The Self-Assembled Hierarchical Interfaces of ZnO Nanotubes

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The Self-Assembled Hierarchical Interfaces of ZnO Nanotubes/Graphene Heterostructures for Efficient Room Temperature Hydrogen Sensors Deepa Kathiravan, Bohr-Ran Huang, and Adhimoorthy Saravanan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00338 • Publication Date (Web): 17 Mar 2017 Downloaded from http://pubs.acs.org on March 19, 2017

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ACS Applied Materials & Interfaces

The Self-Assembled Hierarchical Interfaces of ZnO Nanotubes/Graphene Heterostructures for Efficient Room Temperature Hydrogen Sensors Deepa Kathiravan†, Bohr-Ran Huang†* and Adhimoorthy Saravanan† †

Graduate Institute of Electro-Optical Engineering and Department of Electronic and Computer

Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan, R.O.C.

ABSTRACT Herein, we report the novel nanostructural interfaces of self-assembled hierarchical ZnO nanotubes/graphene (ZNTs/G) with three different growing times of ZNTs on graphene substrates (namely, SH1, SH2 and SH3). Each sample was fabricated with interdigitated electrodes to form hydrogen sensors and their hydrogen sensing properties were comprehensively studied. The systematical investigation reveals that SH1 sensor exhibits an ultra-high sensor response even at low detection level of 10 ppm (14.3%) to 100 ppm (28.1%) compared to SH2 and SH3 sensor. The SH1 sensor was also found to be well retained with repeatability, reliability and long-term stability of 90 days under hydrogenation/dehydrogenation process. This outstanding enhancement in sensing properties of SH1 is attributed to the formation of strong metalized region in the ZNTs/G interface due to the inner/outer surfaces of ZNTs, establishing a multiple depletion layer. Furthermore, the respective band models of each nanostructure were also purposed to describe their heterostructure which illustrates the hydrogen sensing properties. Moreover, the long-term stability can be ascribed by the heterostructured combination of ZNTs and graphene via a spillover effect. The salient feature of this self-assembled nanostructure is that their reliability, simple synthesize methods and long-term stability, which makes them a promising candidate for new generation hydrogen sensors and hydrogen storage materials.

Keywords. Graphene hydrogen sensor, hydrogen storage, self-assembled ZNTs/G nanostructures, spillover effect.

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1. INTRODUCTION Because of the environmental concerns in all over the world, a clean source of energy is required to make pollute free atmosphere. In that order, hydrogen is reflecting as a main ecological energy source that is considered to be an alternative fuel for future generation. Thereby, the indispensable production of hydrogen results a numerous energy storage and chemical industries certainly leads to the extensive safety concerns. Hence it is crucial to detect the inadvertent leakage of hydrogen, when the flammable range exceeds 4% in the atmosphere.14

Accordingly, the long-life hydrogen sensor requires a reliable material to sense hydrogen gas

even at low concentration in rapid response and recovery. Up to now, metal oxide semiconductor based hydrogen sensors are showing good sensing properties, however, high operating temperature and stability are their significant disadvantages.5,6 Recently, H. Gu et.al demonstrated a comprehensive review of semiconductor oxide thin films and one dimensional (1D) nanostructures based hydrogen sensors, and reported that 1D nanostructures exhibit excellent sensing properties of hydrogen gas at room temperature.7 In the recent decades, 1D ZnO nanostructures are the mostly studied semiconductor materials in the resistive type gas sensors, which shows high selectivity to explosive and toxic gases.8-10 More specifically, the oxygen surface from 1D ZnO nanostructures induce metallic behavior when reacted with hydrogen gas, which assist to decrease the semiconductor resistance and thereby increase the hydrogen sensor response.11,12 However, ZnO nanotube (ZNT) receives great attention among 1D ZnO nanostructures due to their potential carrier transport on both inner and outer surface. As a result, the large surface area of nanotube prevents the trapping of electrons in any site and delivers the good electron mobility while gas adsorption/desorption, and exhibits high sensor response at room temperature.13-15 On the other hand, graphene, a well-known fascinating material from carbon family exhibits excellent mechanical stability and electrical properties at room temperature. Over the last few decades, several literatures were reported that graphene materials can be designed to sense suitable gases by the presence of defects and functional groups in it, resulting a higher interaction with gas molecules during adsorption/desorption process.16-19 However, the pure graphene materials (including single layer and multilayer graphene) themselves are not sensitive to hydrogen and requires other material combinations (such as Pd3, Pt19, ZnO20, TiO221, SnO222, ITO23 etc.,) to make them sensitive. It is because, a small binding energy exists when the graphene surface contact


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with hydrogen molecules.24-25 Among such combinations, ZnO and graphene shows remarkable gas sensing performances since the integrated nanostructure exhibits combined properties such as high carrier mobility, and metallization effect with local heterojunction. However, a few works have been studied on the combination of graphene and ZnO in the field of hydrogen sensor applications.26, 27 The development of ZnO nanotubes and graphene based hydrogen sensor has not been reported so far. In this context, it is believed that the structure–property correlation is a potential concept in material chemistry that inspires to synthesize new kind of materials with unique nanostructures.28-32 Due to the extensive focus on the improvement of graphene materials and the significant advances of ZNTs in gas sensing applications, it would be highly desired to combine these nanomaterials as advance hybrid structure for high performance gas sensors. Hitherto, to the best of our knowledge, no effort has been made to study the hierarchical properties of ZNTs and graphene with proper band models. Herein, we report the gas sensing properties of novel ZNTs/G nanohybrid heterostructures, and also investigated their nanostructural defects while increase the growing time of ZNTs. A simple hydrothermal route combined with the subsequent CVD method was devised in the preparation of ZNTs/G nanohybrid heterostructures. The as-obtained samples were then fabricated as gas sensors and their gas sensing behaviors were studied. It was revealed that ZNTs/G based sensors exhibit high response even at low ppm, reproducibility, long-term stability, ultra-fast response and recovery to hydrogen, especially at room temperature. 2. EXPERIMENTAL DETAILS 2.1 Synthesis of ZNTs/G nanohybrid heterostructures All the chemicals used in this work were analytical grade without further purification. Firstly, a monolayer graphene was synthesized on Cu foils (Alfa Aesar) using CVD system. Prior to synthesize graphene, Cu foils were cleansed with acetone, distilled water, isopropanol and distilled water in sequence of 10 min. The cleansed Cu foils were kept in a quartz furnace tube of CVD to grow graphene using a mixture of CH4 and H2. More briefly, Cu foils were first heated in a mixture of Ar and H2 gas flow at 800 ºC for 35 min under a pressure of 5.33 kPa and, then heated up to 1000 ºC. After the temperature reached at 1000 ºC, CH4 was introduced into the furnace for 10 min to grow a monolayer graphene. Then the furnace was cooled and H2/Ar flow was stopped, subsequently, the as-grown graphene on Cu foil is transferred to SiO2/Si substrates by the steps as



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follows: a polymethylmethacrylate (PMMA) membrane was first coated on graphene samples and baked for 3 min at 180 ºC. The Cu foil is then removed by using 0.5 M iron nitrate solution and PMMA coated graphene membrane was washed with a solution of dilute HCl and distilled water. The PMMA-graphene was subsequently transferred on SiO2/Si substrates and baked for 10 min at 50 ºC. Finally, the PMMA on graphene substrate was removed by acetone bath and the as-obtained graphene samples were cleansed in sequence of acetone and distilled water, to remove the unwanted particles on it. Followed by that, vertically aligned ZNTs/G nanohybrid heterostructure was obtained by hydrothermal method with different growing time of ZNTs. All the graphene substrates were first coated by ZnO target (with 99.9% purity) under 20:5 sccm ratio of Ar/O2 at 50 W power using radio frequency (RF) sputtering at room temperature for 30 min, to grow the ZnO seed layers. Subsequently, ZnO seed layered graphene substrates were dipped in 35 mM equimolar precursor solution of zinc acetate (ZnAc) and hexamethylenetetramine (HMTA) with distilled water to primarily synthesize ZNRs at 90 ⁰C for 3 hours. In order to obtain ZNTs, all the graphene substrates were kept under same precursor solution (after the growth of ZNRs) with the respective time durations of 5 hours to 15 hours at 50 ⁰C to self-etch. Furthermore, the samples were completely rinsed with distilled water and heated at 120 ⁰C for 10 min. 2.2 Characterization of materials The surface morphology and microstructures of the as-obtained samples were characterized by field emission scanning electron microscopy, FESEM (JSM-6500F) and transmission electron microscopy, TEM (Joel 2100F). The Raman spectra were recorded with a iHR550 instrument (Taiwan). The X-ray diffraction (XRD) spectra were conducted on X-ray Powder Diffraction, (D2 PHASER BRUKER) using CuKα1 radiation (λ = 1.54056 Å ). The surface topologies of the asobtained samples were examined by Atomic force microscopy (AFM, Bruker Dimension ICON). 2.3 Sensor Fabrication and Hydrogen testing All the samples were fabricated with platinum (Pt) interdigitated electrodes to form hydrogen sensors. The hydrogen sensing studies were carried out in a home-built closed chamber with a controlled flow of hydrogen gas (10 ppm to 100 ppm) via mass flow controller at room temperature. Each sensor electrodes on the holder were connected to a computer controlled source


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unit (Keithley 237). The response curves were obtained by periodically allowed and stopped the hydrogen gas (99.99% diluted with dry air) into the chamber. 3. RESULTS AND DISCUSSION 3.1 Surface morphologies and topologies of ZNTs/G nanohybrid heterostructures The different time duration used for growing ZNTs strongly influence the morphology of the samples, which were studied using FESEM. Initially, the growing times of ZNTs on graphene substrate was optimized for every 2 hours and their FESEM images are given in the supporting information (Figure S1). Among that, the different time periods of 8 hours, 12 hours and 18 hours were selected due to their novel heterostructure and named as: SH1, SH2 and SH3. The micrographs of Figure 1a-c indicates the surface morphology of SH1, SH2 and SH3, respectively. From Figure 1a, it is markedly seen that the ZNTs are well-etched and there is no graphene on the top of the ZNTs. On the contrary, micrographs of Figure 1b and 1c markedly shows the structural defects occur between graphene and ZNTs. More briefly, SH1 indicates the defect-free structure of ZNTs and graphene due to the appropriate growing time of 8 hours, so that ZNTs were allowed to self-etch properly. However, SH2 and SH3 shows the defective structure due to the increased growing time of ZNTs up to 12 and 18 hours. Moreover, when the growing time is increased to 12 hours, the graphene starts to peel-off from the substrate and led to dissociate the ZNTs structure as bunches. Similarly, for 18 hours, most of the graphene was elevated from the substrate to the top of ZNTs and remaining was blended with ZNTs on the substrate. Therefore, the FESEM cross-sectional images were also investigated to confirm the exact structure of as-obtained samples SH1, SH2 and SH3, respectively. The cross-sectional images correspond to SH1, SH2 and SH3 are shown in the inset of Figure 1d-f unambiguously conﬁrmed the well-aligned structure of SH1, peel-off structure of SH2 and sandwich like structure of SH3. To understand further, the schematics of SH1, SH2 and SH3 structure is illustrated in Figure 2 a-c, respectively. In addition, TEM cross-section was also conducted to study the exact position and hierarchical changes of graphene on SH1, SH2 and SH3, respectively. Thus, the TEM cross-sectional structure corresponds to SH1 in Figure 1d clearly indicates the defect-free structure of graphene and ZNTs, where the red marked region reveals the honey-comb structure of graphene. Similarly, TEM micrographs of Figure 1e and 1f markedly represents peel-off structure and sandwich-like structure of SH2 and SH3. Thus, Figure 1e minutely represents the self-peel off of graphene from the



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substrates, which collapse the structure of ZNTs and simultaneously wrapped on ZNTs. Nevertheless, the structure of SH3 in Figure 1f is quite similar to SH2, however after the growing time of 18 hours, few graphene is elevated itself to the top of ZNTs and few are blended at bottom, eventually sandwich-like structure was observed. The FESEM and TEM microstructure therefore exhibits the hierarchical growth of SH1, SH2 and SH3. Furthermore, AFM was conducted to analyze the topography and surface roughness of the as-obtained samples SH1, SH2 and SH3. Figure 3a-c displays the AFM images of SH1, SH2 and SH3, respectively. While the well-aligned structure of ZNTs is evident from SH1, the AFM images from SH2 and SH3 displays the defective structure of ZNTs. In addition, the three-dimensional AFM images of the samples are shown in Figure 3d-f. It is markedly seen that ZNTs of SH1 shows uniform growth, in contrast, SH2 and SH3 exhibits the coalescence of ZNTs due to the hierarchical changes of graphene. The surface topologies of SH1, SH2 and SH3 are therefore consistent with the FESEM images (Figure 1a-c). The surface roughness was also measured for the aforementioned samples with the scan range of 10 µm x 10 µm. The RMS (root-mean square) surface roughness of SH1 (65.3 nm) is smaller than SH2 (146 nm) and SH3 (163 nm), which shows the high surface uniformity of SH1. The smallest surface roughness of a material normally exhibits homogeneous structure and makes a highly desirable material for electronic device applications.33, 34 3.2 Structural characteristics of ZNTs/G nanohybrid heterostructures Figure. 4a shows the XRD pattern of SH1, SH2 and SH3, respectively. The diffraction features present at the 2θ values (crystalline plane) of 31.7⁰ (100), 34.45⁰ (002), 36.2⁰ (101), 47.5⁰ (102), 62.95⁰ (103) and 67.85⁰ (200) is in good accordance with a hexagonal wurtzite structure of ZnO (JCPDS: 65-3411). However, the diffraction peaks at 2θ= 31.7⁰ and 36.2⁰ corresponding to (100) and (101) exhibits low intensity as compared to the diffraction peak of 34.45⁰ (002) due to the well-etched ZNTs.35 Besides, the diffraction peaks obtained from the 2θ values of 26.35⁰ (002), 44.25⁰ (101) and 51.5⁰ (204) are well matched with a hexagonal Carbon (JCPDS: 75-1621).36 The weak diffraction peaks of SH1, SH2 and SH3 in Figure 4a indicates the concentration of graphene is comparatively lower than crystalline ZNTs. However, compared to SH1 and SH2, SH3 shows the much weaker diffraction peaks due to the amalgamation of ZNTs and graphene after certain growth time. Thus, the XRD analysis evidently confirms the crystalline growth of ZNTs on a monolayer graphene substrate.


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Figure. 4b shows the Raman spectra of as-obtained samples SH1, SH2 and SH3, respectively. The Raman features obtained from all the samples exhibit broad peaks at 435 cm-1, 517 cm-1, 1321 cm-1 and 1574 cm-1. The peaks at 435 cm-1 and 517 cm-1 corresponds to the hexagonal wurtzite phase of ZNTs, respectively.37 Thus, the peak at 435 cm-1 is attributed to the high frequency vibration modes of E2 and the high intense peak at 517 cm-1 related to the longitudinal optical (LO) phonon mode of A1, which reveals the formation of defects (or) oxygen vacancies. The larger defects (or) oxygen vacancies obtained from the ZnO nanostructures make it a suitable material for gas sensor applications.38, 39 The inset image of Figure 4b shows the broad peaks of D-band and G-band, respectively. While the D-band (1321 cm-1) is corresponds to the disordered carbon from a monolayer graphene and the in-plane stretching vibrational G-band (1574 cm-1) is associated with sp2 C-C bonds, respectively.40, 41 3.3 Hydrogen sensing properties and mechanism To explore the hydrogen sensing properties of SH1, SH2 and SH3, all the as-fabricated samples were tested under different concentration (ppm) of hydrogen gas at room temperature. Figure 5a-c shows the dynamic sensor response curves of SH1, SH2 and SH3, respectively. The sensor response of a sensor is normally defined as S (%)= Ra/Rg x 100 for reducing gases and S (%)= Rg/Ra x 100 for oxidizing gases, where Rg is the resistance of hydrogen gas and Ra is the resistances measured in air.42 It is observed that SH1 shows an ultra-high response even at low hydrogen concentration of 10 ppm and, also it shows the enhanced response when the ppm level is gradually increased. In contrast, the response curves of SH3 exhibits slightly lower value than SH1, however, SH2 shows very poor response which can be attributed by their materials’ structure. In addition to high response, fast recovery time also has much attention to ensure the sensor material is suitable for practical application. The sensor response curves in Figure 6a is shown to compare the response and recovery time of SH1, SH2 and SH3 at 100 ppm, respectively. It is apparently seen that the gas response of SH1 is very stable after the response reached 90% of its saturation value and exhibits fast recovery in vacuum when the response reduced to 10% of its saturation value. Similarly, it is well-known that selectivity is an important factor of gas sensing properties, subsequently, the as-fabricated samples (SH1, SH2 and SH3) were investigated with other test gases such as acetone and ammonia at 100 ppm. The bar diagram in Figure 6b summarizes the cross sensitivity of hydrogen, ammonia and acetone. It clearly demonstrates that S H1 has high



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sensitivity of hydrogen over other gases. However, SH2 and SH3 shows the response of other gases depend on their growth condition and structural properties. Table 1 shows the hydrogen sensing results obtained from this study including ZnO nanorods/graphene (ZNRs/G). Accordingly, in Figure 5a-c and 6a-b, it is more evident to see that the sensing properties of SH1 is strongly greater than SH2 and SH3. Hereinafter, SH1 was applied in all studies, however, it is important to elucidate how the sensing properties of as-fabricated SH1 is superior to as-fabricated SH2 and SH3. The hydrogen sensing mechanism of ZNTs/graphene nanostructure based sensors (SH1, SH2 and SH3) can be elucidated in terms of spill over process and metallic layer formation. Specifically, when the hydrogen gas is exposed to ZNTs/G, the metal electrode dissociates the hydrogen molecules into hydrogen atoms and thereby interact with graphene followed by ZNTs. As reported, Pt is a good metal electrode to dissociate hydrogen molecules into atoms at room temperature and led to spill over mechanism.43 Subsequently, the adsorbed oxygen species (O- or O2-) on the surface of ZNTs/G react with hydrogen atoms, which increase the free electron concentration on the conduction band and thereby decrease the resistance of the sensor. In contrast, when the sensor is exposed to air, the oxygen adsorbed surface of ZNTs/G extract free electrons from the conduction band of ZNTs, and ionize the oxygen molecules into oxygen species (O- or O2-). This decreases the free electron concentration which results the formation of electron depletion layer, and thereby increase the sensor resistance. From Figure 5 and 6, it is observed that SH1 sensor shows superb sensing properties, which can be attributed to the proper electron transportation between graphene and ZNTs heterostructure. A spill over process occurs, when the hydrogen gas is injected to each as-fabricated sensor. That is, initially the hydrogen molecules interact with metal electrodes, which dissociates hydrogen molecules into hydrogen atoms. Subsequently, the hydrogen atoms were interacted with the surface of ZNTs/G (act as receptor) and finally migrated throughout the surface of the receptor. On the other side, a heterojunction barrier is formed between the interface of ZNTs and graphene due to their different work functions, governs the electron transportation of ZNTs/G nanostructure. Because, when a hydrogen gas is exposed to ZNTs/G nanostructure, a metallic Zn layer is induced and generate the potential barrier on not only the ZNTs/G, but also on the Zn/ZNTs interface. To equalize the EF (Fermi energy) level of ZNTs and graphene, electron transportation occurs from the conduction band of ZNTs to graphene without any obstruction, since the work function of


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graphene is less than that of ZNTs. This, in sequence, leads to the free charge carrier electrons on their conduction band, which decrease the SH1 sensor resistance and thereby highly increase the sensitivity. While the band diagram of SH1 sensor is depicted in Figure 7a. On the contrary, in SH2 sensor, the electron transportation is occurred from graphene to ZNTs due to its structure, evident from SEM image (Figure 1b). In this case, electrons in the conduction band cannot be easily transferred from graphene to ZNTs (as shown in Figure 7b). This confines the free charge carrier concentration in the conduction band and leads to the weak formation of depletion region, which poorly reduce the resistance of the sensor. Besides, SH3 sensor exhibits slightly good response as compared to SH1 sensor due to the sandwich like structure as revealed from TEM microstructure. More specifically, the SH3 sensor can be ascribed from the structure of SH1 and SH2 sensor, where the band diagram is given in Figure 7c. It is clearly shown that the structure of SH3 sensor has two regions, one with graphene to ZNTs and another one deals with ZNTs to graphene. In this case, the first region struggles to form the depletion layer as likely to be mentioned in SH2 sensor, however the working of second region is similar to SH1 sensor. The second region is thus working as sensor region by forming the depletion layer (due to the free charge carrier concentration on the conduction band) when exposed to the hydrogen gas. The abovementioned mechanism and results are consistent, which clearly demonstrates the selectivity of SH1 sensor among SH2 and SH3 sensor. Because stability and repeatability are the two remarkable aspects of gas sensors, consequently, the as-fabricated SH1 sensor was investigated to check out the repeatability and stability of the sensor device. Figure 8a-c exhibits the reliability, reproducibility and long-time response curves of SH1 over hydrogen gas, respectively. As in Figure 8a, the SH1 sensor was tested under arbitrary concentrations of 10 ppm, 30 ppm and 50 ppm with cycles of 5 min gas response and 5 min recovery. The as-fabricated SH1 sensor shows a good consistent response for each concentrations’ on-state and off-state. Utilizing the properties of repeatability, the cycling performance of SH1 sensor was exposed at 30 ppm over 20 cycles of 3 min on/off state (Figure 8b). The as-fabricated SH1 sensor had a good repeatability, demonstrating a rapid and stable response in each on/off cycle. To evaluate the long-time response, the as-fabricated SH1 sensor was exposed to 30 ppm hydrogen gas for ~80 min (Figure 8c), which suggested that the nanohybrid heterostructure of ZNTs/G had superb hydrogen stability. This outstanding enhancement in



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sensing properties of SH1 sensor is ascribed to the formation of strong metalized region in the ZNTs/G interface (due to the surface of ZNTs) under the exposure of hydrogen gas. Aside from the stability and repeatability, the long-term stability is also a significant factor for sensor devices from the view of practical applications. The sensor response of SH1 sensor was thus evaluated at 10 ppm of hydrogen over a period of 90 days with the continuous cycle of every 5 days, which is shown in Figure 8d. This, in turn, the insets of Figure 8d exhibit the splendid response of first day to 90 day, which confirms the long-term stability of SH1 sensor. The large surface area of graphene and the high oxygen vacancies of ZNTs can be attributed to the long-term stability of SH1 sensor. Generally speaking, graphene material offers a large surface area for the dispersion of ZNTs and thereby produce a large number of active sites for hydrogen sensing. The adsorbed oxygen molecules on the SH1 surface gain electrons from the conduction band and form a metallic region, which creates the electron accumulation layer. However, in comparison, the ZNTs/G is more sensitive to hydrogen gas than ZNRs/G (the sensor response is given in Table 1) due to the higher amount of oxygen vacancies present on the surface of ZNTs. The process reaction44 of oxygen adsorption on ZNTs/G is written as: O2 (gas.) O2 (adsn.) + eO2- (adsn.) + e-

O2 (adsn.) O2- (adsn.) 2O- (adsn.)

(i) (ii) (iii)

From these equations (i-iii), it is cleared that the charge carrier concentration of ZNTs/G reduces in air and therefore, the SH1 sensor resistance increases. In contrast, while the interaction of SH1 sensor with hydrogen gas, the adsorbed oxygen ions revert the electrons back to ZNTs/G. The reaction process is given as: H2 + O-(adsn.)

H2O + e-

(iv)

As a consequence, the charge carriers of ZNTs/G increase according to equation (iv) and the SH1 sensor resistance decrease, which gives the high sensitivity. Furthermore, the ZNTs/G material based sensors possess better hydrogen sensing properties compared to previous studies of different materials based hydrogen sensors, which are listed in Table 2. There are several factors that make the ZNTs/G hydrogen sensors perform better than other graphene hybrid with metal and metal


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oxide semiconductor such as G-ZnO, G-Pd, G-Pt, G-SnO2, GNR-Pd, CNT-Pd/Ni, and bare ZNTs. First, graphene itself cannot give high sensitivity towards hydrogen very effectively. However, graphene offers good robustness for sensor from the prospective of device application due to their large surface area. Correspondingly, ZNTs offers high sensitivity, still, long-life time is one of its most significant drawback. Hence, the ZNTs/G materials hybridization can therefore effectively adsorb and desorb the hydrogen gas and, subsequently gives the ultra-high response with longterm stability at room temperature. Due to the aforementioned features of ZNTs/G heterostructures, it can serve as long lifetime hydrogen sensors with high reproducibility even at different concentrations of hydrogenation/dehydrogenation, which is highly advantageous compared with other materials based hydrogen sensors up-to-date. 4. CONCLUSION The self-assembled hierarchical ZNTs/G nanohybrid heterostructures were effectively prepared by a facile hydrothermal solution route followed by CVD growth. The present study focused on the different nanostructures of ZNTs/G obtained from hydrothermal method, subsequently, named as SH1, SH2 and SH3. The SH1 sensor thus gives 2 and 10 times higher sensitivity than SH3 and SH2 sensors, also exhibit fast response time of 30 s and recovery time of 38 s at room temperature. The obtained sensing performance of ZNTs/G is overwhelmingly better than as-grown ZnO and graphene materials based previous studies. The possible mechanism involved in the improvement of sensitivity is ascribed to the formation of metalized Zn layer, which forms the multiple depletion layers on the ZNTs/graphene interfaces. Aside from sensitivity, the SH1 sensor is also well retained the sensor’s repeatability and reliability even exposed at low hydrogen concentrations. Importantly, another mechanism of spill over process was also involved to produce the striking long-term stability of SH1 sensor, which can be probably used as a hydrogen storage material. Therefore, a practical approach of both physical and chemical route tailored the defect-free ZNTs/G nanostructure and, demonstrates a potential for developing an efficient candidate for toxic/explosive gas sensors.



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 ASSOCIATED CONTENT Supporting Information SEM images.  AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Bohr-Ran Huang) Notes The authors declare no competing financial interest.  ACKNOWLEDGEMENTS The authors like to thank the financial support of Ministry of Science and Technology of Republic of China through the project No. MOST 105-2221-E-011-012.


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Figure captions: Figure 1: The FESEM images of (a). SH1, (b). SH2 and (c). SH3. The TEM microstructure cross sections of (d). SH1, (e). SH2 and (f). SH3 and FESEM cross section images of inset (d). SH1, (e). SH2 and (f). SH3. Figure 2: The schematic illustration of (a). SH1, (b). SH2 and (c) SH3. Figure 3: The AFM images of (a). SH1, (b). SH2 and (c). SH3. The 3-Dimensional AFM cross section view of (d). SH1, (e). SH2 and (f). SH3. Figure 4: (a) The XRD spectra of (I). SH1, (II). SH2 and (III). SH3, (*indicates the crystal plane of graphene) and (b) The Raman spectra of (I). SH1, (II). SH2 and (III). SH3. Figure 5: The sensor response curves of (a). SH1, (b). SH2 and (c). SH3. Figure 6: (a). The response and recovery curves of SH1, SH2 and SH3 and (b). The cross-sensitivity curves of SH1, SH2 and SH3 towards other gases. Figure 7: The respective band models of (a) SH1, (b) SH2 and (c) SH3. Figure 8: (a) The repeatability, (b) reliability (c) stable response of hydrogen and (d) long-term stability curves of SH1 sensor.



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Table 1. Comparison of sensor response of as-fabricated sensors in this study. As-fabricated sensors

Sensor response (%) 10ppm

30ppm

50ppm

80ppm

100ppm

ZNRs/G

1.5

2.1

2.7

3.0

3.3

ZNTs/G (SH1)

14.3

18.31

21.27

24.4

28.08

ZNTs/G (SH2)

0.98

1.99

2.98

4.39

5.69

ZNTs/G (SH3)

8.68

11.08

13.43

15.47

18.76


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Table 2. Comparison on hydrogen sensing properties in 100 ppm of various hydrogen sensors operating at room temperature. Hydrogen sensors 1

CNT- Pd-Ni MWNT-Pt19 G-Pt19 G-Pd2 GNRs-Pd3 G-PMMA-Pd24 G-Pt-TiO2 21* G-Pd-TiO2 21* G-SnO222 G-ZnO25* ITO23 ZNRs-Mg8 ZNRs- Pd9 TiO2 NTs-Pt11 ZNTs 15 G-ZNRs this study G-ZNTs this study

Sensitivity (%)

Tres

Trec

Stability

7.5 8 16 5 2.5 17 1.2 1.5 2.8 2.3 18 18 9 6.9 16.2 3.3 28.1

312 s 9m 7m 10 m 60 s 1.8 m ---22 s -85 s 1.8 m --1.2 m 30 s

150 s --20 m 90 s 5.5 m ---90 s -70 s 2.04 m --1.8 m 38 s

Good Acceptable Acceptable Very Poor Acceptable Good Acceptable Good Acceptable Good Acceptable Very Poor Acceptable Acceptable Very Poor Acceptable Very good

@180⁰C and 25*@150⁰C Response time (Tres) and Recovery time (Trec)

21*



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Figure 1


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Figure 2



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Figure 3


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Figure 4



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Figure 5


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Figure 6



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Figure 7


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Figure 8



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TABLE OF CONTENTS


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The Self-Assembled Hierarchical Interfaces of ZnO Nanotubes

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