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Nitrogen-doped activated carbon-based ammonia sensors: effect of specific surface functional groups on carbon electronic properties Nikolina A Travlou, Christopher Ushay, Mykola Seredych, Enrique Rodriguez-Castellon, and Teresa J Bandosz ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.6b00093 • Publication Date (Web): 18 Mar 2016 Downloaded from http://pubs.acs.org on March 21, 2016
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Nitrogen-doped activated carbon-based ammonia sensors: effect of specific surface functional groups on carbon electronic properties Nikolina A. Travlou1,2, Christopher Ushay1, Mykola Seredych1, Enrique Rodríguez-Castellón3 and Teresa J. Bandosz1,2* 1
Department of Chemistry, The City College of New York 160 Convent Ave, New York, NY, 10031, USA. 2 Ph.D. Program in Chemistry, The Graduate Center of the City University of New York, New York, NY 10016. E-mail:
[email protected]; Fax: +1 212 650 6107; Tel: +1 212 650 6017 3 Departamento de Química Inorgánica, Facultad de Ciencias, Universidad de Málaga, 29071 Málaga, Spain.
Supporting Information Placeholder ABSTRACT: Wood-based commercial activated carbon (BAX) and its oxidized counterpart (BAX-O) were treated with melamine and then heated at 450oC in nitrogen. Further oxidation with nitric acid was also applied. The carbons were tested for ammonia sensing (45-500 ppm of NH3). Even though all samples exhibit p-type conduction, their exposure to NH3 led to different electrical outcomes. It was found that the electronic and transport properties of the carbons strongly depend on the type of nitrogen groups/surface defects, their concentration, and distribution in the carbon matrix. Pyridines and nitropyridines are the most important. A competition between the structural and chemical features of the carbons as those governing the sensing signals was observed. Exposure to ammonia altered the surface chemistry of the samples, and therefore their electrical properties. When sensitivity to H2S was tested to evaluate the selectivity of our sensors, the results showed that the chips are selective to NH3 in terms of the response time and magnitude of the signal changes.
Carbon based materials act as p-type semiconductors, with positively charged holes (h+) being the main charge carriers. 1-5 Their gas sensing mechanism is based on an electrical change due to the reduction of the charge carriers (holes) when an electron-donating species such as NH3 or H2S interacts with them.5–8 Such carbonbased sensors, even though are highly sensitive, lack the selectivity. This limitation can be overcome through their chemical modification and/or their doping with metals/metal oxides.7,9–12 Nanoporous carbons, having a great number of desirable features such as an extensive surface area, highly developed pore structure, rich surface chemistry and high degree of surface reactivity, might be advantageous in gas sensing devices.13,14 We have recently shown that when used as gas sensors, their electrical response is affected by both physical adsorption of gases in their pore system and also reactive adsorption on surface functional groups.14–16 The introduction of various heteroatoms such as N and O to the carbon matrix offers an effective way of altering the electronic structure of porous carbons, manipulating their surface chemistry and inserting local changes to their matrices.17–21 In the case of Ndoping, the modified electronic properties arise upon the replacement of a carbon atom by nitrogen that contains three valence electrons and an additional lone e- pair. Its incorporation into the sp2 carbon lattice leads to several chemical configurations, among which pyrrole-like, pyridine-like, and graphite-like/quaternary are the most commonly found.22-26 Based on the structural variety of
N-defects, donor or acceptor properties can arise. Previous studies on N-doped graphene and carbon nanotubes (CNTs) have shown that pyridinic nitrogen is considered to be a p-type doping impurity, while nitrogen atoms with a coordination number of 3 are considered to be n-type dopants.22,23,25-27 The former is related to the electron deficiency of the pyridinic configurations, compared to pristine graphene/CNTs, which therefore induce acceptor-type states.25,26 Hence, depending on the type of N-doping, carbonbased materials with various electronic properties can arise. Battie and coworkers tested ammonia gas sensing properties of single-walled carbon nanotube (SWCNTs) films. They found that the pyridinic defects determine the response of the sensors.28 Bai and Zhou, using DFT computations, examined an NH3 and NO2 adsorption and sensing capability of -B and N-doped singlewalled carbon nanotubes (SWCNTs).29 They showed that while boron doped SWCNTs were able to detect only NH3, SWCNTs doped with nitrogen showed a good detection capability towards both gasses. Interestingly, in a previous study by Y. Fujimoto and S. Saito30-32 it was shown that upon the adsorption of NH3 and H2 on N-doped graphene, pyridine-type defects are highly reactive towards the target gas, causing a transition in the doping properties of the materials. In our previous study where wood-based activated carbons were used as ammonia gas-sensors14 it was found that oxidation of the initial carbon with nitric acid induced nitro-type complexes to the carbon matrix. This led to materials with altered electronic properties. A conversion of their conduction type from predominantly -p to predominantly –n was observed.14 Finally, in our most recent study where the sensing capability for ammonia of S and N co-doped nanoporous carbons was tested, we demonstrated that the synergistic effect of both heteroatoms enhanced the response of the carbons as ammonia sensors compared to the previously studied carbons doped solely with sulfur.15 The role of nitrogen was linked to the ability of the surface to activate oxygen and thus to form superoxide ions (O2).33,34 The introduction of N-atoms to a nanoporous carbon matrix can be done using a N-containing precursor or through chemical modifications of the carbon surface.35,36 The most commonly used methods are: i) oxidation of carbons followed by ii) impregnation of the carbons with melamine/urea suspensions in ethanol, and a subsequent heat treatment at elevated temperatures,21,37 and iii) the use of nitrogen-containing compounds as precursors, which can be either commercially available monomers, such as melamine, polyaniline etc. or N-containing waste products and biomass.38-40 In the second case, the type of the nitrogen-containing
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surface groups depends on the temperature at which carbonization is carried out.41 The objective of this paper is an evaluation of the role of specific nitrogen functional groups in the ammonia sensing capability of nanoporous carbons. For this purpose chips made of melamine modified nanoporous carbons were exposed to NH3 concentrations varying from 45-500 ppm. Factors such as nitrogen content, its configurations, surface acidity and pore accessibility are extensively studied. We examine how the type of nitrogen defects, their concentration and distribution in the carbon matrix affect the electronic and transport properties of nanoporous carbons. An important feature of a sensor is its selectivity that indicates its ability to detect a particular analyte. Therefore to test this, our carbons were also exposed to H2S, which, similarly to NH3, is a reducing gas, but of a different chemistry. The initial and exhausted materials were extensively characterized using, XPS analysis, sorption of N2, thermal analysis and potentiometric titration. Details on their preparation, characterization and electrochemical measurements are provided in SI (Supplementary Information). RESULTS AND DISCUSSION Seven different samples were tested. They were derived by the melamine treatment and further oxidation of a wood-based commercial activated carbon (BAX) and its oxidized counterparts with 20% or 50% of HNO3 (BAX-O1 or BAX-O2, respectively). A brief description of the treatments applied is given below:
Figure 1 demonstrates the changes in the normalized resistance of the coated chips upon their initial exposure to 500 ppm of NH3, which is followed by a subsequent air purging. During this initial stabilization step, both reactive and physical adsorption of NH3 gas occur resulting in quasi-equilibrium (stabilization). The complexity of these processes results in a slow recovery of the sensors upon air purging.
Figure 1. Changes in in the normalized resistance for the activated carbon samples upon their initial exposure to NH3 and subsequent air purging (after the signal stabilization), A) for BAX-M,
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B) for BAX-MO1 and BAX-MO2, C) BAX-O1-M and BAX-O2M and D) BAX-O1-M[O1] and BAX-O2-M[O2]. To examine the reversible sensing capability of the carbons and determine the optimal performing sample the chamber was purged with ammonia gas of concentrations ranging from 45 to500 ppm. At this sensing stage, only weak interactions between the target gas and the carbons’ surface generate the reversible sensing and are responsible for the fast recovery in the normalized resistance between the consecutive cycles. For each carbon pair, (BAXMO1/BAX-MO2, BAX-O1M/BAX-O2M and BAX-O1M[O1]/BAX-O2-M[O1]) the changes in the normalized resistance of the counterpart with the optimal electrical response (highest sensitivity) are presented (Figure 2). All chips exhibit an excellent repeatability and reversibility in their electrical response, which varies linearly with an increased ammonia concentration (Figure 2E). Error bars indicate standard deviations from the mean upon testing three different chips of each sample. The relative σ (% RSD) was found to be between 3.3-10.5 %. The effective response time was defined as 6, 2, 8, and 2 min for BAX-M, BAXM-O series, BAX-O-M series and BAX-O-M[O] series, respectively. The responses of the carbon-based chips addressed in this study are compared in Figure 3. The results of the initial BAX and BAX-O, which were tested in our previous study14 are also included. As seen: i) The melamine treated samples exhibit an opposite trend in their normalized resistance compared to the samples where oxidation with ΗΝΟ3 was the last treatment step, and ii) the carbons which were oxidized with HNO3 in the last treatment step (BAX-M-O1, BAX-M-O2, BAX-O1-M[O1], BAX-O2M[O2]) exhibit more pronounced signal changes compared to those for their initial counterparts (BAX-M, BAX-O1-M, BAXO2-M). This behavior suggests that melamine treatment at 450oC followed by the subsequent oxidation with HNO3 affected their electronic properties. This is most likely related to their specific surface chemistry, and therefore differences in the type and distribution of N-type defects in the carbon matrix. Since the electronic and transport properties of activated carbons (ACs) strongly depend on the above mentioned features, NH3 breakthrough tests were run to collect a sufficient quantity of samples for further characterization of their surface. The breakthrough and desorption curves along with the breakthrough capacity values are presented in Figure S-1 of Supporting Information (SI). The structural features of the carbons tested are listed in Table 1. For BAX the contributions of micropores and especially mesopores to the total porosity are significant (41.4 % and 59 % of the total pore volume, respectively). Treatment of this carbon with melamine led to decreased volumes of these pores and ultramicropores (V < 1nm), due to the deposition of bulky melamine resins at their entrances.37 Upon oxidation of the melamine treated samples with HNO3, the volumes of micro- and ultramicropores increased with the largest effect for pores smaller than 0.7 nm (~ 46%). This effect is linked to the removal of some carbon atoms from the walls of these pores (micro- and ultramicropores) and of bulky groups previously deposited at their entrances, as a result of surface oxidation. As expected, treatment of the oxidized samples with melamine (BAX-O1M, BX-O2M), decreased the pore volumes. Interestingly, unlike in the case of the BAX-M, further oxidation of the BAX-O1-M and BAX-O2-M carbons led to a decrease in their micro- and ultramicropore volume. For BAX-O1-M, this decrease is accompanied by an increase in the volume of mesopores (~ 30%). As discussed above, oxidation of the samples likely removed some carbon atoms from the walls of smaller pores, and this resulted the increased mesopore volume. Another reason for the decreased micropore volume of these carbons might be the
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formation of bulky functional groups owing to the chemical reaction of HNO3 with the carbons’ surfaces. Such surface groups could be nitro-type complexes, whose presence and nature will be analyzed later in this paper.
Figure 3. The comparison of the normalized resistance changes of all carbon samples tested upon exposure to 500 ppm of NH3. aRe-
Figure 2. Response curves for the best performing carbons (A, B, C, D). Dependence of ∆R/Ro (%) of the chips on various NH3 concentrations (error bars indicate σ)(E). printed with Permission from Reference [14]. Copyright 2014, Royal Society of Chemistry. Among all melamine treated samples, BAX-O1-M[O1] which has the second highest volume of micropores and ultramicropores (comparable/slightly lower than that of BAX-O1M), exhibits the highest sensitivity in ammonia sensing (Figure 3). Ultramicropores (V BAX-MO2> BAX-O2-M> BAX-M, which agrees with the corresponding surface pH values (Figure S-3 of the SI).
of the sensors), and B) in terms of the response time of the sensors toward NH3 sensing. Interestingly, even though both gases are electron donating, opposite electrical responses were recorded in the case of BAX-M, BAX-O1-M, and BAX-O2-M. This is related to the surface chemistry of these carbons and the chemistry of the target gases. In the case of the melamine-treated carbons, the increased contribution of pyridine-like nitrogen is responsible for their increased surface polarity,53,54 and therefore an increased water adsorption. Even though the sensing tests were performed in dry conditions, water molecules can still be present in very small pores of carbons. In their presence, and in vicinity of basic groups, H2S dissociates into H+ and HS- ions. The pyridinic groups facilitate the direct contact of HS- ions with the carbon matrix and their further oxidation to SO2 can take place.53,55 The latter species being electron withdrawing can cause a decrease in the normalized resistance. The dissociated HS- ions, by providing ionic conductive paths through the carbon matrix, may also lead to the same electrical outcome.
Selectivity study To evaluate the selectivity of our sensors, they were exposed to 500 ppm of H2S, which similarly to ammonia is an electron donating gas, but of different chemistry. The selectivity was tested in two ways: (a) in terms of the changes of the normalized resistance (sensitivity of the sensors), and (b) in terms of the response time, which indicates how fast a chip can respond to an environmental change. Even though the melamine impregnated samples exposed to H2S shows larger total changes in their normalized resistance compared to NH3 (Figure 5), their response time to the latter gas is much shorter. Since our carbons exhibit much higher sensitivity to NH3 than to H2S, the changes in their normalized resistance were also evaluated based on the response time of the sensors to NH3. Thus the same exposure time for both ammonia and hydrogen sulfide caused much greater changes in the normalized resistance for the former species. Considering that both molecules are of similar sizes, and therefore have similar accessibility to the porenetwork, the higher selectivity of our carbons to NH3 is likely related to the presence of certain surface functional groups and their surface acidity that enhance their affinity toward NH3 adsorption. Undoubtedly, nitrogen-type basic groups should attract H2S,53 and they do, but kinetics of adsorption/reactive adsorption are apparently much slower.
CONCLUSIONS During the reversible ammonia sensing opposite trends in the electrical signal of those carbons where oxidation with HNO3 was their last treatment step and those where melamine impregnation was their last treatment step were observed. For BAX-M, BAXO1M and BAX-O2M, the decrease in their conductivity was attributed to the increased contribution of pyridine-type impurities after ammonia exposure. For those samples where oxidation with HNO3 was their last treatment step, on the other hand, the conversion of nitropyridines to amino-nitropyridines caused an enhanced distribution/delocalization of the electron charge density and therefore an increase in the conductivity. Furthermore, oxidation of ammonia to NO2 caused an increase in the concentration of the charge carries (h+), which also led to an increased conductivity. The reactivity of ammonia with the carbons’ surface depends on the specific chemical arrangement of the N-species and their basicity/acidity. Changes in the surface chemistry occur simultaneously with changes in porosity. Even though it has been shown that there is a direct relationship between the volume of micro/ ultramicropores and the sensitivity of the chips, in this work the indirect relationship of these parameters for some samples reveals the competitive role between the structural and chemical features of the carbons. The role of the nitrogen functionalities on the electrical performance of the carbons was further investigated by testing their selectivity with respect to H2S sensing. Pyridinic groups, acting as p-type impurities, were found to be responsible for the observed opposite electrical responses of the melamine impregnated samples upon exposure to NH3/H2S. The facilitate H2S dissociation into H+ and HS- ions. The latter ions, either by providing ionic conductive paths through the carbon matrix, or through their oxidation to SO2 may cause a decrease of the normalized resistance. ACKNOWLEDGEMENTS This work was supported by the ARO (Army Research Office) grant W911NF-13-1-0225, and NSF collaborative CBET Grant no. 1133112, and project P12-RNM 1565 (Excelencia, Junta de Andalucía) and FEDER funds. Special thanks are addressed to Mr. Anmol Jadvani for his experimental contribution. ASSOCIATED CONTENT
Figure 5. Comparison of the selectivity of the carbon sensors. A) in terms of the changes of their normalized resistance (sensitivity
Supporting Information: The Supporting Information is available free of charge on the ACS Publications website.
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Details on the preparation of the materials, the characterization methods and electrochemical measurements. Figure S1: The breakthrough and desorption curves along with the breakthrough capacity values. Figure S2. DTG curves for the samples studied. Figure S3: Distributions of the acidity constants for the species present on the carbons’ surfaces, along with pH values of the initial and exhausted samples. Figure S4. Mott-Schottky plots. Tables S1-S4: The content of the elements on the surface in atomic % and the deconvolution results of C 1s, O 1s and N 1s core energy levels. REFERENCES (1) Huang, X.; Hu, N.; Gao, R.; Yu, Y.; Wang, Y.; Yang, Z.; Siu-Wai Kong, E.; Wei, H.; Zhang, Y. Reduced Graphene Oxide–Polyaniline Hybrid: Preparation, Characterization and its Applications for Ammonia Gas Sensing. J. Mater. Chem. 2012, 22, 22488- 22495. (2) Mattson, E. C.; Pande, K.; Unger, M.; Cui, S.; Lu, G.; Weinert, M.; Chen, J.; Hirschmugl, C. J. Exploring Adsorption and Reactivity of NH3 on Reduced Graphene Oxide. J. Phys. Chem. C 2013, 117, 10698–10707. (3) Teerapanich, P.; Myint, M. T. Z.; Joseph, C. M.; Hornyak, G. L.; Dutta, J. Development and Improvement of Carbon Nanotube-Based Ammonia Gas Sensors Using Ink- Jct Printed Interdigitated Electrodes. IEEE Trans. Nanotechnol. 2013, 12, 255–262. (4) Zhang, R.; Alecrim, V.; Hummelgård, M.; Andres, B.; Forsberg, S.; Andersson, M.; Olin, H. Thermally Reduced Kaolin-Graphene Oxide Nanocomposites for Gas Sensing. Sci. Rep. 2015, 5, 7676. (5) Ghosh, R.; Singh, A.; Santra, S.; Ray, S. K.; Chandra, A.; Guha, P. K. Highly Sensitive Large-Area Multi-Layered Graphene-Based Flexible Ammonia Sensor. Sens. Actuators, B 2014, 205, 67–73. (6) Wu, J.; Tao, K.; Miao, J.; Norford, L. K. Improved Selectivity and Sensitivity of Gas Sensing Using a 3D Reduced Graphene Oxide Hydrogel with an Integrated Microheater. ACS Appl. Mater. Interfaces 2015, 7, 27502–27510. (7) Abdulla, S.; Mathew, T. L.; Pullithadathil, B. Highly Sensitive, Room Temperature Gas Sensor Based on Polyaniline-Multiwalled Carbon Nanotubes (PANI/MWCNTs) Nanocomposite for Trace-Level Ammonia Detection. Sens. Actuators, B 2015, 221, 1523–1534. (8) Latif, U.; Dickert, F. Graphene Hybrid Materials in Gas Sensing Applications. Sensors 2015, 15, 30504–30524. (9) 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. (10) Yang, Y.; Li, S.; Yang, W.; Yuan, W.; Xu, J.; Jiang, Y. In Situ Polymerization Deposition of Porous Conducting Polymer on Reduced Graphene Oxide for Gas Sensor. ACS Appl. Mater. Interfaces 2014, 6, 13807–13814. (11) Gautam, M.; Jayatissa, A. H. Ammonia Gas Sensing Behavior of Graphene Surface Decorated with Gold Nanoparticles. Solid-State Electron. 2012, 78, 159–165. (12) Ghosh, R.; Nayak, A. K.; Santra, S.; Pradhan, D.; Guha, P. K. Enhanced Ammonia Sensing at Room Temperature with Reduced Graphene Oxide/Tin Oxide Hybrid Film. RSC Adv. 2015, 5, 50165–50173. (13) Liu, H.; Zhang, Y.; Ke, Q.; Ho, K. H.; Hu, Y.; Wang, J. Tuning the Porous Texture and Specific Surface Area of Nanoporous Carbons for Supercapacitor Electrodes by Adjusting the Hydrothermal Synthesis Temperature. J. Mater. Chem. A 2013, 1, 12962–12970. (14) Travlou N. A.; Seredych M.; Rodríguez-Castellón E.; Bandosz T. J. Activated Carbon-Based Gas Sensors: Effects of Surface Features on the Sensing Mechanism. J. Mater. Chem. A 2015, 3, 3821-3831. (15) Travlou N. A.; Seredych M.; Rodríguez-Castellón E.; Bandosz T. J. Insight into Ammonia Sensing on Heterogeneous S- and N- co-Doped Nanoporous Carbons. Carbon 2016, 96, 1014–1021. (16) Singh, K.; Travlou, N. A.; Bashkova, S.; Rodríguez-Castellón, E.; Bandosz, T. J. Nanoporous Carbons as Gas Sensors: Exploring the Surface Sensitivity. Carbon 2014, 80, 183–192. (17) Hulicova-Jurcakova, D.; Seredych, M.; Lu, G. Q.; Bandosz, T. J. Combined Effect of Nitrogen- and Oxygen-Containing Functional Groups
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concentrations (error bars indicate the standard deviation from the mean) (E). Figure 3. The comparison of the normalized resistance changes of all carbon samples tested upon exposure to 500 ppm of NH3. aReprinted with Permission from Reference [14]. Copyright 2014, Royal Society of Chemistry. Figure 4. A) Content of elements; B-D) Surface concentration of the specific carbon, oxygen, and nitrogen groups. Figure 5. The comparison of the selectivity of the carbon sensors. A) in terms of the changes of their normalized resistance (sensitivity of the sensors), and B) in terms of the response time of the sensors toward NH3 sensing.
Captions to the Tables Table 1. Parameters of the porous structure calculated from the nitrogen adsorption isotherms Captions to the Figures Figure 1. Changes in the normalized resistance for the activated carbon samples upon their initial exposure to NH3 and subsequent air purging (after the signal stabilization). Figure 2. Response curves for the best performing carbons (A, B, C, D). Dependence of ∆R/Ro (%) of the chips on various NH3
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