Graphene-Oxide Nano Composites for Chemical Sensor Applications
Abstract
:1. Introduction
2. Synthesis and Characterization of Graphene Based Nanocomposites
3. Mechanism of Gas Sensing and Device Fabrication
4. Sensor Response of the Metal Oxide-Graphene Nanocomposites
5. Conclusions and Future Outlook
Conflicts of Interest
References
- Basu, S.; Bhattacharyya, P. Recent developments on graphene and graphene oxide based solid state gas sensors. Sens. Actuators B 2012, 173, 1–21. [Google Scholar] [CrossRef]
- Anasori, B.; Beidaghi, M.; Gogotsi, Y. Graphene—Transition metal oxide hybrid materials. Mater. Today 2014, 17, 253–254. [Google Scholar]
- Sun, M.; Liu, H.; Liu, Y.; Qu, J.; Li, J. Graphene-based transition metal oxide nanocomposites for the oxygen reduction reaction. Nanoscale 2015, 7, 1250–1269. [Google Scholar] [CrossRef] [PubMed]
- Zhang, W.; Liu, F.; Li, Q.; Shou, Q.; Cheng, J.; Zhang, L.; Nelson, B.J.; Zhang, X. Transition metal oxide and graphene nanocomposites for high-performance electrochemical capacitors. Phys. Chem. Chem. Phys. 2012, 14, 16331–16337. [Google Scholar] [CrossRef] [PubMed]
- Khan, M.; Tahir, M.N.; Adil, S.F.; Khan, H.U.; Siddiqui, M.R.H.; Al-warthan, A.A.; Tremel, W. Graphene based metal and metal oxide nanocomposites: Synthesis, properties and their applications. J. Mater. Chem. A 2015, 3, 18753–18808. [Google Scholar] [CrossRef]
- Hu, C.; Lu, T.; Chen, F.; Zhang, R. A brief review of graphene–metal oxide composites synthesis and applications in photocatalysis. J. Chin. Adv. Mater. Soc. 2013, 1, 21–39. [Google Scholar] [CrossRef]
- Joshi, M.K.; Pant, H.R.; Kim, H.J.; Kim, J.H.; Kim, C.S. One-pot synthesis of Ag-iron oxide/reduced graphene oxide nanocomposite via hydrothermal treatment. Colloids Surf. A Physicochem. Eng Asp. 2014, 446, 102–108. [Google Scholar] [CrossRef]
- Stankovich, S.; Dikin, D.A.; Dommett, G.H.B.; Kohlhaas, K.M.; Zimney, E.J.; Stach, E.A.; Piner, R.D.; Nguyen, S.B.T.; Ruoff, R.S. Graphene-based composite materials. Nature 2006, 442, 282–286. [Google Scholar] [CrossRef] [PubMed]
- Huang, X.; Qi, X.; Boey, F.; Zhang, H. Graphene-based composites. Chem. Soc. Rev. 2012, 41, 666–686. [Google Scholar] [CrossRef] [PubMed]
- Zhang, K.; Zhang, L.L.; Zhao, X.S.; Wu, J.S. Graphene/polyaniline nanofiber composites as supercapacitor electrodes. Chem. Mater. 2010, 22, 1392–1401. [Google Scholar] [CrossRef]
- Paek, S.-M.; Yoo, E.; Honma, I. Enhanced cyclic performance and lithium storage capacity of SnO2/graphene nanoporous electrodes with three-dimensionally delaminated flexible structure. Nano Lett. 2009, 9, 72–75. [Google Scholar] [CrossRef] [PubMed]
- Williams, G.; Seger, B.; Kamat, P.V. TiO2-graphene nanocomposites, UV-assisted photocatalytic reduction of graphene oxide. ACS Nano 2008, 2, 4348–4350. [Google Scholar] [CrossRef] [PubMed]
- Novoselov, K.S.; Geim, A.K.; Morozov, S.V.; Jiang, D.; Zhang, Y.; Dubonos, S.V.; Grigorieva, I.V.; Firsov, A.A. Electric field in atomically thin carbon films. Science 2004, 306, 666–669. [Google Scholar] [CrossRef] [PubMed]
- Park, S.; Ruoff, R.S. Chemical methods for the production of graphenes. Nat. Nanotechnol. 2009, 4, 217–224. [Google Scholar] [CrossRef] [PubMed]
- Reina, A.; Jia, X.; Ho, J.; Nezich, D.; Son, H.; Bulovic, V.; Dresselhaus, M.S.; Kong, J. Large area, few-layer graphene films on arbitrary substrates by chemical vapor deposition. Nano Lett. 2009, 9, 30–35. [Google Scholar] [CrossRef] [PubMed]
- Campos-Delgado, J.; Romo-Herrera, J.M.; Jia, X.; Cullen, D.A.; Muramatsu, H.; Kim, Y.A.; Hayashi, T.; Ren, Z.; Smith, D.J.; Okuno, Y.; et al. Bulk production of a new form of sp2 carbon: Crystalline graphene nanoribbons. Nano Lett. 2008, 8, 2773–2778. [Google Scholar] [CrossRef] [PubMed]
- Tung, V.C.; Allen, M.J.; Yang, Y.; Kaner, R.B. High-throughput solution processing of large-scale graphene. Nat. Nanotechnol. 2009, 4, 25–29. [Google Scholar] [CrossRef] [PubMed]
- Li, N.; Wang, Z.; Zhao, K.; Shi, Z.; Gu, Z.; Xu, S. Large scale synthesis of N-doped multi-layered graphene sheets by simple arc-discharge method. Carbon 2010, 48, 255–259. [Google Scholar] [CrossRef]
- Elías, A.L.; Botello-Méndez, A.R.; Meneses-Rodríguez, D.; González, V.J.; Ramírez-González, D.; Ci, L.; Muñoz-Sandoval, E.; Ajayan, P.M.; Terrones, H.; Terrones, M. Longitudinal cutting of pure and doped carbon nanotubes to form graphitic nanoribbons using metal clusters as nanoscalpels. Nano Lett. 2010, 10, 366–372. [Google Scholar] [CrossRef] [PubMed]
- Choucair, M.; Thordarson, P.; Stride, J.A. Gram-scale production of graphene based on solvothermal synthesis and sonication. Nat. Nanotechnol. 2009, 4, 30–33. [Google Scholar] [CrossRef] [PubMed]
- Rollings, E.; Gweon, G.-H.; Zhou, S.Y.; Mun, B.S.; McChesney, J.L.; Hussain, B.S.; Fedorov, A.V.; First, P.N.; de Heer, W.A.; Lanzara, A. Synthesis and characterization of atomically thin graphite films on a silicon carbide substrate. J. Phys. Chem. Solids 2006, 67, 2172–2177. [Google Scholar] [CrossRef]
- Li, S.; Deng, D.; Shi, Q.; Liu, S. Electrochemical synthesis of a graphene sheet and gold nanoparticle-based nanocomposite, and its application to amperometric sensing of dopamine. Microchim. Acta 2012, 177, 325–331. [Google Scholar] [CrossRef]
- Emtsev, K.V.; Bostwick, A.; Horn, K.; Jobst, J.; Kellogg, G.L.; Ley, L.; McChesney, J.L.; Ohta, T.; Reshanov, S.A.; Röhrl, J.; et al. Towards wafer-size graphene layers by atmospheric pressure graphitization of silicon carbide. Nat. Mater. 2009, 8, 203–207. [Google Scholar] [CrossRef] [PubMed]
- Sun, J.; Lindvall, N.; Cole, T.M.; Angel, K.T.T.; Teng, W.; Teo, K.B.K.; Chua, D.H.C.; Johan, L.; Yurgens, A. Low partial pressure chemical vapor deposition of graphene on copper. IEEE Trans. Nanotechnol. 2012, 11, 255–260. [Google Scholar] [CrossRef]
- Batzill, M. The surface science of graphene: Metal interfaces, CVD synthesis, nanoribbons, chemical modifications, and defects. Surf. Sci. Rep. 2012, 67, 83–115. [Google Scholar] [CrossRef]
- Hummers, S.; Offeman, R.E. Preparation of graphitic oxide. J. Am. Chem. Soc. 1958, 80, 1339. [Google Scholar] [CrossRef]
- Lian, P.; Wang, J.; Cai, D.; Ding, L.; Jia, Q.; Wang, H. Porous SnO2@C/graphene nanocomposite with 3D carbon conductive network as a superior anode material for lithium-ion batteries. Electrochim. Acta 2014, 116, 103–110. [Google Scholar] [CrossRef]
- Liu, Y.-Z.; Li, Y.-F.; Yang, Y.-G.; Wen, Y.-F.; Wang, M.-Z. A one-pot method for producing ZnO-graphene nanocomposites from graphene oxide for supercapacitors. Scr. Mater. 2013, 68, 301–304. [Google Scholar] [CrossRef]
- Dong, L.; Li, M.; Dong, L.; Zhao, M.; Feng, J.; Han, Y.; Deng, J.; Li, X.; Li, D.; Sun, X. Hydrothermal synthesis of mixed crystal phases TiO2-reduced graphene oxide nanocomposites with small particle size for lithium ion batteries. Int. J. Hydrog. Energ. 2014, 39, 16116–16122. [Google Scholar] [CrossRef]
- Yang, X.; Ding, H.; Zhang, D.; Yan, X.; Lu, C.; Qin, J.; Zhang, R.; Tang, H.; Song, H. Hydrothermal synthesis of MoO3 nanobelt-graphene composites. Cryst. Res. Technol. 2011, 46, 1195–1201. [Google Scholar] [CrossRef]
- Rabieh, S.; Nassimi, K.; Bagheri, M. Synthesis of hierarchical ZnO-reduced graphene oxide nanocomposites with enhanced adsorption-photocatalytic performance. Mater. Lett. 2016, 162, 28–31. [Google Scholar] [CrossRef]
- Dai, J.; Song, M.; Wang, M.; Li, P.; Zhang, C.; Shen, Y.; Xie, A. Freeze-drying growth of Co3O4/N-doped reduced graphene oxide nanocomposite as excellent anode material for lithium-ion batteries. Ceram. Int. 2016, 42, 2410–2415. [Google Scholar] [CrossRef]
- Nguyen-Phan, T.-D.; Pham, V.H.; Shin, E.W.; Pham, H.-D.; Kim, S.; Chung, J.S.; Kim, E.J.; Hur, S.H. The role of graphene oxide content on the adsorption-enhanced photocatalysis of titanium dioxide/graphene oxide composites. Chem. Eng. J. 2011, 170, 226–232. [Google Scholar] [CrossRef]
- Nethravathi, C.; Viswanath, B.; Shivakumara, C.; Mahadevaiah, N.; Rajamathi, M. The production of smectite clay/graphene composites through delamination and co-stacking. Carbon 2008, 46, 1773–1781. [Google Scholar] [CrossRef]
- Nethravathi, C.; Rajamathi, J.T.; Ravishankar, N.; Shivakumara, C.; Rajamathi, M. Graphite oxide-intercalated anionic clay and its decomposition to graphene-inorganic material nanocomposites. Langmuir 2008, 24, 8240–8244. [Google Scholar] [CrossRef] [PubMed]
- Wu, J.; Shen, X.; Jiang, L.; Wang, K.; Chen, K. Solvothermal synthesis and characterization of sandwitch-like graphene/ZnO nanocomposites. Appl. Surf. Sci. 2010, 256, 2826–2830. [Google Scholar] [CrossRef]
- Chu, Y.H.; Yamagishi, M.; Wang, Z.M.; Kanoh, H.; Hirotsu, T. Synthesis of nanoporous graphite-derived carbon/TiO2-SiO2 composites by a mechanochemical intercalation method. Micropor. Mesopor. Mater. 2009, 118, 496–502. [Google Scholar] [CrossRef]
- Wang, D.; Kou, R.; Choi, D.; Yang, Z.; Nie, Z.; Li, J.; Saraf, L.V.; Hu, D.; Zhang, J.; Graff, G.L.; et al. Ternary self-assembly of ordered metal oxide-graphene nanocomposites for electrochemical energy storage. ACS Nano 2010, 4, 1587–1595. [Google Scholar] [CrossRef] [PubMed]
- Jiang, G.; Lin, Z.; Chen, C.; Zhu, L.; Chang, Q.; Wang, N.; Wei, W.; Tang, H. TiO2 nanoparticles assembled on graphene oxide nanosheets with high photocatalytic activity for removal of pollutants. Carbon 2011, 49, 2693–2701. [Google Scholar] [CrossRef]
- Liang, Y.T.; Vijayan, B.K.; Gray, K.A.; Hersam, M.C. Minimizing graphene defects enhances titania nanocomposite-based photocatalytic reduction of CO2 for improved solar fuel production. Nano Lett. 2011, 11, 2865–2870. [Google Scholar] [CrossRef] [PubMed]
- Stengl, V.; Popelkova, D.; Vlacil, P. TiO2-graphene nanocomposite as high performace photocatalysts. J. Phys. Chem. C. 2011, 115, 25209–25218. [Google Scholar] [CrossRef]
- Nurzulaikha, R.; Lim, H.N.; Harrison, I.; Lim, S.S.; Pandikumar, A.; Huang, N.M.; Lim, S.P.; Thien, G.S.H.; Yusoff, N.; Ibrahim, I. Graphene/SnO2 nanocomposite-modified electrode for electrochemical detection of dopamine. Sens. Biosens. Res. 2015, 5, 42–49. [Google Scholar] [CrossRef]
- Dutta, D.; Hazra, S.K.; Das, J.; Sarkar, C.; Basu, S. Studies on p-TiO2/n-graphene heterojunction for hydrogen detection. Sens. Actuators B 2015, 212, 84–92. [Google Scholar] [CrossRef]
- Burkhanov, G.S.; Gorina, N.B.; Kolchugina, N.B.; Roshan, N.R.; Slovetsky, D.I.; Chistov, E.M. Palladium-based alloy membranes for separation of high purity hydrogen from hydrogen-containing gas mixtures. Platin. Metals Rev. 2011, 55, 3–12. [Google Scholar] [CrossRef]
- Singh, N.B.; Bhattacharya, B.; Sarkar, U. Nickel decorated single-wall carbon nanotube as CO sensor. Soft Nanosci. Lett. 2013, 3, 9–11. [Google Scholar] [CrossRef]
- Wang, X.; Tabakman, S.M.; Dai, H. Atomic layer deposition of metal oxides on pristine and functionalized grapheme. J. Am. Chem. Soc. 2008, 130, 8152–8153. [Google Scholar] [CrossRef] [PubMed]
- Zhang, R.; Alecrim, V.; Andres, M.H.B.; Forsberg, S.; Andersson, M.; Olin, H. Thermally reduced kaolin-graphene oxide nanocomposites for gas sensing. Sci. Rep. 2015, 5. [Google Scholar] [CrossRef] [PubMed]
- Srivastava, S.; Jain, K.; Singh, V.N.; Singh, S.; Vijayan, N.; Dilawar, N.; Gupta, G.; Senguttuvan, T.D. Faster response of NO2 sensing in graphene-WO3 nanocomposites. Nanotechnology 2012, 23. [Google Scholar] [CrossRef] [PubMed]
- Lu, G.; Ocola, L.E.; Chen, J. Gas detection using low-temperature reduced graphene oxide sheets. Appl. Phys. Lett. 2009, 94, 083111. [Google Scholar] [CrossRef]
- Esfandiar, A.; Irajizad, A.; Akhavan, O.; Ghasemi, S.; Gholami, M.R. Pd-WO3/reduced graphene oxide hierarchical nanostructures as efficient hydrogen gas sensors. Int. J. Hydrog. Energy 2014, 39, 8169–8179. [Google Scholar] [CrossRef]
- Breysse, M.; Claudel, B.; Faure, L.; Wolkenstein, T. Chemiluminescence during the catalysis of carbon monoxide oxidation on a thoria surface. J. Catal. 1976, 45, 137–144. [Google Scholar] [CrossRef]
- Song, H.; Zhang, L.; He, C.; Qu, Y.; Tiana, Y.; Lv, Y. Graphene sheets decorated with SnO2 nanoparticles: In situ synthesis and highly efficient materials for cataluminescence gas sensors. J. Mater. Chem. 2011, 21, 5972–5977. [Google Scholar] [CrossRef]
- Zhang, D.; Liu, A.; Chang, H.; Xia, B. Room-temperature high-performance acetone gas sensor based on hydrothermal synthesized SnO2-reduced graphene oxide hybrid composite. RSC Adv. 2015, 5, 3016–3022. [Google Scholar] [CrossRef]
- Lin, Q.; Li, Y.; Yang, M. Tin oxide/graphene composite fabricated via a hydrothermal method for gas sensors working at room temperature. Sens. Actuators B Chem. 2012, 173, 139–147. [Google Scholar] [CrossRef]
- Mishra, R.K.; Upadhyay, S.B.; Kushwaha, A.; Kim, T.-H.; Murali, G.; Verma, R.; Srivastava, M.; Singh, J.; Sahay, P.P.; Hee Lee, S. SnO2 quantum dots decorated on RGO: A superior sensitive, selective and reproducible performance for a H2 and LPG sensor. Nanoscale 2015, 7, 11971–11979. [Google Scholar] [CrossRef] [PubMed]
- Nemade, K.R.; Waghuley, S.A. LPG sensing by graphene/ZnO quantum dots composite. Int. J. ChemTech Res. 2014, 6, 3399–3401. [Google Scholar]
- Anand, K.; Singh, O.; Singh, M.P.; Kaur, J.; Singh, R.C. Hydrogen sensor based on graphene/ZnO nanocomposite. Sens. Actuators B 2014, 195, 409–415. [Google Scholar] [CrossRef]
- Iftekhar Uddin, A.S.M.; Phan, D.-T.; Chung, G.-S. Low temperature acetylene gas sensor based on Ag nanoparticles-loaded ZnO-reduced graphene oxide hybrid. Sens. Actuators B 2015, 207, 362–369. [Google Scholar] [CrossRef]
- Mu, H.; Zhang, Z.; Zhao, X.; Liu, F.; Wang, K.; Xie, H. High sensitive formaldehyde graphene gas sensor modified by atomic layer deposition zinc oxide films. Appl. Phys. Lett. 2014, 105. [Google Scholar] [CrossRef]
- Jebreiil Khadem, S.M.; Abdi, Y.; Darbari, S.; Ostovari, F. Investigating the effect of gas absorption on the electromechanical and electrochemical behavior of graphene/ZnO structure, suitable for highly selective and sensitive gas sensors. Curr. Appl. Phys. 2014, 14, 1498–1503. [Google Scholar] [CrossRef]
- Zhong, L.; Yun, K. Graphene oxide-modified ZnO particles: Synthesis, characterization, and antibacterial properties. Int. J. Nanomed. 2015, 10, 79–92. [Google Scholar]
- Kumar, N.; Srivastava, A.K.; Patel, H.S.; Gupta, B.K.; Varma, G.D. Facile synthesis of ZnO-reduced graphene oxide nanocomposites for NO2 gas sensing applications. Eur. J. Inorg. Chem. 2015, 2015, 1912–1923. [Google Scholar] [CrossRef]
- How, G.T.S.; Pandikumar, A.; Ming, H.N.; Ngee, L.H. Highly exposed {001} facets of titanium dioxide modified with reduced graphene oxide for dopamine sensing. Sci. Rep. 2014, 4, 5044. [Google Scholar] [CrossRef] [PubMed]
- Fan, Y.; Lu, H.-T.; Liu, J.-H.; Yang, C.-P.; Jing, Q.-S.; Zhang, Y.-X.; Yang, X.-K.; Huang, K.-J. Hydrothermal preparation and electrochemical sensing properties of TiO2-graphene nanocomposite. Colloids Surf. B 2011, 83, 78–82. [Google Scholar] [CrossRef] [PubMed]
- Xu, C.-X.; Huang, K.-J.; Fan, Y.; Wu, Z.-W.; Li, J.; Gan, T. Simultaneous electrochemical determination of dopamine and tryptophan using a TiO2-graphene/poly(4-aminobenzenesulfonic acid) composite film based platform. Mater. Sci. Eng. C 2012, 32, 969–974. [Google Scholar]
- Jang, H.D.; Kim, S.K.; Chang, H.C.; Jo, E.H.; Roh, K.M.; Choi, J.-H.; Choi, J.-W. Synthesis of 3D silver-graphene-titanium dioxide composite via aerosol spray pyrolysis for sensitive glucose biosensor. Aerosol Sci. Tech. 2015, 49, 538–546. [Google Scholar] [CrossRef]
- Chang, B.Y.S.; Huang, N.M.; An'amt, M.N.; Marlinda, A.R.; Norazriena, Y.; Muhamad, M.R.; Harrison, I.; Lim, H.N.; Chia, C.H. Facile hydrothermal preparation of titanium dioxide decorated reduced graphene oxide nanocomposite. Int. J. Nanomed. 2012, 7, 3379–3387. [Google Scholar]
- Wang, T.; Peng, Z.; Wang, Y.; Tang, J.; Zheng, G. MnO nanoparticle@mesoporous carbon composites grown on conducting substrates featuring high-performance lithium-ion battery, supercapacitor and sensor. Sci. Rep. 2013. [Google Scholar] [CrossRef] [PubMed]
- Xiang, C.; Jiang, D.; Zou, Y.; Chu, H.; Qiu, S.; Zhang, H.; Xu, F.; Sun, L.; Zheng, L. Ammonia sensor based on polypyrrole-graphene nanocomposite decorated with titania nanoparticles. Ceram. Int. 2015, 41, 6432–6438. [Google Scholar] [CrossRef]
- Liu, S.; Wang, Z.; Zhang, Y.; Zhang, C.; Zhang, T. High performance room temperature NO2 sensors based on reduced graphene oxide-multiwalled carbon nanotubes-tin oxide nanoparticles hybrids. Sens. Actuators B 2015, 211, 318–324. [Google Scholar] [CrossRef]
- Liu, S.; Zhou, L.; Yao, L.; Chai, L.; Li, L.; Zhang, G.; Kankan; Shi, K. One-pot reflux method synthesis of cobalt hydroxide nanoflake-reduced graphene oxide hybrid and their NOx gas sensors at room temperature. J. Alloys Compd. 2014, 612, 126–133. [Google Scholar] [CrossRef]
- Zhang, H.; Feng, J.; Fei, T.; Liu, S.; Zhang, T. SnO2 nanoparticles-reduced graphene oxide nanocomposites for NO2 sensing at low operating temperature. Sens. Actuators B 2014, 190, 472–478. [Google Scholar] [CrossRef]
- Li, L.; He, S.; Liu, M.; Zhang, C.; Chen, W. Three-dimensional mesoporous graphene aerogel-supported SnO2 nanocrystals for high-performance NO2 gas sensing at low temperature. Anal. Chem. 2015, 87, 1638–1645. [Google Scholar] [CrossRef] [PubMed]
- Jiang, Z.; Li, J.; Aslan, H.; Li, Q.; Li, Y.; Chen, M.; Huang, Y.; Froning, J.P.; Otyepka, M.; Zboril, R.; Besenbacher, F.; Dong, M. A high efficiency H2S gas sensor material: Paper like Fe2O3/graphene nanosheets and structural alignment dependency of device efficiency. J. Mater. Chem. A 2014, 2, 6714–6717. [Google Scholar] [CrossRef]
- MalekAlaie, M.; Jahangiri, M.; Rashidi, A.M.; HaghighiAsl, A.; Izadi, N. Selective hydrogen sulfide (H2S) sensors based on molybdenum trioxide (MoO3) nanoparticle decorated reduced graphene oxide. Mater. Sci. Semicond. Process. 2015, 38, 93–100. [Google Scholar] [CrossRef]
- Latif, U.; Dickert, F.L. Graphene hybrid materials in gas sensing applications. Sensors 2015, 15, 30504–30524. [Google Scholar] [CrossRef] [PubMed]
- Han, X.; Li, S.; Peng, Z.; Al-Yuobi, A.O.; Omar Bashammakh, A.S.; El-Shahawi, M.S.; Leblanc, R.M. Interactions between carbon nanomaterials and biomolecules. J. Oleo Sci. 2016, 65. [Google Scholar] [CrossRef] [PubMed]
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Hazra, S.K.; Basu, S. Graphene-Oxide Nano Composites for Chemical Sensor Applications. C 2016, 2, 12. https://doi.org/10.3390/c2020012
Hazra SK, Basu S. Graphene-Oxide Nano Composites for Chemical Sensor Applications. C. 2016; 2(2):12. https://doi.org/10.3390/c2020012
Chicago/Turabian StyleHazra, Surajit Kumar, and Sukumar Basu. 2016. "Graphene-Oxide Nano Composites for Chemical Sensor Applications" C 2, no. 2: 12. https://doi.org/10.3390/c2020012