- Email: [email protected]
ZnO nanosheets for highly efficient hydrogen peroxide sensing
Accepted Manuscript Title: Noble metal-free modified electrode of exfoliated graphitic carbon nitride/ZnO nanosheets for highly efficient hydrogen peroxide sensing Authors: Hailin Tian, Huiqing Fan, Jiangwei Ma, Longtao Ma, Guangzhi Dong PII: DOI: Reference:
S0013-4686(17)31501-3 http://dx.doi.org/doi:10.1016/j.electacta.2017.07.083 EA 29900
To appear in:
Electrochimica Acta
Received date: Accepted date:
9-6-2017 13-7-2017
Please cite this article as: Hailin Tian, Huiqing Fan, Jiangwei Ma, Longtao Ma, Guangzhi Dong, Noble metal-free modified electrode of exfoliated graphitic carbon nitride/ZnO nanosheets for highly efficient hydrogen peroxide sensing, Electrochimica Actahttp://dx.doi.org/10.1016/j.electacta.2017.07.083 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Noble metal-free modified electrode of exfoliated graphitic carbon nitride/ZnO nanosheets for highly efficient hydrogen peroxide sensing
Hailin Tian, Huiqing Fan,* Jiangwei Ma, Longtao Ma, Guangzhi Dong
State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, P. R. China
Graphical abstract
We designed and prepared the g-C3N4/ZnO/FTO electrode as a noble metal-free hydrogen peroxide sensor for the first time. The prepared g-C3N4/ZnO/FTO electrode revealed the high sensitivity of 540.8 μA•mM-1•cm-2 and wide linearity range from 0.05 to 14.15 mM, which was comparable with noble metal modified electrodes. And the g-C3N4/ZnO/FTO electrode also exhibited the reliable operational and environmental stability.
Highlights 1. The g-C3N4/ZnO nanosheets were synthesized as a noble metal-free
H2O2 sensor. 2. The high electrocatalytic property of g-C3N4/ZnO nanosheets was 1
demonstrated. 3. The g-C3N4/ZnO/FTO electrode revealed the high sensitivity of 540.8
μA·mM-1·cm-2. 4. The linear range was obtained from 0.05 to 14.15 mM towards H2O2.
ABSTRACT Graphitic carbon nitride (g-C3N4)/ZnO nanosheets were coated on the surface of fluorine-doped tin oxide (FTO) glass as a noble metal-free g-C3N4/ZnO/FTO electrode, which was used for the determination of hydrogen peroxide (H2O2). The exfoliated g-C3N4 nanosheets were firstly prepared and then ZnO nanosheets were grown on g-C3N4 nanosheets by a microwave-assisted hydrothermal synthesis. The g-C3N4 nanosheets combined with ZnO nanosheets can greatly improve the adsorption of hydrogen peroxide and increase the conductivity of materials as well. The g-C3N4/ZnO nanosheets were characterized by powder X-ray diffraction (XRD), scanning electron microscopic (SEM), transmission electron microscopic (TEM), and X-ray photoelectron spectroscopy (XPS) analysis. Meanwhile, electrochemical measurements of the g-C3N4/ZnO/FTO electrode were utilized to investigate the amperometric sensing of hydrogen peroxide. The prepared g-C3N4/ZnO/FTO electrode revealed the sensitivity of 540.8 μA·mM-1·cm-2 at −0.5 V with the linear response range from 0.05 to 14.15 mM, which even was superior to some 2
noble metal-decorated H2O2 sensors. Additionally, the g-C3N4/ZnO/FTO electrode also showed the reliable operational and environmental stability for electrochemical reduction of H2O2.
Keywords: Graphitic carbon nitride; Zinc oxide; Electrocatalysis; Hydrogen peroxide sensor
1. Introduction Hydrogen peroxide (H2O2) is the vital intermediate in chemical and food industries and also involved in our life process. Research on the quantitative detection of H2O2 received considerable attention in recent decades [1−5]. Though many noble metal-based H2O2 sensors possess good sensitivity and limit of detection (LOD), they are environmentally instable and high cost [6−9]. Therefore, the development of noble metal-free electrodes is significant to produce the practical hydrogen peroxide sensor with high sensitivity and wide linearity range. To date, carbonaceous materials or binary heterostructures of metal oxides and carbon materials, such as NCNTs [10], g-C3N4 nanosheets [11], MnO2/SWCNT-Nf [12], and RGO/ZnO [3], have been prepared for the modified electrode of noble metal-free H2O2 sensors. As a fascinating semiconductor material, bulk g-C3N4 can be prepared by the polycondensation of various precursors, such as cyanamide [13−15], 3
melamine [16] and other s-triazine heterocyclic compounds [17]. In particular, the exfoliated g-C3N4 nanosheets from bulk g-C3N4, a material that has not been much looked at from the perspective of 2D layered structure is in fact an excellent metal-free catalyst for various reactions [18−20]. Zinc oxide (ZnO) is a typical n-type oxide semiconductor with a band gap of 3.37 eV and has great potential applications in the determination of H2O2 [21−23]. To the best of our knowledge, there are no reports available for assembling g-C3N4 nanosheets and ZnO as the electrocatalyst of H2O2 sensors. Predictably, g-C3N4/ZnO nanosheets should be a promising electrocatalysis material of noble metal-free H2O2 sensors. Designing the shape and morphology of materials is a key step to extend their applications due to the direct correlations between the morphology and chemical reactivity [24,25]. For the H2O2 sensing applications, fascinating ZnO nanostructures with different morphologies, as well as ZnO heterostructures, have been studied widely [3,26]. Hierarchical nanostructures with controllable morphologies not only increase surface area, but more importantly can facilitate the adsorption and mass transfer in catalysis [27−29]. Therefore, g-C3N4/ZnO nanosheets, as a novel ZnO-based heterostructures, may effectively improve the electrocatalytic properties for H2O2 and elucidate the morphology-controlled properties. 4
In this work, we report the preparation of g-C3N4/ZnO nanosheets at 120 ºC for 2 h by a microwave-assisted hydrothermal synthesis as the modified electrode of noble metal-free H2O2 sensors for the first time. The chemical composition and morphology of g-C3N4/ZnO nanosheets are characterized by XRD, SEM, TEM, and XPS analysis. The g-C3N4/ZnO nanosheets sensor exhibits the high sensitivity and wide linearity range towards H2O2, which are attributed to their large surface area and fantastic morphology. 2. Experimental section 2.1. Materials All raw materials were analytical reagent grade and used without further purification. 2.2 Preparation of g-C3N4/ZnO nanosheets The g-C3N4 nanosheets were exfoliated by bulk g-C3N4 according to the previous work in our group [30]. Briefly, 2 g melamine and 10 g lithium chloride were mixed by the vigorously mechanical stirrer. Lithium chloride was an exfoliated reagent for g-C3N4 nanosheets. The mixed powder was calcined at 380 °С for 2 hours in air at ramp rate 6.7 °С per minute and heated to 550 °С for 4 hours at a same ramp rate. After grinding, the yellow powder of bulk g-C3N4/LiCl was obtained. Then, 0.5 g bulk g-C3N4/LiCl powder was dispersed in 500 mL deionized water under the magnetic stirrer of 1000 rpm for 24 hours. Finally, the 5
light yellow powder of exfoliated g-C3N4 nanosheets was collected from the dispersed suspension by centrifugation at 8000 rpm for 5 minutes. Then, the exfoliated g-C3N4 nanosheets (243 mg) were dispersed in 30 mL deionized water to form a light yellow solution under ultrasonic bath. The weight ratio of g-C3N4/ZnO was 1:1, which was an important reason for the morphology of g-C3N4/ZnO nanosheets. And 0.1 M zinc nitrate hexahydrate (Zn(NO3)2·6H2O) and 0.4 M sodium hydroxide (NaOH) were added to the above solution. After stirring for 30 minutes at room temperature, the solution was transferred into a 100 mL Teflon-lined autoclave and maintained at 120 °С for 2 h in the microwave workstation. (MDS-10, Sineo Microwave, Shanghai, China). The obtained precipitate was separated by centrifuging and washed with deionized water and ethanol several times and then dried at 70 °С for 8 h in an oven. 2.3. Characterizations The morphology of g-C3N4/ZnO nanosheets were observed by using field emission scanning electron microscopy (FE-SEM; JSM-6701F, JEOL, Tokyo, Japan), transmission electron microscopy (TEM; Tecnai F30G, FEI, Hillsboro, OR, USA) operating at an accelerating voltage of 200 kV. And the sample was loaded by Cu grid coated with carbon for TEM measurement. The thickness of the 2D material was analyzed by atomic force microscopy (AFM; SPM-9600, Shimadzu, Tokyo, Japan) in 6
air. AFM tip was Si3N4 and mica substrate was used for AFM work. The crystal structure of the as-synthesized material was analyzed by using powder X-ray diffraction (XRD; X’pert PRO MPD, Philips, Eindhoven, The Netherlands) with Cu Kα radiation (λ = 1.5406 Å) in the range of 10-80°. Surface chemical element analysis of g-C3N4/ZnO nanosheets was carried out by using X-ray photoelectron spectroscopy (XPS; VG ESCALA-B220i-XL, Thermo Scientific, Surrey, UK) with an Al Kα (hv = 1486.6 eV) source at a residual gas pressure below 10-8 Pa. Nitrogen adsorption-desorption was performed on a nitrogen adsorption apparatus at 77 K (V-Sorb 2800P, Gold APP Corp., Beijing, China), the sample was degassed at 120 °С for 2 h before measurement. Electrochemical measurements were all operated on a CHI 660E electrochemical analyzer (Chenhua Co., Shanghai, China) with a conventional three-electrode method, using a Pt plate as the counter electrode and an Ag/AgCl electrode (saturated with KCl) as the reference electrode. The working electrode was fabricated as follows. Briefly, 50 mg of g-C3N4/ZnO nanosheets, 30 mg of ethyl cellulose, 20 mg of lauric acid and 350 mg terpilenol were mixed and ground in an agate mortar to form a paste. And then the paste was coated on FTO glass via a doctor blade method, using Scotch tape as the spacer. Subsequently, the coated film was dried in an infrared lamp for 5 minutes and baked at 350 °С for 1 h. The exposed area of the working electrode was 1 × 1 cm. Phosphate buffer solution 7
(PBS, 0.2 M, pH 7.4) was used as the supporting electrolyte, and the electrolyte was stirred by the magnetic stirrer with 100 per minute when the determination of H2O2 was performed. The oxygen reduction occurred on the g-C3N4/ZnO/FTO electrode may interfere with the response of H2O2 reduction. In order to obtain the accurate quantitative determination of H2O2, high-purity nitrogen was employed to remove oxygen of the solution (Fig. S1). 3. Results and discussion The formation process of g-C3N4/ZnO nanosheets is illustrated in Fig. 1(a). The exfoliated g-C3N4 nanosheets as the substrate can promote the -
growth of ZnO nanosheets. And the concentration ratio of Zn2+ and OH ions is 1:4, which is a key parameter to form the hierarchical ZnO nanosheet flowers. The detection of the H2O2 sensing is operated on an
electrochemical analyzer with a conventional three-electrode method in Fig. 1(b). The catalytic current due to the electrochemical reduction of H2O2 was recorded by using cyclic voltammetry and the amperometric current-time response, when H2O2 molecules were adsorbed and diffused on the surface of the working electrode. Therefore, as shown in Fig. 1(c), the sensitive detection of H2O2 can be widely applied in many fields such as biosensing and chemical sensing applications. The phase composition of g-C3N4/ZnO nanosheets is characterized by XRD in Fig. 2. In the case of g-C3N4 nanosheets, the strong XRD peak at 27.6° is a characteristic 8
interlayer stacking reflection of conjugated aromatic systems, indexed for graphitic materials as the (002) planes [13]. And an additional peak at 13° can be attributed to an in-planar repeat period of 0.678 nm in the crystal [14]. The hexagonal wurtzite structure of ZnO can be confirmed by the standard card (JCPDS 36-1451). In g-C3N4/ZnO nanosheets, all peaks can be assigned to g-C3N4 nanosheets and ZnO phase, respectively. And no diffraction peaks from impurities can be detected. The intensity ratio of the broad peak at 13° and (002) peak at 27.6° is decreased due to ZnO reveals the strong peaks in g-C3N4/ZnO nanosheets. The position of two peaks has no obvious shift in comparison of g-C3N4 nanosheets, indicating that the layered structure of g-C3N4 nanosheets is stable during the preparation of g-C3N4/ZnO nanosheets. As shown in Fig. 3(a), ZnO nanosheets can be recognized from the exfoliated g-C3N4 nanosheets according to color contrast. And the thin ZnO nanosheets are uniformly dispersed on the laminar morphology of g-C3N4 nanosheets. In Fig. 3(b), the morphology of g-C3N4/ZnO nanosheets was further observed by TEM. The thin nanosheets stack up hierarchical ZnO with flower-like nanostructures. And g-C3N4/ZnO nanosheets exhibit the rich inner-channels in the observation of TEM. Compared to g-C3N4/ZnO nanosheets, SEM and TEM images of only g-C3N4 nanosheets are also given in the supplementary material (Fig. S2). The exfoliated g-C3N4 nanosheets are transparent to electron beams, and cross-sectional atomic 9
force microscopy (AFM) image of g-C3N4 nanosheets is given in Fig. 3(c). The typical AFM image and thickness analyses reveal g-C3N4 nanosheets have a uniform thickness of 4-5 nm, as the plane substrate material, g-C3N4 nanosheets are favorable to the growth of ZnO nanosheets in the process of hydrothermal synthesis. XPS investigation was carried out to determine the chemical status and composition of elements in g-C3N4/ZnO nanosheets. XPS survey scan spectrum of g-C3N4/ZnO nanosheets is given in the supplementary material [Fig. S3]. In Fig. 4(a), the binding energy of Zn 2p can be fitted into two distinct peaks of Zn 2p3/2 and 2p2/1, and the spin-orbit splitting of them is 23.3 eV, which is in agreement with the value of ZnO [31]. As shown in Fig. 4(b), the O 1s high-resolution spectrum can be deconvolved into 529.7 and 531.6 eV peaks, which belong to the lattice oxygen of ZnO and the surface hydroxyl groups (−OH) or oxygen (O2) adsorbed on g-C3N4/ZnO nanosheets, respectively. The regional spectrum of C1s shows three peaks at 284.6, 286.5 and 288.4 eV in Fig. 4(c), which are attributed to graphite carbon atoms, carbon atoms attached to the −NH2 group and the sp2-bonded carbon (N−C=N) inside the aromatic structure. In Fig. 4(d), the high resolution N 1s spectrum of g-C3N4/ZnO nanosheets can be deconvoluted into three peaks. The peaks at 398.5 and 400.6 eV are attributed to sp2-hybridized nitrogen (C−N=C) and tertiary nitrogen (N−(C)3) in g-C3N4, whereas the peak at 404.3 eV can be 10
assigned to amino functional groups in aromatic rings (Ar−N−H) [32]. Atomic ratio of all elements in g-C3N4/ZnO nanosheets from XPS spectra is listed in Table S1. Based on the above discussion, it is demonstrated that g-C3N4/ZnO nanosheets have been successfully synthesized and phase compositions of g-C3N4 and ZnO are stable during the preparation of g-C3N4/ZnO nanosheets. Additionally, the prepared g-C3N4/ZnO nanosheets are investigated the electrocatalytic performance as a H2O2 sensor. In Fig. 5(a), it is seen that g-C3N4/ZnO/FTO (curve A) exhibits a weak cathodic current density in the absence of H2O2 and the maximum reduction current density of bare FTO (curve B) is also pretty weak in the presence of 5 mM H2O2. Correspondingly, the maximum reduction current density is gradually increased with the addition of 5 mM H2O2 in the case of ZnO/FTO (curve C), g-C3N4/FTO (curve D), and g-C3N4/ZnO/FTO (curve E) in 0.2 M PBS (pH 7.4). The g-C3N4/ZnO/FTO electrode reveals the biggest current density of 2.15 mA·cm-2 towards the reduction of H2O2. ZnO has a good electroconductivity to promote the electron transfer and then improve the electrochemical reduction of H2O2 in g-C3N4/ZnO nanosheets [Fig. S4]. The above results indicate that g-C3N4/ZnO nanosheets can be used as a highly efficient electrocatalyst towards H2O2 and the synergistic effect of g-C3N4 and ZnO nanosheets can obviously improve the electrocatalytic performance. To study the effect of scan rates on the electrocatalytic 11
performance, cyclic voltammetry (CV) curves of the g-C3N4/ZnO/FTO electrode were further examined in 0.2 M PBS (pH 7.4) containing 5 mM H2O2 at various scan rates of 30, 50, 90, 130, 170, 220, 270, and 350 mV·s-1. As shown in Fig. 5(b), the peak reduction current density increases with the scan rate at an applied potential of –0.5 V. And the linear relationship can be seen between the peak current density and the square root of scan rates in the inset of Fig. 5(b), indicating that the electrochemical reaction is controlled by the diffusion of the analyte from solution to the electrode surface. And the number of electrons transferred in the rate-determining step (nα) and the electrons transfer coefficient (α) can be obtained as the following Eq. (1): αnα = (47.7 mV)/(∣ Ep − Ep/2 ∣)
(1)
where Ep and Ep/2 are the peak potential and the potential at the half maximum peak current, respectively. The value of αnα is 0.318 in g-C3N4/ZnO nanosheets, indicating that the rate-determining step involves one electron transfer, and the value of α range between 0.3 and 0.7 [33]. The mechanism of H2O2 electrochemical reduction on the g-C3N4/ZnO/FTO electrode can be proposed as the following [34,35]: H2 O2 + e− → OHad + OH −
(2)
OHad + e− → OH −
(3)
2OH − + 2H + → 2H2 O
(4)
The formation of OHad is a key step controlling the reaction rate in the 12
reduction of H2O2. The CV curve of the g-C3N4/ZnO/FTO electrode in Fig. 5(a) represents an obvious increase of the current density within the potential range between -0.6 and -0.1 V after adding 5 mM H2O2. The onset potentials of H2O2 electrochemical reduction and chemisorption of OH- anion [34] on the electrode were almost the same, which may support the proposed mechanism in Eqs. (2)-(4) for the g-C3N4/ZnO/FTO electrode. Amperometric current-time plot of g-C3N4/ZnO/FTO was obtained with dropping different H2O2 concentrations into 0.2 M PBS (pH 7.4) at an applied potential of −0.5 V. Fig. 6(a) shows the reduction current density of g-C3N4/ZnO/FTO is increased with the addition of H2O2 concentrations and presents a rapid response time about 3−4 s, which is contributed to the abundant inner-channels of g-C3N4/ZnO nanosheets. The linear range can be obtained between logarithm current density (log Ip) and logarithm H2O2 concentration (log C) from 0.05 to 14.15 mM in the inset of Fig. 6(a). From the slope of 0.571 in the calibration curve, the sensitivity of g-C3N4/ZnO/FTO is determined to be 540.8 μA·mM-1·cm-2 [10,36]. Additionally, nitrogen adsorption isotherms of g-C3N4/ZnO nanosheets and the corresponding pore size distributions are presented in Fig. 6(b). The isotherms are type IV with an H1 loop according to the IUPAC classification [37]. And a small desorption branch shift at the high relative pressure region reflects some opening pores (interconnection) 13
exist in g-C3N4/ZnO nanosheets, which is in accordance with the analytic result of TEM observation and the previous report [38]. The BET surface area and the total pore volume are 16.5 m2·g-1 and 0.33 cm3·g-1, respectively. The average pore size of g-C3N4/ZnO nanosheets is 15.8 nm in the inset of Fig. 6(b). Therefore, H2O2 molecule can easily transfer on the surface of g-C3N4/ZnO nanosheets through those apertures. In Fig. 7(a), the limit of detection (LOD), based on signal-to-noise ratio of 3, is calculated to be 1.7 μM for the reduction of H2O2 using the following Eq. (5): LOD = SDbackground /S
(5)
where S is the sensitivity of the g-C3N4/ZnO/FTO electrode, and SD is the standard deviation of the background current density [12]. Table 1 shows a partial list of properties on the amperometric H2O2 sensor, using noble metal modified and noble metal-free modified electrodes. It can be seen that the g-C3N4/ZnO/FTO electrode in this study reveals the good sensitivity and wide linearity range towards H2O2, which is comparable with or higher than some noble metal modified electrodes. The selectivity of H2O2 sensor was also evaluated against many probable interferential analytes, such as ethanol, Na2SO4, KCl, ascorbic acid (AA), glucose, and urea. As shown in Fig. 7(b), the reduction current density of g-C3N4/ZnO/FTO is apparently changed in 0.2 M PBS (pH 7.4) containing 0.5 mM H2O2 at −0.5 V. In contrast, no obvious current 14
response is observed to the addition of other analytes, except 3 mM Na2SO4. In order to accurate quantitative determination of H2O2, the high-concentration Na2SO4 solution need be avoided for the g-C3N4/ZnO/FTO electrode in practical applications. The pH value of the buffer solution is an important factor to affect the electrocatalytic reduction of H2O2. We have investigated the effect of the pH value of the buffer solution on the electrochemical response of H2O2 for the g-C3N4/ZnO/FTO electrode. Fig. 8 shows the maximum reduction current density of the g-C3N4/ZnO nanosheets electrode in 5 mM H2O2 at different pH values of the solution. The current density is gradually increased in the pH value from 5.7 to 7.4. As we know, the g-C3N4/ZnO nanostructures will be destroyed due to ZnO is unstable in the acidic solution. On the other hand, when the pH value is added to 8.0, the current density will decrease again because the catalytic activity and stability of H2O2 are inhibited in alkaline solution [42]. Therefore, the pH value of 7.4 was optimized for the g-C3N4/ZnO/FTO electrode on the electrochemical response of H2O2. Additionally, we have also studied the related experiments with different solution temperatures when the g-C3N4/ZnO nanosheets electrode existed in 0.2 M PBS (pH 7.4) containing 5 mM H2O2. As shown in Fig. 8, the reduction current density of H2O2 at the same potential is increased in the whole temperature range from 22 to 60 °С because the activation energy of H2O2 reduction is low 15
in the elevated temperature [34]. Compared to enzyme-modified sensors of H2O2, the g-C3N4/ZnO/FTO electrode can act as the stable sensor of H2O2 at a wide temperature range. The reproducibility of the g-C3N4/ZnO/FTO electrode for electrochemical response of hydrogen peroxide was conducted by using various equivalent electrodes in 0.2 M PBS (pH 7.4) containing 5 mM H2O2 at 100 mV·s-1 (Fig. S5). It can be found that a deviation is less than 12%, indicating that the g-C3N4/ZnO/FTO electrode possess acceptable reproducibility. To evaluate the potential of the g-C3N4/ZnO/FTO electrode for determination of hydrogen peroxide in real samples, different real samples such as drinking water, tape water and serum sample containing certain H2O2 were tested by using the same electrochemical method in the paper. The detection results are listed in Table S2. It is demonstrated that the g-C3N4/ZnO/FTO electrode revealed good selectivity with a reasonable deviation lower than 16% even in real samples. The long-term stability of the prepared g-C3N4/ZnO/FTO electrode is a critical parameter in practical applications. To evaluate the stability of the g-C3N4/ZnO/FTO electrode, the stability was examined by periodical measurements for number of cycles and time in 0.2 M PBS (pH 7.4) containing 5 mM H2O2 at a scan rate of 100 mV·s-1. As shown in Fig. 9(a), the maximum reduction current density is stable on the 11 cycle and no apparent shift can be seen after 100 cycles. This result indicated 16
the reaction was an adsorption controlled process and it obeyed the Nernstian condition for adsorption controlled reactions [43]. The variation of the maximum response current density is about 9% of its initial value after 37 days in Fig. 9(b). The reason of the enhanced stability can be attributed to the stable structure of g-C3N4/ZnO nanosheets, indicating that the collapse of nanostructures has been prevented during the electrochemical reduction of H2O2. And the g-C3N4/ZnO nanosheet is the nonenzymatic and noble metal-free electrode, which means the g-C3N4/ZnO/FTO electrode shows the reliable operational and environmental stability. 4. Conclusions The g-C3N4/ZnO nanosheets were prepared by a simple microwave-assisted hydrothermal synthesis. The exfoliated g-C3N4 nanosheets promoted the grown process of ZnO nanosheets. And the layer structure of g-C3N4/ZnO nanosheets was favorable to the adsorption and diffusion of H2O2 molecules. The morphology and chemical compositions of g-C3N4/ZnO nanosheets were confirmed. As a green and low-cost noble metal-free modified H2O2 sensor, the g-C3N4/ZnO/FTO electrode exhibited the high sensitivity, wide linearity range, and reliable stability, which might be replaced the noble metal modified H2O2 sensors in some ways.
17
Acknowledgment This work was supported by the National Natural Science Foundation (51672220), and 111 Program (B08040) of MOE, the National Defense Science Foundation (32102060303), the Fundamental Research Funds for the Central Universities (3102014JGY01004) of China.
References [1] K.K. Tang, X.F. Wu, G.D. Wang, L.X. Li, S.Y. Wu, X.T. Dong, Z.L. Liu, B. Zhao, One-step preparation of silver nanoparticle embedded amorphous carbon for nonenzymatic hydrogen peroxide sensing, Electrochem. Commun. 68 (2016) 90−94. [2] L.C. Jiang, W.D. Zhang, Electrodeposition of TiO2 nanoparticles on multiwalled carbon nanotube arrays for hydrogen peroxide sensing, Electroanalysis 21 (2009) 988−993. [3] S. Palanisamy, S.M. Chen, R. Sarawathi, A novel nonenzymatic hydrogen peroxide sensor based on reduced graphene oxide/ZnO composite modified electrode, Sens. Actuators B 166−167 (2012) 372−377. [4] K. Dhara, T. Ramachandran, B.G. Nair, T.G. Satheesh, Au nanoparticles decorated reduced graphene oxide for the fabrication of disposable nonenzymatic hydrogen peroxide sensor, J. Electroanal. Chem. 764 (2016) 64−70. 18
[5] W. Chen, S. Cai, Q.Q. Ren, W. Wen, Y.D Zhao, Recent advances in electrochemical sensing for hydrogen peroxide: a review, Analyst 137 (2012) 49−58. [6] B. Zhao, Z. Liu, W. Fu, H. Yang, Construction of 3D electrochemically reduced graphene oxide-silver nanocomposite film and application as nonenzymatic hydrogen peroxide sensor, Electrochem. Commun. 27 (2013) 1−4. [7] J. Xu, X.W. Wei, X.J. Song, X.J. Lu, C.C. Ji, Y.H. Ni, G.C. Zhao, Synthesis and electrocatalytic activity of multi-walled carbon nanotubes/Cu-Ag nanocomposites, J. Mater. Sci. 42 (2007) 6972−6976. [8] J. Wang, L. Wang, J. Di, Y. Tu, Electrodeposition of gold nanoparticles on indium/tin oxide electrode for fabrication of a disposable hydrogen peroxide biosensor, Talanta 77 (2009) 1454−1459. [9] H. Cao, X. Sun, Y. Zhang, N. Jia, Electrochemical sensing based on gold nanoparticle-decorated halloysite nanotube composites, Anal. Biochem. 430 (2012) 111−115. [10] X. Xu, S.J. Jiang, Z. Hu, S.Q. Liu, Nitrogen-doped carbon nanotubes: high electrocatalytic activity toward the oxidation of hydrogen peroxide and its application for biosensing, ACS Nano 4 (2010) 4292−4298. 19
[11] J.Q. Tian, Q. Liu, C.J. Ge, Z.C. Xing, A.M. Asiri, A.O. Al-Youbi, X.P. Sun, Ultrathin graphitic carbon nitride nanosheets: a low-cost, green, and highly efficient electrocatalyst toward the reduction of hydrogen peroxide and its glucose biosensing application, Nanoscale 5 (2013) 8921−8924. [12] A.B.M. Zakaria, D. Leszczynska, Novel design of non-enzymatic sensor for rapid monitoring of hydrogen peroxide in water matrix, J. Electroanal. Chem. 766 (2016) 30−36. [13] X.C. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J.M. Carlsson, K. Domen, M. Antonietti, A metal-free polymeric photocatalyst for hydrogen production from water under visible light, Nat. Mater. 8 (2009) 76−80. [14] F. Goettmann, A. Fischer, M. Antonietti, A. Thomas, Chemical synthesis of mesoporous carbon nitrides using hard templates and their use as a metal-free catalyst for Friedel-Craftes reaction of benzene, Angew. Chem. Int. Ed. 45 (2006) 4467–4471. [15] M. Groenewolt, M. Antonietti, Synthesis of g-C3N4 nanoparticles in mesoporous silica host matrices, Adv. Mater. 17 (2005) 1789–1792. [16] J.Q. Tian, Q. Liu, A.M. Asiri, A.O. Al-Youbi, X.P. Sun, Ultrathin graphitic carbon nitride nanosheets: a highly efficient fluorosensor for rapid, ultrasensitive detection of Cu2+, Anal. Chem. 85 (2013) 5595–5599. 20
[17] B. Jurgens, E. Irran, J. Senker, P. Kroll, H. Muller, W. Schnick, Melem (2,5,8-triamino-tri-s-triazine), an important intermediate during condensation of melamine rings to graphitic carbon nitride: synthesis, structure determination by X-ray powder diffractometry, solid-state NMR, and theoretical studies, J. Am. Chem. Soc. 125 (2003) 10288–10300. [18] P. Niu, L.L. Zhang, G. Liu, H.M. Cheng, Graphene-like carbon nitride nanosheets for improved photocatalytic activities, Adv. Funct. Mater. 22 (2012) 4763–4770. [19] S.B. Yang, X.L. Feng, X.C. Wang, K. Müllen, Graphene-based carbon nitride nanosheets as efficient metal-free electrocatalysts for oxygen reduction reaction, Angew. Chem. Int. Ed. 50 (2011) 5339– 5343. [20] K.K.R. Datta, B.V.S. Reddy, K. Ariga, A. Vinu, Gold nanoparticles embedded in a mesoporous carbon nitride stabilizer for highly efficient three-component coupling reaction, Angew. Chem. Int. Ed. 49 (2010) 5961–5965. [21] N.H.A. Hardan, M.A.A. Hamid, R. Shamsudin, N.K. Othman, L.K. Keng, Amperometric non-enzymatic hydrogen peroxide sensor based on aligned zinc oxide nanorods, Sensors 16 (2016) 1004– 1012. [22] X.L. Zhu, I. Yuri, X. Gan, I. Suzuki, G.X. Li, Electrochemical study 21
of the effect of nano-zinc oxide on microperoxidase and its application to more sensitive hydrogen peroxide biosensor preparation, Biosens. Bioelectron. 22 (2007) 1600–1604. [23] M. Hussain, H. Sun, S. Karim, A. Nisar, M. Khan, A.U. Haq, M. Iqbal, M. Ahmad, Noble metal nanoparticle-functionalized ZnO nanoflowers for photocatalytic degradation of RhB dye and electrochemical sensing of hydrogen peroxide, J. Nanopart. Res. 18 (2016) 95–109. [24] J.I. Sohn, W.K. Hong, S. Lee, S. Lee, J. Ku, Y.J. Park, J. Hong, S. Hwang, K.H. Park, J.H. Warner, Surface energy-mediated construction of anisotropic semiconductor wires with selective crystallographic polarity, Sci. Rep. 4 (2014) 1–7. [25] N. Tian, Z.Y. Zhou, S.G. Sun, Y. Dong, Z.L. Wang, Synthesis of tetrahexahedral platinum nanocrystals with high-index facets and high electro-oxidation activity, Science 316 (2007) 732–735. [26] Q. Wang, J. Zheng, Electrodeposition of silver nanoparticles on a zinc oxide film: improvement of amperometric sensing sensitivity and stability for hydrogen peroxide determination, Microchim Acta 169 (2010) 361–365. [27] Q. Zeng, J. Cheng, L. Tang, X. Liu, Y. Liu, J. Li, J. Jiang, Self-assembled graphene-enzyme hierarchical nanostructures for electrochemical biosensing, Adv. Funct. Mater. 20 (2010) 3366– 22
3372. [28] W. Zhang, R. Li, L. Xing, X. Wang, X. Gou, Carnation-like CuO hierarchical nanostructures assembled by porous nanosheets for nonenzymatic glucose sensing, Electroanalysis 28 (2016) 2214−2221. [29] H.L. Tian, H.Q. Fan, M.M. Li, L.T. Ma, Zeolitic imidazolate framework coated ZnO nanorods as molecular sieving to improve selectivity of formaldehyde gas sensor, ACS Sens. 1 (2016) 243– 250. [30] L.T. Ma, H.Q. Fan, J. Wang, Y.W. Zhao, H.L. Tian, G.Z. Dong, Water-assisted ions in situ intercalation for porous polymeric graphitic carbon nitride nanosheets with superior photocatalytic hydrogen evolution performance, Appl. Catal. B 190 (2016) 93–102. [31] L. Xu, R.F. Zheng, S.H. Liu, J. Song, J.S. Chen, B. Dong, H.W. Song, NiO@ZnO heterostructured nanotubes: coelectrospinning fabrication, characterization, and highly enhanced gas sensing properties, Inorg. Chem. 51 (2012) 7733–7740. [32] S.B. Yang, Y.J. Gong, J.S. Zhang, L. Zhan, L.L. Ma, Z.Y. Fang, R. Vajtai, X.C. Wang, P.M. Ajayan, Exfoliated graphitic carbon nitride nanosheets as efficient catalysts for hydrogen evolution under visible light, Adv. Mater. 25 (2013) 2452–2456. [33] C.Y. Lin, Y.H. Lai, A. Balamurugan, R. Vittal, C.W. Lin, K.C. Ho, 23
Electrode modified with a composite film of ZnO nanorods and Ag nanoparticles as a sensor for hydrogen peroxide, Talanta 82 (2010) 340–347. [34] F.H. Meng, X.L. Yan, J.G. Liu, J. Gu, Z.G. Zou, Nanoporous gold as non-enzymatic sensor for hydrogen peroxide, Electrochim. Acta 56 (2011) 4657–4662. [35] Y.X. Li, J. Ma, Z.F. Ma, Synthesis of gold nanostars with tunable morphology and their electrochemical application for hydrogen peroxide sensing, Electrochim. Acta 108 (2013) 435–440. [36] W.J. Kang, X. Pei, C.A. Rusinek, A. Bange, E.N. Haynes, W.R. Heineman, I. Papautsky, Determination of lead with a copper-based electrochemical sensor, Anal. Chem. 89 (2017) 3345–3352. [37] K.S.W. Sing, Reporting physisorption data for gas/solid systems-with special reference to the determination of surface area and porosity, Pure & Appl. Chem. 57 (1985) 603–619. [38] S.B. Yoon, G.S. Chai, S.K. Kang, J.S. Yu, K.P. Gierszal, M. Jaroniec, Graphitized pitch-based carbons with ordered nanopores synthesized by using colloidal crystals as templates, J. Am. Chem. Soc. 127 (2005) 4188–4189. [39] M.R. Guascito, E. Filippo, C. Malitesta, D. Manno, A. Serra, A. Turco, A new amperometric nanostructured sensor for the analytical determination of hydrogen peroxide, Biosens. Bioelectron. 24 (2008) 24
1057–1063. [40] S. Wu, H.T. Zhao, H.X. Ju, C.G. Shi, J.W Zhao, Electrodeposition of silver-DNA hybrid nanoparticles for electrochemical sensing of hydrogen peroxide and glucose, Electrochem. Commun. 8 (2006) 1197–1203. [41] S.B. Khan, M.M. Rahman, A.M. Asiri, S.A.B. Asif, S.A.S. Al-Qarni, A.G. Al-Sehemi, S.A. Al-Sayari, M.S. Al-Assiri, Fabrication of non-enzymatic sensor using Co doped ZnO nanoparticles as a maker of H2O2, Phys. E Low Dimens. Syst. Nanostruct. 62 (2014) 21–27. [42] Y.Q. Sun, L.F. Hang, D.D. Men, H.L. Li, D.L. Liu, X.Y. Li, L.L. Wen, Y. Li, Periodic nanostructured Au arrays on an Si electrode for high-performance electrochemical detection of hydrogen peroxide without an enzyme, J. Mater. Chem. C 4 (2016) 9864–9871. [43] X.P. Zhang, D. Liu, B. Yu, T.Y. You, A novel nonenzymatic hydrogen peroxide sensor based on electrospun nitrogen-doped carbon nanoparticles-embedded carbon nanofibers film, Sens. Actuators B 224 (2016) 103–109.
25
Figure captions Fig. 1 (a) Schematic diagrams of the g-C3N4/ZnO nanosheets formation process. (b) Photograph of the g-C3N4/ZnO powder and g-C3N4/ZnO/FTO electrode, and the electrochemical measurement and (c) applications of H2O2 sensing are illustrated.
Fig. 2 XRD patterns of g-C3N4 nanosheets, ZnO, and g-C3N4/ZnO nanosheets.
26
Fig. 3 (a) SEM image of g-C3N4/ZnO nanosheets. (b) TEM image of g-C3N4/ZnO nanosheets. (c) AFM image of g-C3N4 nanosheets and the thickness of g-C3N4 nanosheets.
Fig. 4 XPS high-resolution spectra and fitted curves of Zn 2p (a), O 1s (b), C 1s (c), and N 1s (d).
27
Fig. 5 (a) Cyclic voltammetry of the g-C3N4/ZnO nanosheets electrode (A) in the absence of H2O2, and cyclic voltammetry of bare FTO (B), ZnO (C), g-C3N4 nanosheets (D), and g-C3N4/ZnO nanosheets (E) electrodes in 0.2 M PBS (pH 7.4) in the presence of 5 mM H2O2. Scan rate is 100 mV·s-1. (b) Cyclic voltammetry of the g-C3N4/ZnO nanosheets electrode in 0.2 M PBS (pH 7.4) containing 5 mM H2O2 at different scan rates. Inset is the relationship between the peak current density and the square root of scan rates.
28
Fig. 6 (a) Amperometric current-time plot of the g-C3N4/ZnO nanosheets electrode to successive addition of H2O2 in different concentrations in 0.2 M PBS (pH 7.4) at an applied potential of –0.5 V. Inset is the calibration curve of steady-state current against H2O2 concentration. (b) Nitrogen adsorption isotherms at 77 K for g-C3N4/ZnO nanosheets, inset shows the pore size distribution and pore volume of g-C3N4/ZnO nanosheets.
29
Fig. 7 (a) The limit of detection of the g-C3N4/ZnO nanosheets electrode measured in 0.2 M PBS (pH 7.4) at –0.5 V. (b) Amperometric response of the g-C3N4/ZnO nanosheets electrode for various concentrations of analytes in 0.2 M PBS (pH 7.4) at an applied potential of –0.5 V.
30
Fig. 8 The variation of the maximum response current density for the g-C3N4/ZnO nanosheets electrode in 5 mM H2O2 at different pH values and temperatures of PBS.
31
Fig. 9 (a) The variation of the maximum response current density for the g-C3N4/ZnO nanosheets electrode in 0.2 M PBS (pH 7.4) containing 5 mM H2O2 at a scan rate of 100 mV·s-1 after 100 cycles. Inset is cyclic voltammetry of the g-C3N4/ZnO nanosheets electrode in 0.2 M PBS (pH 7.4) containing 5 mM H2O2 at different cycles. (b) The stability of the g-C3N4/ZnO nanosheets electrode in 0.2 M PBS (pH 7.4) containing 5 mM H2O2 at a scan rate of 100 mV·s-1 for 37 days. Inset is cyclic voltammetry of the g-C3N4/ZnO nanosheets electrode in 0.2 M PBS (pH 7.4) containing 5 mM H2O2 at different days.
32
Table 1 Comparison of g-C3N4/ZnO/FTO with other modified electrodes on electrochemical H2O2 sensing. Type of the electrode
Potential (V)
Sensitivity (μA·mM-1·cm-2)
LOD (μM)
Linear range (mM)
Ref.
AgNPs/PVA/Pt AgNPs/DNA/GCE PDDA/AuNSs/GCE AgNPs/ZnONRs/FTO g-C3N4/GCE Co doped ZnO/GCE g-C3N4/ZnO/FTO
-0.5 -0.45 -0.4 -0.55 -0.3 -0.3 -0.5
4090 773 60.2 152.1 92.4 540.8
1.0 0.6 9.7 0.9 2 14.3 1.7
0.04-6 0.002-2.5 0.02-2.5 0.008-0.983 0.1-90 5-20 0.05-14.15
[39] [40] [35] [33] [11] [41] This work
33
S0013-4686(17)31501-3 http://dx.doi.org/doi:10.1016/j.electacta.2017.07.083 EA 29900
To appear in:
Electrochimica Acta
Received date: Accepted date:
9-6-2017 13-7-2017
Please cite this article as: Hailin Tian, Huiqing Fan, Jiangwei Ma, Longtao Ma, Guangzhi Dong, Noble metal-free modified electrode of exfoliated graphitic carbon nitride/ZnO nanosheets for highly efficient hydrogen peroxide sensing, Electrochimica Actahttp://dx.doi.org/10.1016/j.electacta.2017.07.083 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Noble metal-free modified electrode of exfoliated graphitic carbon nitride/ZnO nanosheets for highly efficient hydrogen peroxide sensing
Hailin Tian, Huiqing Fan,* Jiangwei Ma, Longtao Ma, Guangzhi Dong
State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, P. R. China
Graphical abstract
We designed and prepared the g-C3N4/ZnO/FTO electrode as a noble metal-free hydrogen peroxide sensor for the first time. The prepared g-C3N4/ZnO/FTO electrode revealed the high sensitivity of 540.8 μA•mM-1•cm-2 and wide linearity range from 0.05 to 14.15 mM, which was comparable with noble metal modified electrodes. And the g-C3N4/ZnO/FTO electrode also exhibited the reliable operational and environmental stability.
Highlights 1. The g-C3N4/ZnO nanosheets were synthesized as a noble metal-free
H2O2 sensor. 2. The high electrocatalytic property of g-C3N4/ZnO nanosheets was 1
demonstrated. 3. The g-C3N4/ZnO/FTO electrode revealed the high sensitivity of 540.8
μA·mM-1·cm-2. 4. The linear range was obtained from 0.05 to 14.15 mM towards H2O2.
ABSTRACT Graphitic carbon nitride (g-C3N4)/ZnO nanosheets were coated on the surface of fluorine-doped tin oxide (FTO) glass as a noble metal-free g-C3N4/ZnO/FTO electrode, which was used for the determination of hydrogen peroxide (H2O2). The exfoliated g-C3N4 nanosheets were firstly prepared and then ZnO nanosheets were grown on g-C3N4 nanosheets by a microwave-assisted hydrothermal synthesis. The g-C3N4 nanosheets combined with ZnO nanosheets can greatly improve the adsorption of hydrogen peroxide and increase the conductivity of materials as well. The g-C3N4/ZnO nanosheets were characterized by powder X-ray diffraction (XRD), scanning electron microscopic (SEM), transmission electron microscopic (TEM), and X-ray photoelectron spectroscopy (XPS) analysis. Meanwhile, electrochemical measurements of the g-C3N4/ZnO/FTO electrode were utilized to investigate the amperometric sensing of hydrogen peroxide. The prepared g-C3N4/ZnO/FTO electrode revealed the sensitivity of 540.8 μA·mM-1·cm-2 at −0.5 V with the linear response range from 0.05 to 14.15 mM, which even was superior to some 2
noble metal-decorated H2O2 sensors. Additionally, the g-C3N4/ZnO/FTO electrode also showed the reliable operational and environmental stability for electrochemical reduction of H2O2.
Keywords: Graphitic carbon nitride; Zinc oxide; Electrocatalysis; Hydrogen peroxide sensor
1. Introduction Hydrogen peroxide (H2O2) is the vital intermediate in chemical and food industries and also involved in our life process. Research on the quantitative detection of H2O2 received considerable attention in recent decades [1−5]. Though many noble metal-based H2O2 sensors possess good sensitivity and limit of detection (LOD), they are environmentally instable and high cost [6−9]. Therefore, the development of noble metal-free electrodes is significant to produce the practical hydrogen peroxide sensor with high sensitivity and wide linearity range. To date, carbonaceous materials or binary heterostructures of metal oxides and carbon materials, such as NCNTs [10], g-C3N4 nanosheets [11], MnO2/SWCNT-Nf [12], and RGO/ZnO [3], have been prepared for the modified electrode of noble metal-free H2O2 sensors. As a fascinating semiconductor material, bulk g-C3N4 can be prepared by the polycondensation of various precursors, such as cyanamide [13−15], 3
melamine [16] and other s-triazine heterocyclic compounds [17]. In particular, the exfoliated g-C3N4 nanosheets from bulk g-C3N4, a material that has not been much looked at from the perspective of 2D layered structure is in fact an excellent metal-free catalyst for various reactions [18−20]. Zinc oxide (ZnO) is a typical n-type oxide semiconductor with a band gap of 3.37 eV and has great potential applications in the determination of H2O2 [21−23]. To the best of our knowledge, there are no reports available for assembling g-C3N4 nanosheets and ZnO as the electrocatalyst of H2O2 sensors. Predictably, g-C3N4/ZnO nanosheets should be a promising electrocatalysis material of noble metal-free H2O2 sensors. Designing the shape and morphology of materials is a key step to extend their applications due to the direct correlations between the morphology and chemical reactivity [24,25]. For the H2O2 sensing applications, fascinating ZnO nanostructures with different morphologies, as well as ZnO heterostructures, have been studied widely [3,26]. Hierarchical nanostructures with controllable morphologies not only increase surface area, but more importantly can facilitate the adsorption and mass transfer in catalysis [27−29]. Therefore, g-C3N4/ZnO nanosheets, as a novel ZnO-based heterostructures, may effectively improve the electrocatalytic properties for H2O2 and elucidate the morphology-controlled properties. 4
In this work, we report the preparation of g-C3N4/ZnO nanosheets at 120 ºC for 2 h by a microwave-assisted hydrothermal synthesis as the modified electrode of noble metal-free H2O2 sensors for the first time. The chemical composition and morphology of g-C3N4/ZnO nanosheets are characterized by XRD, SEM, TEM, and XPS analysis. The g-C3N4/ZnO nanosheets sensor exhibits the high sensitivity and wide linearity range towards H2O2, which are attributed to their large surface area and fantastic morphology. 2. Experimental section 2.1. Materials All raw materials were analytical reagent grade and used without further purification. 2.2 Preparation of g-C3N4/ZnO nanosheets The g-C3N4 nanosheets were exfoliated by bulk g-C3N4 according to the previous work in our group [30]. Briefly, 2 g melamine and 10 g lithium chloride were mixed by the vigorously mechanical stirrer. Lithium chloride was an exfoliated reagent for g-C3N4 nanosheets. The mixed powder was calcined at 380 °С for 2 hours in air at ramp rate 6.7 °С per minute and heated to 550 °С for 4 hours at a same ramp rate. After grinding, the yellow powder of bulk g-C3N4/LiCl was obtained. Then, 0.5 g bulk g-C3N4/LiCl powder was dispersed in 500 mL deionized water under the magnetic stirrer of 1000 rpm for 24 hours. Finally, the 5
light yellow powder of exfoliated g-C3N4 nanosheets was collected from the dispersed suspension by centrifugation at 8000 rpm for 5 minutes. Then, the exfoliated g-C3N4 nanosheets (243 mg) were dispersed in 30 mL deionized water to form a light yellow solution under ultrasonic bath. The weight ratio of g-C3N4/ZnO was 1:1, which was an important reason for the morphology of g-C3N4/ZnO nanosheets. And 0.1 M zinc nitrate hexahydrate (Zn(NO3)2·6H2O) and 0.4 M sodium hydroxide (NaOH) were added to the above solution. After stirring for 30 minutes at room temperature, the solution was transferred into a 100 mL Teflon-lined autoclave and maintained at 120 °С for 2 h in the microwave workstation. (MDS-10, Sineo Microwave, Shanghai, China). The obtained precipitate was separated by centrifuging and washed with deionized water and ethanol several times and then dried at 70 °С for 8 h in an oven. 2.3. Characterizations The morphology of g-C3N4/ZnO nanosheets were observed by using field emission scanning electron microscopy (FE-SEM; JSM-6701F, JEOL, Tokyo, Japan), transmission electron microscopy (TEM; Tecnai F30G, FEI, Hillsboro, OR, USA) operating at an accelerating voltage of 200 kV. And the sample was loaded by Cu grid coated with carbon for TEM measurement. The thickness of the 2D material was analyzed by atomic force microscopy (AFM; SPM-9600, Shimadzu, Tokyo, Japan) in 6
air. AFM tip was Si3N4 and mica substrate was used for AFM work. The crystal structure of the as-synthesized material was analyzed by using powder X-ray diffraction (XRD; X’pert PRO MPD, Philips, Eindhoven, The Netherlands) with Cu Kα radiation (λ = 1.5406 Å) in the range of 10-80°. Surface chemical element analysis of g-C3N4/ZnO nanosheets was carried out by using X-ray photoelectron spectroscopy (XPS; VG ESCALA-B220i-XL, Thermo Scientific, Surrey, UK) with an Al Kα (hv = 1486.6 eV) source at a residual gas pressure below 10-8 Pa. Nitrogen adsorption-desorption was performed on a nitrogen adsorption apparatus at 77 K (V-Sorb 2800P, Gold APP Corp., Beijing, China), the sample was degassed at 120 °С for 2 h before measurement. Electrochemical measurements were all operated on a CHI 660E electrochemical analyzer (Chenhua Co., Shanghai, China) with a conventional three-electrode method, using a Pt plate as the counter electrode and an Ag/AgCl electrode (saturated with KCl) as the reference electrode. The working electrode was fabricated as follows. Briefly, 50 mg of g-C3N4/ZnO nanosheets, 30 mg of ethyl cellulose, 20 mg of lauric acid and 350 mg terpilenol were mixed and ground in an agate mortar to form a paste. And then the paste was coated on FTO glass via a doctor blade method, using Scotch tape as the spacer. Subsequently, the coated film was dried in an infrared lamp for 5 minutes and baked at 350 °С for 1 h. The exposed area of the working electrode was 1 × 1 cm. Phosphate buffer solution 7
(PBS, 0.2 M, pH 7.4) was used as the supporting electrolyte, and the electrolyte was stirred by the magnetic stirrer with 100 per minute when the determination of H2O2 was performed. The oxygen reduction occurred on the g-C3N4/ZnO/FTO electrode may interfere with the response of H2O2 reduction. In order to obtain the accurate quantitative determination of H2O2, high-purity nitrogen was employed to remove oxygen of the solution (Fig. S1). 3. Results and discussion The formation process of g-C3N4/ZnO nanosheets is illustrated in Fig. 1(a). The exfoliated g-C3N4 nanosheets as the substrate can promote the -
growth of ZnO nanosheets. And the concentration ratio of Zn2+ and OH ions is 1:4, which is a key parameter to form the hierarchical ZnO nanosheet flowers. The detection of the H2O2 sensing is operated on an
electrochemical analyzer with a conventional three-electrode method in Fig. 1(b). The catalytic current due to the electrochemical reduction of H2O2 was recorded by using cyclic voltammetry and the amperometric current-time response, when H2O2 molecules were adsorbed and diffused on the surface of the working electrode. Therefore, as shown in Fig. 1(c), the sensitive detection of H2O2 can be widely applied in many fields such as biosensing and chemical sensing applications. The phase composition of g-C3N4/ZnO nanosheets is characterized by XRD in Fig. 2. In the case of g-C3N4 nanosheets, the strong XRD peak at 27.6° is a characteristic 8
interlayer stacking reflection of conjugated aromatic systems, indexed for graphitic materials as the (002) planes [13]. And an additional peak at 13° can be attributed to an in-planar repeat period of 0.678 nm in the crystal [14]. The hexagonal wurtzite structure of ZnO can be confirmed by the standard card (JCPDS 36-1451). In g-C3N4/ZnO nanosheets, all peaks can be assigned to g-C3N4 nanosheets and ZnO phase, respectively. And no diffraction peaks from impurities can be detected. The intensity ratio of the broad peak at 13° and (002) peak at 27.6° is decreased due to ZnO reveals the strong peaks in g-C3N4/ZnO nanosheets. The position of two peaks has no obvious shift in comparison of g-C3N4 nanosheets, indicating that the layered structure of g-C3N4 nanosheets is stable during the preparation of g-C3N4/ZnO nanosheets. As shown in Fig. 3(a), ZnO nanosheets can be recognized from the exfoliated g-C3N4 nanosheets according to color contrast. And the thin ZnO nanosheets are uniformly dispersed on the laminar morphology of g-C3N4 nanosheets. In Fig. 3(b), the morphology of g-C3N4/ZnO nanosheets was further observed by TEM. The thin nanosheets stack up hierarchical ZnO with flower-like nanostructures. And g-C3N4/ZnO nanosheets exhibit the rich inner-channels in the observation of TEM. Compared to g-C3N4/ZnO nanosheets, SEM and TEM images of only g-C3N4 nanosheets are also given in the supplementary material (Fig. S2). The exfoliated g-C3N4 nanosheets are transparent to electron beams, and cross-sectional atomic 9
force microscopy (AFM) image of g-C3N4 nanosheets is given in Fig. 3(c). The typical AFM image and thickness analyses reveal g-C3N4 nanosheets have a uniform thickness of 4-5 nm, as the plane substrate material, g-C3N4 nanosheets are favorable to the growth of ZnO nanosheets in the process of hydrothermal synthesis. XPS investigation was carried out to determine the chemical status and composition of elements in g-C3N4/ZnO nanosheets. XPS survey scan spectrum of g-C3N4/ZnO nanosheets is given in the supplementary material [Fig. S3]. In Fig. 4(a), the binding energy of Zn 2p can be fitted into two distinct peaks of Zn 2p3/2 and 2p2/1, and the spin-orbit splitting of them is 23.3 eV, which is in agreement with the value of ZnO [31]. As shown in Fig. 4(b), the O 1s high-resolution spectrum can be deconvolved into 529.7 and 531.6 eV peaks, which belong to the lattice oxygen of ZnO and the surface hydroxyl groups (−OH) or oxygen (O2) adsorbed on g-C3N4/ZnO nanosheets, respectively. The regional spectrum of C1s shows three peaks at 284.6, 286.5 and 288.4 eV in Fig. 4(c), which are attributed to graphite carbon atoms, carbon atoms attached to the −NH2 group and the sp2-bonded carbon (N−C=N) inside the aromatic structure. In Fig. 4(d), the high resolution N 1s spectrum of g-C3N4/ZnO nanosheets can be deconvoluted into three peaks. The peaks at 398.5 and 400.6 eV are attributed to sp2-hybridized nitrogen (C−N=C) and tertiary nitrogen (N−(C)3) in g-C3N4, whereas the peak at 404.3 eV can be 10
assigned to amino functional groups in aromatic rings (Ar−N−H) [32]. Atomic ratio of all elements in g-C3N4/ZnO nanosheets from XPS spectra is listed in Table S1. Based on the above discussion, it is demonstrated that g-C3N4/ZnO nanosheets have been successfully synthesized and phase compositions of g-C3N4 and ZnO are stable during the preparation of g-C3N4/ZnO nanosheets. Additionally, the prepared g-C3N4/ZnO nanosheets are investigated the electrocatalytic performance as a H2O2 sensor. In Fig. 5(a), it is seen that g-C3N4/ZnO/FTO (curve A) exhibits a weak cathodic current density in the absence of H2O2 and the maximum reduction current density of bare FTO (curve B) is also pretty weak in the presence of 5 mM H2O2. Correspondingly, the maximum reduction current density is gradually increased with the addition of 5 mM H2O2 in the case of ZnO/FTO (curve C), g-C3N4/FTO (curve D), and g-C3N4/ZnO/FTO (curve E) in 0.2 M PBS (pH 7.4). The g-C3N4/ZnO/FTO electrode reveals the biggest current density of 2.15 mA·cm-2 towards the reduction of H2O2. ZnO has a good electroconductivity to promote the electron transfer and then improve the electrochemical reduction of H2O2 in g-C3N4/ZnO nanosheets [Fig. S4]. The above results indicate that g-C3N4/ZnO nanosheets can be used as a highly efficient electrocatalyst towards H2O2 and the synergistic effect of g-C3N4 and ZnO nanosheets can obviously improve the electrocatalytic performance. To study the effect of scan rates on the electrocatalytic 11
performance, cyclic voltammetry (CV) curves of the g-C3N4/ZnO/FTO electrode were further examined in 0.2 M PBS (pH 7.4) containing 5 mM H2O2 at various scan rates of 30, 50, 90, 130, 170, 220, 270, and 350 mV·s-1. As shown in Fig. 5(b), the peak reduction current density increases with the scan rate at an applied potential of –0.5 V. And the linear relationship can be seen between the peak current density and the square root of scan rates in the inset of Fig. 5(b), indicating that the electrochemical reaction is controlled by the diffusion of the analyte from solution to the electrode surface. And the number of electrons transferred in the rate-determining step (nα) and the electrons transfer coefficient (α) can be obtained as the following Eq. (1): αnα = (47.7 mV)/(∣ Ep − Ep/2 ∣)
(1)
where Ep and Ep/2 are the peak potential and the potential at the half maximum peak current, respectively. The value of αnα is 0.318 in g-C3N4/ZnO nanosheets, indicating that the rate-determining step involves one electron transfer, and the value of α range between 0.3 and 0.7 [33]. The mechanism of H2O2 electrochemical reduction on the g-C3N4/ZnO/FTO electrode can be proposed as the following [34,35]: H2 O2 + e− → OHad + OH −
(2)
OHad + e− → OH −
(3)
2OH − + 2H + → 2H2 O
(4)
The formation of OHad is a key step controlling the reaction rate in the 12
reduction of H2O2. The CV curve of the g-C3N4/ZnO/FTO electrode in Fig. 5(a) represents an obvious increase of the current density within the potential range between -0.6 and -0.1 V after adding 5 mM H2O2. The onset potentials of H2O2 electrochemical reduction and chemisorption of OH- anion [34] on the electrode were almost the same, which may support the proposed mechanism in Eqs. (2)-(4) for the g-C3N4/ZnO/FTO electrode. Amperometric current-time plot of g-C3N4/ZnO/FTO was obtained with dropping different H2O2 concentrations into 0.2 M PBS (pH 7.4) at an applied potential of −0.5 V. Fig. 6(a) shows the reduction current density of g-C3N4/ZnO/FTO is increased with the addition of H2O2 concentrations and presents a rapid response time about 3−4 s, which is contributed to the abundant inner-channels of g-C3N4/ZnO nanosheets. The linear range can be obtained between logarithm current density (log Ip) and logarithm H2O2 concentration (log C) from 0.05 to 14.15 mM in the inset of Fig. 6(a). From the slope of 0.571 in the calibration curve, the sensitivity of g-C3N4/ZnO/FTO is determined to be 540.8 μA·mM-1·cm-2 [10,36]. Additionally, nitrogen adsorption isotherms of g-C3N4/ZnO nanosheets and the corresponding pore size distributions are presented in Fig. 6(b). The isotherms are type IV with an H1 loop according to the IUPAC classification [37]. And a small desorption branch shift at the high relative pressure region reflects some opening pores (interconnection) 13
exist in g-C3N4/ZnO nanosheets, which is in accordance with the analytic result of TEM observation and the previous report [38]. The BET surface area and the total pore volume are 16.5 m2·g-1 and 0.33 cm3·g-1, respectively. The average pore size of g-C3N4/ZnO nanosheets is 15.8 nm in the inset of Fig. 6(b). Therefore, H2O2 molecule can easily transfer on the surface of g-C3N4/ZnO nanosheets through those apertures. In Fig. 7(a), the limit of detection (LOD), based on signal-to-noise ratio of 3, is calculated to be 1.7 μM for the reduction of H2O2 using the following Eq. (5): LOD = SDbackground /S
(5)
where S is the sensitivity of the g-C3N4/ZnO/FTO electrode, and SD is the standard deviation of the background current density [12]. Table 1 shows a partial list of properties on the amperometric H2O2 sensor, using noble metal modified and noble metal-free modified electrodes. It can be seen that the g-C3N4/ZnO/FTO electrode in this study reveals the good sensitivity and wide linearity range towards H2O2, which is comparable with or higher than some noble metal modified electrodes. The selectivity of H2O2 sensor was also evaluated against many probable interferential analytes, such as ethanol, Na2SO4, KCl, ascorbic acid (AA), glucose, and urea. As shown in Fig. 7(b), the reduction current density of g-C3N4/ZnO/FTO is apparently changed in 0.2 M PBS (pH 7.4) containing 0.5 mM H2O2 at −0.5 V. In contrast, no obvious current 14
response is observed to the addition of other analytes, except 3 mM Na2SO4. In order to accurate quantitative determination of H2O2, the high-concentration Na2SO4 solution need be avoided for the g-C3N4/ZnO/FTO electrode in practical applications. The pH value of the buffer solution is an important factor to affect the electrocatalytic reduction of H2O2. We have investigated the effect of the pH value of the buffer solution on the electrochemical response of H2O2 for the g-C3N4/ZnO/FTO electrode. Fig. 8 shows the maximum reduction current density of the g-C3N4/ZnO nanosheets electrode in 5 mM H2O2 at different pH values of the solution. The current density is gradually increased in the pH value from 5.7 to 7.4. As we know, the g-C3N4/ZnO nanostructures will be destroyed due to ZnO is unstable in the acidic solution. On the other hand, when the pH value is added to 8.0, the current density will decrease again because the catalytic activity and stability of H2O2 are inhibited in alkaline solution [42]. Therefore, the pH value of 7.4 was optimized for the g-C3N4/ZnO/FTO electrode on the electrochemical response of H2O2. Additionally, we have also studied the related experiments with different solution temperatures when the g-C3N4/ZnO nanosheets electrode existed in 0.2 M PBS (pH 7.4) containing 5 mM H2O2. As shown in Fig. 8, the reduction current density of H2O2 at the same potential is increased in the whole temperature range from 22 to 60 °С because the activation energy of H2O2 reduction is low 15
in the elevated temperature [34]. Compared to enzyme-modified sensors of H2O2, the g-C3N4/ZnO/FTO electrode can act as the stable sensor of H2O2 at a wide temperature range. The reproducibility of the g-C3N4/ZnO/FTO electrode for electrochemical response of hydrogen peroxide was conducted by using various equivalent electrodes in 0.2 M PBS (pH 7.4) containing 5 mM H2O2 at 100 mV·s-1 (Fig. S5). It can be found that a deviation is less than 12%, indicating that the g-C3N4/ZnO/FTO electrode possess acceptable reproducibility. To evaluate the potential of the g-C3N4/ZnO/FTO electrode for determination of hydrogen peroxide in real samples, different real samples such as drinking water, tape water and serum sample containing certain H2O2 were tested by using the same electrochemical method in the paper. The detection results are listed in Table S2. It is demonstrated that the g-C3N4/ZnO/FTO electrode revealed good selectivity with a reasonable deviation lower than 16% even in real samples. The long-term stability of the prepared g-C3N4/ZnO/FTO electrode is a critical parameter in practical applications. To evaluate the stability of the g-C3N4/ZnO/FTO electrode, the stability was examined by periodical measurements for number of cycles and time in 0.2 M PBS (pH 7.4) containing 5 mM H2O2 at a scan rate of 100 mV·s-1. As shown in Fig. 9(a), the maximum reduction current density is stable on the 11 cycle and no apparent shift can be seen after 100 cycles. This result indicated 16
the reaction was an adsorption controlled process and it obeyed the Nernstian condition for adsorption controlled reactions [43]. The variation of the maximum response current density is about 9% of its initial value after 37 days in Fig. 9(b). The reason of the enhanced stability can be attributed to the stable structure of g-C3N4/ZnO nanosheets, indicating that the collapse of nanostructures has been prevented during the electrochemical reduction of H2O2. And the g-C3N4/ZnO nanosheet is the nonenzymatic and noble metal-free electrode, which means the g-C3N4/ZnO/FTO electrode shows the reliable operational and environmental stability. 4. Conclusions The g-C3N4/ZnO nanosheets were prepared by a simple microwave-assisted hydrothermal synthesis. The exfoliated g-C3N4 nanosheets promoted the grown process of ZnO nanosheets. And the layer structure of g-C3N4/ZnO nanosheets was favorable to the adsorption and diffusion of H2O2 molecules. The morphology and chemical compositions of g-C3N4/ZnO nanosheets were confirmed. As a green and low-cost noble metal-free modified H2O2 sensor, the g-C3N4/ZnO/FTO electrode exhibited the high sensitivity, wide linearity range, and reliable stability, which might be replaced the noble metal modified H2O2 sensors in some ways.
17
Acknowledgment This work was supported by the National Natural Science Foundation (51672220), and 111 Program (B08040) of MOE, the National Defense Science Foundation (32102060303), the Fundamental Research Funds for the Central Universities (3102014JGY01004) of China.
References [1] K.K. Tang, X.F. Wu, G.D. Wang, L.X. Li, S.Y. Wu, X.T. Dong, Z.L. Liu, B. Zhao, One-step preparation of silver nanoparticle embedded amorphous carbon for nonenzymatic hydrogen peroxide sensing, Electrochem. Commun. 68 (2016) 90−94. [2] L.C. Jiang, W.D. Zhang, Electrodeposition of TiO2 nanoparticles on multiwalled carbon nanotube arrays for hydrogen peroxide sensing, Electroanalysis 21 (2009) 988−993. [3] S. Palanisamy, S.M. Chen, R. Sarawathi, A novel nonenzymatic hydrogen peroxide sensor based on reduced graphene oxide/ZnO composite modified electrode, Sens. Actuators B 166−167 (2012) 372−377. [4] K. Dhara, T. Ramachandran, B.G. Nair, T.G. Satheesh, Au nanoparticles decorated reduced graphene oxide for the fabrication of disposable nonenzymatic hydrogen peroxide sensor, J. Electroanal. Chem. 764 (2016) 64−70. 18
[5] W. Chen, S. Cai, Q.Q. Ren, W. Wen, Y.D Zhao, Recent advances in electrochemical sensing for hydrogen peroxide: a review, Analyst 137 (2012) 49−58. [6] B. Zhao, Z. Liu, W. Fu, H. Yang, Construction of 3D electrochemically reduced graphene oxide-silver nanocomposite film and application as nonenzymatic hydrogen peroxide sensor, Electrochem. Commun. 27 (2013) 1−4. [7] J. Xu, X.W. Wei, X.J. Song, X.J. Lu, C.C. Ji, Y.H. Ni, G.C. Zhao, Synthesis and electrocatalytic activity of multi-walled carbon nanotubes/Cu-Ag nanocomposites, J. Mater. Sci. 42 (2007) 6972−6976. [8] J. Wang, L. Wang, J. Di, Y. Tu, Electrodeposition of gold nanoparticles on indium/tin oxide electrode for fabrication of a disposable hydrogen peroxide biosensor, Talanta 77 (2009) 1454−1459. [9] H. Cao, X. Sun, Y. Zhang, N. Jia, Electrochemical sensing based on gold nanoparticle-decorated halloysite nanotube composites, Anal. Biochem. 430 (2012) 111−115. [10] X. Xu, S.J. Jiang, Z. Hu, S.Q. Liu, Nitrogen-doped carbon nanotubes: high electrocatalytic activity toward the oxidation of hydrogen peroxide and its application for biosensing, ACS Nano 4 (2010) 4292−4298. 19
[11] J.Q. Tian, Q. Liu, C.J. Ge, Z.C. Xing, A.M. Asiri, A.O. Al-Youbi, X.P. Sun, Ultrathin graphitic carbon nitride nanosheets: a low-cost, green, and highly efficient electrocatalyst toward the reduction of hydrogen peroxide and its glucose biosensing application, Nanoscale 5 (2013) 8921−8924. [12] A.B.M. Zakaria, D. Leszczynska, Novel design of non-enzymatic sensor for rapid monitoring of hydrogen peroxide in water matrix, J. Electroanal. Chem. 766 (2016) 30−36. [13] X.C. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J.M. Carlsson, K. Domen, M. Antonietti, A metal-free polymeric photocatalyst for hydrogen production from water under visible light, Nat. Mater. 8 (2009) 76−80. [14] F. Goettmann, A. Fischer, M. Antonietti, A. Thomas, Chemical synthesis of mesoporous carbon nitrides using hard templates and their use as a metal-free catalyst for Friedel-Craftes reaction of benzene, Angew. Chem. Int. Ed. 45 (2006) 4467–4471. [15] M. Groenewolt, M. Antonietti, Synthesis of g-C3N4 nanoparticles in mesoporous silica host matrices, Adv. Mater. 17 (2005) 1789–1792. [16] J.Q. Tian, Q. Liu, A.M. Asiri, A.O. Al-Youbi, X.P. Sun, Ultrathin graphitic carbon nitride nanosheets: a highly efficient fluorosensor for rapid, ultrasensitive detection of Cu2+, Anal. Chem. 85 (2013) 5595–5599. 20
[17] B. Jurgens, E. Irran, J. Senker, P. Kroll, H. Muller, W. Schnick, Melem (2,5,8-triamino-tri-s-triazine), an important intermediate during condensation of melamine rings to graphitic carbon nitride: synthesis, structure determination by X-ray powder diffractometry, solid-state NMR, and theoretical studies, J. Am. Chem. Soc. 125 (2003) 10288–10300. [18] P. Niu, L.L. Zhang, G. Liu, H.M. Cheng, Graphene-like carbon nitride nanosheets for improved photocatalytic activities, Adv. Funct. Mater. 22 (2012) 4763–4770. [19] S.B. Yang, X.L. Feng, X.C. Wang, K. Müllen, Graphene-based carbon nitride nanosheets as efficient metal-free electrocatalysts for oxygen reduction reaction, Angew. Chem. Int. Ed. 50 (2011) 5339– 5343. [20] K.K.R. Datta, B.V.S. Reddy, K. Ariga, A. Vinu, Gold nanoparticles embedded in a mesoporous carbon nitride stabilizer for highly efficient three-component coupling reaction, Angew. Chem. Int. Ed. 49 (2010) 5961–5965. [21] N.H.A. Hardan, M.A.A. Hamid, R. Shamsudin, N.K. Othman, L.K. Keng, Amperometric non-enzymatic hydrogen peroxide sensor based on aligned zinc oxide nanorods, Sensors 16 (2016) 1004– 1012. [22] X.L. Zhu, I. Yuri, X. Gan, I. Suzuki, G.X. Li, Electrochemical study 21
of the effect of nano-zinc oxide on microperoxidase and its application to more sensitive hydrogen peroxide biosensor preparation, Biosens. Bioelectron. 22 (2007) 1600–1604. [23] M. Hussain, H. Sun, S. Karim, A. Nisar, M. Khan, A.U. Haq, M. Iqbal, M. Ahmad, Noble metal nanoparticle-functionalized ZnO nanoflowers for photocatalytic degradation of RhB dye and electrochemical sensing of hydrogen peroxide, J. Nanopart. Res. 18 (2016) 95–109. [24] J.I. Sohn, W.K. Hong, S. Lee, S. Lee, J. Ku, Y.J. Park, J. Hong, S. Hwang, K.H. Park, J.H. Warner, Surface energy-mediated construction of anisotropic semiconductor wires with selective crystallographic polarity, Sci. Rep. 4 (2014) 1–7. [25] N. Tian, Z.Y. Zhou, S.G. Sun, Y. Dong, Z.L. Wang, Synthesis of tetrahexahedral platinum nanocrystals with high-index facets and high electro-oxidation activity, Science 316 (2007) 732–735. [26] Q. Wang, J. Zheng, Electrodeposition of silver nanoparticles on a zinc oxide film: improvement of amperometric sensing sensitivity and stability for hydrogen peroxide determination, Microchim Acta 169 (2010) 361–365. [27] Q. Zeng, J. Cheng, L. Tang, X. Liu, Y. Liu, J. Li, J. Jiang, Self-assembled graphene-enzyme hierarchical nanostructures for electrochemical biosensing, Adv. Funct. Mater. 20 (2010) 3366– 22
3372. [28] W. Zhang, R. Li, L. Xing, X. Wang, X. Gou, Carnation-like CuO hierarchical nanostructures assembled by porous nanosheets for nonenzymatic glucose sensing, Electroanalysis 28 (2016) 2214−2221. [29] H.L. Tian, H.Q. Fan, M.M. Li, L.T. Ma, Zeolitic imidazolate framework coated ZnO nanorods as molecular sieving to improve selectivity of formaldehyde gas sensor, ACS Sens. 1 (2016) 243– 250. [30] L.T. Ma, H.Q. Fan, J. Wang, Y.W. Zhao, H.L. Tian, G.Z. Dong, Water-assisted ions in situ intercalation for porous polymeric graphitic carbon nitride nanosheets with superior photocatalytic hydrogen evolution performance, Appl. Catal. B 190 (2016) 93–102. [31] L. Xu, R.F. Zheng, S.H. Liu, J. Song, J.S. Chen, B. Dong, H.W. Song, NiO@ZnO heterostructured nanotubes: coelectrospinning fabrication, characterization, and highly enhanced gas sensing properties, Inorg. Chem. 51 (2012) 7733–7740. [32] S.B. Yang, Y.J. Gong, J.S. Zhang, L. Zhan, L.L. Ma, Z.Y. Fang, R. Vajtai, X.C. Wang, P.M. Ajayan, Exfoliated graphitic carbon nitride nanosheets as efficient catalysts for hydrogen evolution under visible light, Adv. Mater. 25 (2013) 2452–2456. [33] C.Y. Lin, Y.H. Lai, A. Balamurugan, R. Vittal, C.W. Lin, K.C. Ho, 23
Electrode modified with a composite film of ZnO nanorods and Ag nanoparticles as a sensor for hydrogen peroxide, Talanta 82 (2010) 340–347. [34] F.H. Meng, X.L. Yan, J.G. Liu, J. Gu, Z.G. Zou, Nanoporous gold as non-enzymatic sensor for hydrogen peroxide, Electrochim. Acta 56 (2011) 4657–4662. [35] Y.X. Li, J. Ma, Z.F. Ma, Synthesis of gold nanostars with tunable morphology and their electrochemical application for hydrogen peroxide sensing, Electrochim. Acta 108 (2013) 435–440. [36] W.J. Kang, X. Pei, C.A. Rusinek, A. Bange, E.N. Haynes, W.R. Heineman, I. Papautsky, Determination of lead with a copper-based electrochemical sensor, Anal. Chem. 89 (2017) 3345–3352. [37] K.S.W. Sing, Reporting physisorption data for gas/solid systems-with special reference to the determination of surface area and porosity, Pure & Appl. Chem. 57 (1985) 603–619. [38] S.B. Yoon, G.S. Chai, S.K. Kang, J.S. Yu, K.P. Gierszal, M. Jaroniec, Graphitized pitch-based carbons with ordered nanopores synthesized by using colloidal crystals as templates, J. Am. Chem. Soc. 127 (2005) 4188–4189. [39] M.R. Guascito, E. Filippo, C. Malitesta, D. Manno, A. Serra, A. Turco, A new amperometric nanostructured sensor for the analytical determination of hydrogen peroxide, Biosens. Bioelectron. 24 (2008) 24
1057–1063. [40] S. Wu, H.T. Zhao, H.X. Ju, C.G. Shi, J.W Zhao, Electrodeposition of silver-DNA hybrid nanoparticles for electrochemical sensing of hydrogen peroxide and glucose, Electrochem. Commun. 8 (2006) 1197–1203. [41] S.B. Khan, M.M. Rahman, A.M. Asiri, S.A.B. Asif, S.A.S. Al-Qarni, A.G. Al-Sehemi, S.A. Al-Sayari, M.S. Al-Assiri, Fabrication of non-enzymatic sensor using Co doped ZnO nanoparticles as a maker of H2O2, Phys. E Low Dimens. Syst. Nanostruct. 62 (2014) 21–27. [42] Y.Q. Sun, L.F. Hang, D.D. Men, H.L. Li, D.L. Liu, X.Y. Li, L.L. Wen, Y. Li, Periodic nanostructured Au arrays on an Si electrode for high-performance electrochemical detection of hydrogen peroxide without an enzyme, J. Mater. Chem. C 4 (2016) 9864–9871. [43] X.P. Zhang, D. Liu, B. Yu, T.Y. You, A novel nonenzymatic hydrogen peroxide sensor based on electrospun nitrogen-doped carbon nanoparticles-embedded carbon nanofibers film, Sens. Actuators B 224 (2016) 103–109.
25
Figure captions Fig. 1 (a) Schematic diagrams of the g-C3N4/ZnO nanosheets formation process. (b) Photograph of the g-C3N4/ZnO powder and g-C3N4/ZnO/FTO electrode, and the electrochemical measurement and (c) applications of H2O2 sensing are illustrated.
Fig. 2 XRD patterns of g-C3N4 nanosheets, ZnO, and g-C3N4/ZnO nanosheets.
26
Fig. 3 (a) SEM image of g-C3N4/ZnO nanosheets. (b) TEM image of g-C3N4/ZnO nanosheets. (c) AFM image of g-C3N4 nanosheets and the thickness of g-C3N4 nanosheets.
Fig. 4 XPS high-resolution spectra and fitted curves of Zn 2p (a), O 1s (b), C 1s (c), and N 1s (d).
27
Fig. 5 (a) Cyclic voltammetry of the g-C3N4/ZnO nanosheets electrode (A) in the absence of H2O2, and cyclic voltammetry of bare FTO (B), ZnO (C), g-C3N4 nanosheets (D), and g-C3N4/ZnO nanosheets (E) electrodes in 0.2 M PBS (pH 7.4) in the presence of 5 mM H2O2. Scan rate is 100 mV·s-1. (b) Cyclic voltammetry of the g-C3N4/ZnO nanosheets electrode in 0.2 M PBS (pH 7.4) containing 5 mM H2O2 at different scan rates. Inset is the relationship between the peak current density and the square root of scan rates.
28
Fig. 6 (a) Amperometric current-time plot of the g-C3N4/ZnO nanosheets electrode to successive addition of H2O2 in different concentrations in 0.2 M PBS (pH 7.4) at an applied potential of –0.5 V. Inset is the calibration curve of steady-state current against H2O2 concentration. (b) Nitrogen adsorption isotherms at 77 K for g-C3N4/ZnO nanosheets, inset shows the pore size distribution and pore volume of g-C3N4/ZnO nanosheets.
29
Fig. 7 (a) The limit of detection of the g-C3N4/ZnO nanosheets electrode measured in 0.2 M PBS (pH 7.4) at –0.5 V. (b) Amperometric response of the g-C3N4/ZnO nanosheets electrode for various concentrations of analytes in 0.2 M PBS (pH 7.4) at an applied potential of –0.5 V.
30
Fig. 8 The variation of the maximum response current density for the g-C3N4/ZnO nanosheets electrode in 5 mM H2O2 at different pH values and temperatures of PBS.
31
Fig. 9 (a) The variation of the maximum response current density for the g-C3N4/ZnO nanosheets electrode in 0.2 M PBS (pH 7.4) containing 5 mM H2O2 at a scan rate of 100 mV·s-1 after 100 cycles. Inset is cyclic voltammetry of the g-C3N4/ZnO nanosheets electrode in 0.2 M PBS (pH 7.4) containing 5 mM H2O2 at different cycles. (b) The stability of the g-C3N4/ZnO nanosheets electrode in 0.2 M PBS (pH 7.4) containing 5 mM H2O2 at a scan rate of 100 mV·s-1 for 37 days. Inset is cyclic voltammetry of the g-C3N4/ZnO nanosheets electrode in 0.2 M PBS (pH 7.4) containing 5 mM H2O2 at different days.
32
Table 1 Comparison of g-C3N4/ZnO/FTO with other modified electrodes on electrochemical H2O2 sensing. Type of the electrode
Potential (V)
Sensitivity (μA·mM-1·cm-2)
LOD (μM)
Linear range (mM)
Ref.
AgNPs/PVA/Pt AgNPs/DNA/GCE PDDA/AuNSs/GCE AgNPs/ZnONRs/FTO g-C3N4/GCE Co doped ZnO/GCE g-C3N4/ZnO/FTO
-0.5 -0.45 -0.4 -0.55 -0.3 -0.3 -0.5
4090 773 60.2 152.1 92.4 540.8
1.0 0.6 9.7 0.9 2 14.3 1.7
0.04-6 0.002-2.5 0.02-2.5 0.008-0.983 0.1-90 5-20 0.05-14.15
[39] [40] [35] [33] [11] [41] This work
33