and electrical gas sensing properties of sol-gel-based ZnO thin film sensors, IEEE Sens. J. 15 (2015) 1068–1076. [3] J. Zhang, J. Liu, Q. Peng, X. Wang, Y. Li, Nearly monodisperse Cu2O and CuO nanospheres: preparation and applications for sensitive gas sensors, Chem. Mater. 18 (2006) 867–871. [4] S. Deng, V. Tjoa, H.M. Fan, H.R. Tan, D.C. Sayle, M. Olivo, S. Mhaisalkar, J. Wei, C.H. Sow, Reduced graphene oxide conjugated Cu2O nanowire mesocrystals for highperformance NO2 gas sensor, J. Am. Chem. Soc. 134 (2012) 4905–4917. [5] H. Meng, W. Yang, K. Ding, L. Feng, Y. Guan, Cu2O nanorods modified by reduced graphene oxide for NH3 sensing at room temperature, J. Mater. Chem. A 3 (2015) 1174–1181. [6] L. Wang, R. Zhang, T. Zhou, Z. Lou, J. Deng, T. Zhang, Concave Cu2O octahedral nanoparticles as an advanced sensing material for benzene (C6H6) and nitrogen dioxide (NO2) detection, Sens. Actuators B 223 (2016) 311–317. [7] D. Yan, S. Li, M. Hu, S. Liu, Y. Zhu, M. Cao, Electrochemical synthesis and the gassensing properties of the Cu2O nanofilms/porous silicon hybrid structure, Sens. Actuators B 223 (2016) 626–633. [8] F. Loffredo, A. De. Girolamo Del Mauro, G. Burrasca, V. La Ferrara, L. Quercia, E. Massera, G. Di Francia, D.D. Sala, Ink-jet printing technique in polymer/carbon black sensing device fabrication, Sens. Actuators B 143 (2009) 421–429. [9] Y. Zhou, Y. Jiang, G. Xie, X. Du, H. Tai, Gas sensors based on multiple-walled carbon nanotubes-polyethylene oxide films for toluene vapor detection, Sens. Actuators B 191 (2014) 24–30. [10] Y. Su, G. Xie, J. Chen, H. Du, H. Zhang, Z. Yuan, Z. Ye, X. Du, H. Tai, Y. Jiang, Reduced graphene oxide-polyethylene oxide hybrid films for toluene sensing at room temperature, RSC Adv. 6 (2016) 97840–97847. [11] Y. Zhou, X. Lin, Y. Wang, G. Liu, X. Zhu, Y. Huang, Y. Guo, C. Gao, M. Zhou, Study on gas sensing of reduced graphene oxide/ZnO thin film at room temperature, Sens. Actuators B 240 (2017) 870–880. [12] E. Llobet, Gas sensors using carbon nanomaterials: a review, Sens. Actuators B 179 (2013) 32–45. [13] F. Schedin, A.K. Geim, S.V. Morozov, E.W. Hill, P. Blake, M.I. Katsnelson, K.S. Novoselov, Detection of individual gas molecules adsorbed on graphene, Nat. Mater. (2007) 652–655. [14] X. Yan, X. Tong, Y. Zhang, X. Han, Y. Wang, G. Jin, Y. Qin, X. Guo, Cuprous oxide nanoparticles dispersed on reduced graphene oxide as an efficient electrocatalyst for oxygen reduction reaction, Chem. Commun. 48 (2012) 1892–1894. [15] A.C. Ferrari, Raman spectroscopy of graphene and graphite: disorder, electron-phonon coupling, doping and nonadiabatic effects, Solid State Commun. 143 (2007) 47–57. [16] Y.X. Xu, K.X. Sheng, C. Li, G.Q. Shi, Highly conductive chemically converted graphene prepared from mildly oxidized graphene oxide, J. Mater. Chem. 21 (2011) 7376–7380. [17] H.L. Wang, J.T. Robinson, X.L. Li, H.J. Dai, Solvothermal reduction of chemically exfoliated graphene sheets, J. Am. Chem. Soc. 131 (2009) 9910–9911. [18] N.A.M. Shanid, M.A. Khadar, V.G. Sathe, Frohlich interaction and associated resonance enhancement in nanostructured copper oxide films, J. Raman Spectrosc. 42 (2011) 1769–1773. [19] A. Chakravarty, K. Bhowmik, A. Mukherjee, G. De, Cu2O nanoparticles anchored on amine-functionalized graphite nanosheet: a potential reusable catalyst, Langmuir 31 (2015) 5210–5219. [20] B.X. Li, T.X. Liu, L.Y. Hu, Y.F. Wang, A facile one-pot synthesis of Cu2O/RGO nanocomposite for removal of organic pollutant, J. Phys. Chem. Solids 74 (2013) 635–640. [21] S. Sun, X. Zhang, X. Song, S. Liang, L. Wang, Z. Yang, Bottom-up assembly of hierarchical Cu2O nanospheres: controllable synthesis, formation mechanism and enhanced photochemical activities, CrystEngComm 14 (2012) 3545–3553. [22] G. Lu, L. Ocola, J. Chen, Gas detection using low-temperature reduced graphene oxide sheets, Appl. Phys. Lett. (2009) 083111. [23] M. Takata, D. Tsubone, H. Yanagida, Dependence of electrical conductivity of ZnO on degree of sintering, J. Am. Ceram. 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Considering that the adsorption of NO2 molecules was proceeded in the form of NO2-(ads) displayed in Eqs. (3) and (4), HALs would be extended accompanied with a decreased resistance due to increased density of majority carriers (holes). RGO+NO2 (gas)=RGO+NO2−+h+
(3)
Cu2O+NO2 (gas)=Cu2O+NO2−+h+
(4)
As for RGO/Cu2O composites, nonconductive property of Cu2O QDs meant that RGO predominantly contributed to overall conductivity of the composites by transferring all charge carriers through RGO conducting paths. As also seen from Fig. 5(a), the initial resistance increased from 1.5 to 100 kΩ after Cu2O introduction, which evidenced the poor conductivity of Cu2O QDs as well. Besides, inset in Fig. 5(b) indicated that the sensing response of RGO/Cu2O sensors was at least 580% larger than that of RGO ones. Moreover, this enhancement would become larger with increasing NO2 concentration. We then exposed RGO/Cu2O sensors to trace NO2 gas with concentration ranging from 20 ppb to 300 ppb at 60 °C (Fig. 5(c)). The sensing response attained 4.7% and 29.3% toward 20 ppb and 100 ppb NO2, which was larger than most Cu2O based resistive gas sensors. Further response comparison (Fig. 5(d)) exhibited an excellent selectivity of RGO/Cu2O sensors for NO2 gas against a range of interfering gas species such as NH3, H2O, HCHO and H2S, despite much smaller concentration of NO2 gas than the others. To probe the long-term stability, another three RGO/Cu2O sensors were prepared and then exposed to 100 ppb NO2 gas at room temperature every two days to record the variation tendency of sensing response. In this investigation, each test period was designed with 16 min exposure and 23 min purification intervals to ensure a sufficient gas adsorption and detachment. As shown in Fig. 6(a) and (b), an analogous resistance and response transient for all sensors was displayed as mentioned earlier in Fig. 5. After five exposures within ten days, Sensors 1, 2 and 3 showed a 94.5%, 88.5% and 98.1% response retention respectively, meaning a small susceptibility to environmental factors such as humidity and dusts, and thus a favorable stability of RGO/Cu2O sensors.
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