Low temperature thermal treatment of hexamethylenetetramine to synthesize nitrogen-doped carbon for non-enzymatic H2O2 sensing

Sensors and Actuators B 201 (2014) 240–245

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Low temperature thermal treatment of hexamethylenetetramine to synthesize nitrogen-doped carbon for non-enzymatic H2 O2 sensing Sen Liu, Bo Yu, Teng Fei, Tong Zhang ∗ State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, PR China

a r t i c l e

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Article history: Received 3 February 2014 Received in revised form 8 May 2014 Accepted 10 May 2014 Available online 17 May 2014 Keywords: Nitrogen-doped carbon Low temperature Heat treatment Metal-free H2 O2 sensing

a b s t r a c t Nitrogen-doped carbon (N-carbon) materials have been prepared by low temperature (300 ◦ C) heat treatment of hexamethylenetetramine in air. The combined characterizations of X-ray diffraction pattern, scanning electron microscopy, and X-ray photoelectron spectroscopy indicate the successful preparation of N-carbon materials. Most importantly, the N-carbon exhibits catalytic activity toward electrochemical reduction of H2 O2 , leading to a novel metal-free non-enzymatic H2 O2 sensor. The linear detection range is estimated to be from 0.1 mM to 40 mM (r = 0.996), and the detection limit is estimated to be 90 ␮M at a signal-to-noise ratio of 3. © 2014 Elsevier B.V. All rights reserved.

1. Introduction During the past few years, nitrogen-doped carbon (N-carbon) materials have received considerable attention because the introduction of N atom into carbon matrix can largely improve the physicochemical properties of the carbon materials, such as conductivity, basicity, oxidation stability, and catalytic activity, which ensure the wide applications of N-carbon materials in the fields of oxygen reduction reaction (ORR) [1], electrochemical sensing [2], Li ion battery [3], fluorescent sensing [4], supercapacitors [5], catalysis [6], and so on. Up to now, numerous N-carbon materials with various structures including graphene-based materials [7,8], carbon nanotubes (CNTs) [9,10], microporous carbon [11], mesoporous carbon [12,13], carbon nanodots [14,15], as well as carbon nanofibers [16] have been successfully prepared. These N-carbon materials have been used as novel functional materials for various applications. Therefore, much attention has been paid on developing simple and efficient approaches to prepare N-carbon materials. Currently, numerous synthesis methods have been successfully developed to synthesize N-carbon materials, which can be normally classified into two main types: post-doping and in situ direct-doping. The post-doping methods involve the preparation of carbon-based materials firstly, followed by treatment of them in the N-containing atmospheres, such as NH3 , N-ion plasma [17,18].

∗ Corresponding author. Tel.: +86 431 85168385. E-mail address: [email protected] (T. Zhang). http://dx.doi.org/10.1016/j.snb.2014.05.032 0925-4005/© 2014 Elsevier B.V. All rights reserved.

However, these methods suffer from obvious drawbacks, such as high energy consumption, expensive hardware, and multi-step processes etc. The in situ direct-doping methods are normally performed by high temperature pyrolysis of N-containing materials in inert gases, where the N-containing precursors consist of resin [19], conducting polymers [20,21], organic compounds [22,23], natural plants [24,25], etc. Unfortunately, high temperature is necessary for preparation of these N-carbon materials, which not only brings out the troubles by rigorous experimental condition, but also results in the relatively low content of N in the final samples. More recently, our [26] and other groups [27–29] have developed a novel method for preparation of N-carbon materials by hydrothermal treatment of N-containing organic compounds, such as grass, soy milk, and silk. Although these methods are low-cost and green, it still suffers from a pretty low yield. Therefore, development of simple and efficient method for preparation of N-carbon materials is highly desired. On the other hand, detection of H2 O2 has also attracted much attention because H2 O2 plays an important role in chemistry, biology, clinical control and environmental protection [30–32]. Particularly, electrochemical technique is a promising tool for the construction of simple and low-cost sensors due to the high sensitivity, good selectivity, and ease of operation [33]. Up to now, the electrochemical H2 O2 sensors are involved of using enzymes, noble metals, metal oxides, porous material as sensing materials [34–37]. Recent research has shown that N-carbon materials exhibit electrocatalytic activity and can be used for detection of H2 O2 [2,18,38,39]. However, there are few reports on development of electrochemical

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H2 O2 sensors based on N-carbon [2,18,38,39]. Additionally, these sensors also suffer from the drawbacks for preparation of N-carbon materials mentioned above. In this paper, a simple and efficient method has been demonstrated to prepare N-carbon by low temperature heat treatment of hexamethylenetetramine (HMT) at 300 ◦ C in air. It should be noted that the method for preparation of N-carbon in present study exhibits some advantages of low synthesis temperature, simple synthesis process, without using insert gas, and high N content in final samples, compared to the methods previously reported. Most importantly, it is also found that N-carbon thus obtained exhibits electrocatalytic activity for reduction of H2 O2 , leading to a highperformance metal-free non-enzymatic H2 O2 sensor. 2. Experimental 2.1. Materials HMT, H2 O2 (30 wt.%), NaH2 PO4 , Na2 HPO4 , ascorbic acid (AA), uric acid (UA), glucose, ethanol and dopamine (DA) were purchased from Beijing Chemical Comp. All chemicals were used as received without further purification. The water used throughout all experiments was purified through a Millipore system. Phosphate buffer saline (PBS) was prepared by mixing stock solutions of NaH2 PO4 and Na2 HPO4 and a fresh solution of H2 O2 was prepared daily. 2.2. Preparation of the N-carbon N-carbon was prepared by heat treatment of HMT at 300 ◦ C for 30 min in air. In a typical run, 8 g of HMT was added into a 20 mL glassy bottle. After that, the mixture was heated in a box furnace in air to 300 ◦ C at a constant rate of 5 ◦ C min−1 and maintained for 30 min. After cooling to room temperature, the products were washed with water and ethanol twice by centrifugation at 8000 rpm for 10 min. The N-carbon materials were obtained by dryness under vacuum. 2.3. Characterizations Powder X-ray diffraction (XRD) datum was recorded on a RigakuD/MAX 2550 diffractometer with Cu K␣ radiation ˚ Infrared (IR) spectra were recorded with a Bruker ( = 1.5418 A). 66 V FTIR spectrometer. X-ray photoelectron spectroscopy (XPS) analysis was measured on an ESCALAB MK II X-ray photoelectron spectrometer using Mg as the exciting source. The morphology of the samples was observed by field emission scanning electron microscope (FE-SEM) on a JSM-6700F electron microscope (JEOL, Japan). Elemental analysis (C, H, N) was performed using a Perkin Elmer 2400 analyzer. Electrochemical measurements were performed with a CHI 660D electrochemical analyzer (CH Instruments, Inc., Shanghai). A conventional three-electrode cell was used, including a glassy carbon electrode (GCE, geometric area = 0.07 cm2 ) as the working electrode, a Ag/AgCl (saturated KCl) electrode as the reference electrode, and a platinum foil as the counter electrode. All the experiments were carried out at room temperature. 2.4. Electrode preparation and electrochemical measurements The modified electrodes were prepared by a simple casting method. Prior to the surface coating, the GCE was polished with 1.0 and 0.3 ␮m alumina powder, respectively, and rinsed with water, followed by sonication in ethanol solution and water successively. Then, the electrode was allowed to dry in a stream of N2 . After that, 3 ␮L of N-carbon aqueous dispersion (5 mg/mL) and 1 ␮L of 1%

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chitosan solution (in 1% acetic acid aqueous solution) was dropped on the clean surface of GCE, and dried at room temperature.

3. Results and discussion In this paper, a new method has been developed to prepare Ncarbon materials, which avoids the use of complicated equipment and high synthesis temperature, compared to the conventional synthesis methods. Fig. 1a shows the photographs of HMT (left) and the samples obtained by heat treatment of HMT at 300 ◦ C for 30 min in air (right). It is seen that HMT shows a typical white color. In contrast, the samples after heat treatment exhibit an obvious dark red color, which is different from the carbon materials obtained by the conventional high temperature pyrolysis method. Fig. 1b shows the XRD pattern of the samples thus obtained. It is seen that the samples exhibit a broad diffraction peak at 2 of 17.68◦ , which is different from that of amorphous carbon or graphitic carbon. According to the previous report on heat treatment of glucose at various temperatures, the samples thus obtained are composed of amorphous carbon composed of aromatic carbon sheets oriented in a considerably random fashion [40]. Fig. 1c shows SEM image of the samples, revealing that the size of samples are about several micrometers. Furthermore, there are some macropores in the samples, which is further confirmed by the corresponding high magnification SEM image, as shown in Fig. 1d. The formation of macropores may be attributed to the release of gases by decomposition of HMT during the heat treatment. Normally, the heat treatment of HMT at high temperature results in no solid products along with formation of large amount of gases. In the present work, HMT was heated at relatively low temperature (300 ◦ C), where HMT is partially decomposed along with releasing gases, leading to formation of carbon-based materials with macropores. In order to determine the structure, the samples were further examined by the XPS technique. Fig. 2a shows the XPS spectrum of samples. It is seen that samples exhibit three strong peaks at 284.0 eV, 398.2 eV and 532.2 eV, which are attributed to C1s, N1s and O1s, respectively, indicating that the samples are consisting of C, N and O elements [41]. Furthermore, other two weak peaks at 101.1 eV and 151.9 eV attributed to Si2p and Si2s are also observed, which are associated with the substrate used. Fig. 2b shows the C1s spectrum of the samples, revealing that the samples exhibit several peaks, which can be deconvoluted into four peaks at 284.6 eV, 285.6 eV, 286.2 eV and 288.4 eV, associated with C C, C O, C N, and C O/C N bands, respectively [42]. The corresponding N1s spectrum (Fig. 2c) exhibits a broad peak, which can be deconvoluted into three peaks at 398.4 eV, 400.0 eV, and 401.1 eV, associated with pyridinic N, pyrrolic N, and quaternary N, respectively [43]. The O1s spectrum of the samples also indicates that there are C O and C O bands in the samples (Fig. 2d) [33]. The composition of the N-carbon was tested by the elemental analysis, revealing that the composition of the N-carbon is C 52.36 wt.%, H 5.29 wt.%, N 27.86 wt.%, and O (calculated) 14.19 wt.%. The presence of O element in N-carbon may be attributed to the oxidation of HMT in air or H2 O, O2 , or CO2 molecules adsorbed on the surfaces of N-carbon. All these observations indicate the formation of N-carbon by low temperature heat treatment of HMT. It is well known that HMT is a typical N-containing organic molecule with content of N element about 40% (the chemical formula is C6 H12 N4 ). Upon heating in air, HMT can decompose and the final production is subjected to the heating conditions. To study the reaction mechanism, FT-TR spectra of HMT and N-carbon are examined, as shown in Fig. 3. It is seen that HMT exhibits several peaks attributed to the various functional groups, which are similar to those of HMT previously reported [44]. Although N-carbon also

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Fig. 1. (a) Photographs of HMT and N-carbon obtained by heat treatment of HMT at 300 ◦ C for 30 min, (b) XRD pattern, (c) low and (d) high magnification SEM images of N-carbon thus obtained.

Fig. 2. (a) XPS, (b) C1s, (c) N1s and (d) O1s spectra of N-carbon obtained by heat treatment of HMT at 300 ◦ C for 30 min.

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Fig. 3. (a) FT-IR spectra of HMT and N-carbon obtained by heat treatment of HMT at 300 ◦ C for 30 min, respectively.

shows several peaks similar to HMT, there are some peaks obviously changed after heat treatment. For example, the peak at 1463 cm−1 attributed to CH2 group [45] and the peak at 813 cm−1 attributed to CN heterocycle [46] disappear. The peaks at 1010 cm−1 and 1237 cm−1 attributed to C N band [46], at 687 cm−1 and 780 cm−1 attributed to N H or C H bands also obviously decrease [47]. Additionally, a new peak at 1159 cm−1 attributed to aromatic C H band is also observed [48], which is agreement with the XRD pattern for the formation of amorphous carbon consisting of aromatic carbon sheets. All these observations indicate that the formation of N-carbon may be attributed to the decomposition of N H or C H

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bands and simultaneously formation of aromatic structures. However, the exact formation mechanism of N-carbon from HMT is not completely understood at present time and requires further study. It is well known that the N-carbon materials are good candidates for electrochemical sensing, and thus the N-carbon thus obtained is used for construction of electrochemical sensors. To demonstrate the sensing application of the N-carbon, a non-enzymatic H2 O2 sensor was constructed by deposition of N-carbon dispersion on a GCE. Fig. 4a shows the cyclic voltammetrys (CVs) of a bare GCE in N2 -saturated 0.2 M PBS at pH 7.5 in the absence and presence of 1 mM of H2 O2 . It is seen that the response of bare GCE toward the reduction of H2 O2 is pretty weak, indicating no electrocatalytic activity of bare GCE toward reduction of H2 O2 . Fig. 4b shows the CVs of N-carbon modified GCE (designated as N-carbon/GCE) in the absence and presence of various concentrations of H2 O2 . It is seen that N-carbon/GCE exhibits no reduction peak in the absence of H2 O2 . Upon introduction of 1 mM H2 O2 , N-carbon/GCE exhibits a weak peak about −0.5 ␮A in intensity at −0.57 V. Further increasing H2 O2 concentration to 5 mM, a strong peak at −0.57 V with the intensity about −0.79 ␮A is observed. Additionally, the intensities of the reduction current for electrochemical reduction of H2 O2 significantly increase by increasing the concentrations of H2 O2 from 5 mM to 30 mM, indicating that the reduction current is related to the concentrations of H2 O2 . All these observations indicate that N-carbon thus obtained exhibits electrocatalytic ability toward reduction of H2 O2 and can be used to detect H2 O2 . Note that our H2 O2 sensor based on N-carbon avoids of using any noble metals or metal oxides, and process for preparation of N-carbon is much simpler than other conventional N-carbon-based electrochemical H2 O2 sensors [2].

Fig. 4. (a) Cyclic voltammetrys (CVs) of a bare GCE in N2 saturated 0.2 M PBS at pH 7.5 in the presence and absence of 1.0 mM H2 O2 ; (b) CVs of N-carbon/GCE in PBS 7.5 in the absence and in the presence of H2 O2 with the concentrations ranging from 1 to 30 mM; (c) typical steady-state response of the N-carbon/GCE to the successive injection of H2 O2 (from 0.1 mM to 40 mM) into stirred N2 saturated 0.2 M PBS at pH 7.5 at the applied potential of −0.3 V (inset shows the corresponding calibration plot for H2 O2 detection); (d) current-time curve for the N-carbon/GCE exposed to H2 O2 (1 mM) AA, DA, UA (0.1 mM) and glucose (5 mM) each.

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Table 1 Comparison of analytical performance of our proposed H2 O2 sensor with other published non-enzymatic H2 O2 sensors based on N-carbon materials. Materials

N-doped graphene N-doped graphene N-doped CNT Carbon nitride nanosheets Mesoporous carbon nitride N-carbon

Performance

References

LOD

Linear range

1 mM 10 nM 0.37 ␮M 2.0 ␮M 1.52 ␮M 90 ␮M

1–5 mM 10 nM to 2.8 mM 1.76–139 ␮M 0.1–90 mM 4–40 ␮M 0.1–40 mM

[2] [18] [38] [39] [51] This work

Fig. 4c shows the typical amperometric response of H2 O2 at the N-carbon/GCE in N2 -saturated 0.2 M PBS 7.5 buffer. When an aliquot of H2 O2 was added into the stirring PBS solution, Ncarbon/GCE responded rapidly to the substrate and the current rose steeply to reach a stable value. At the applied potential of −0.30 V, the cathode current of the sensor increased dramatically and achieved 95% of the steady state current within 2 s, revealing a fast amperometric response behavior. The inset in Fig. 4c shows the calibration curve of the sensor. The linear detection range is estimated to be from 0.1 mM to 40 mM (r = 0.996), and the detection limit is estimated to be 90 ␮M at a signal-to-noise ratio of 3. The catalytic activity of H2 O2 sensor based on N-carbon was also tested in the presence of interferences such as DA, AA, UA and glucose as shown in Fig. 4d. It is found that the current response was hardly discernible after the addition of DA, AA, UA and glucose respectively. In contrast, a well-defined H2 O2 response was obtained after the addition of H2 O2 , indicating the high selectivity of N-carbon/GCE toward detection of H2 O2 . It is well known that the HMT possesses high content of N element, which is a good candidate for preparation of N-carbon with high nitrogen content. Additionally, HMT is also useful in the synthesis of other chemical compounds, such as plastics, pharmaceuticals, rubber additives, indicating that HMT exhibits the advantage of low-cost. Therefore, HMT was used for preparation of N-carbon. Although the N-carbon materials can be used for fabrication of H2 O2 sensor, the sensing performance of sensitivity and selectivity is worse than those of catalse [49,50]. The sensing performances of our present H2 O2 sensors are compared with other previously reported non-enzymatic H2 O2 sensors based on carbon materials, as shown in Table 1. It is seen that the linear range of the sensor based on N-carbon is much larger than that of N-doped graphene [2] and the maximum detection concentration for H2 O2 of the sensor thus fabricated is also larger than that of N-doped graphene [18], N-doped CNT [38], and mesoporous carbon nitride [51]. Additionally, the H2 O2 sensor based on N-carbon exhibits good selectivity toward electrochemical reduction of H2 O2 , compared to other interferences (as shown in Fig. 4d). However, the effect of interferences on the detection of H2 O2 for the previous reported H2 O2 sensors based on other N-doped carbon materials has not been examined. Especially, the detection of H2 O2 by Ndoped carbon was performed at +0.30 V, which may be interfered by organic molecules such as DA, AA and UA [38]. Thus, the good selectivity of H2 O2 ensures that N-carbon can be used for fabrication of high performance H2 O2 sensor for practical applications. 4. Conclusions In summary, N-carbon has been successfully prepared by low temperature heat treatment (300 ◦ C) of HMT in air. The N-carbon exhibits good performance for electrochemical detection of H2 O2 . Our present study is important because it provides a simple and efficient method for preparation of N-carbon materials for electrochemical detection.

Acknowledgement This research work was financially supported by the National Natural Science Foundation of China (Grant No. 51202085).

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Biographies Sen Liu received his BS degree in 2005 in Chemistry and PhD degree in 2010 in Inorganic Chemistry from Jilin University. During the period of 2010–2012, he worked in Prof. Xuping Sun’s group as a postdoctoral research associate in State Key Lab of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. He joined in College of Electronic Science and Engineering at Jilin University in 2012. Now he is an associate professor in Jilin University and his current research is focused on the preparation of functional materials and their sensing applications. Bo Yu received his BS degree from the College of Electronics Science and Engineering, Jilin University, China in 2012. As an MS student, his research interests include sensing functional materials and devices. Teng Fei received his BS degree in 2005 in chemical engineering and technology and PhD degree in 2010 in polymer chemistry and physics from Jilin University, China. He is currently an associate professor in the College of Electronics Science and Engineering, Jilin University. His research interests include sensing functional materials and devices. Tong Zhang completed her MS degree in semiconductor materials in 1992 and her PhD in the field of microelectronics and solid-state electronics in 2001 from Jilin University. She was appointed as a full-time professor in the College of Electronics Science and Engineering, Jilin University in 2001. Her research interest is sensing functional materials, gas sensors, and humidity sensors.