Natural Biowaste-Cocoon-Derived Granular Activated Carbon-Coated

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Natural Biowaste-Cocoons Derived Granular Activated Carbon-Coated ZnO Nanorods: A Simple Route to Synthesis Core-Shell Structure and Their Highly Enhanced UV and Hydrogen Sensing Properties Adhimoorthy Saravanan, Bohr-Ran Huang, Deepa Kathiravan, and Adhimoorthy Prasannan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11051 • Publication Date (Web): 20 Oct 2017 Downloaded from http://pubs.acs.org on October 21, 2017

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ACS Applied Materials & Interfaces

Natural Biowaste-Cocoons Derived Granular Activated Carbon-Coated ZnO Nanorods: A Simple Route to Synthesis Core-Shell Structure and Their Highly Enhanced UV and Hydrogen Sensing Properties Adhimoorthy Saravanan †, Bohr-Ran Huang†*, Deepa Kathiravan† and Adhimoorthy Prasannan$ †

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. $

Soft Matter Lab, Materials Science Engineering, National Taiwan University of Science and Technology,

Taipei 106, Taiwan, R.O.C.

ABSTRACT: Granular activated carbon (GAC) materials were prepared via simple gas activation of silkworm cocoon, and coated on ZnO nanorods (ZNRs) by facile hydrothermal method. The present combination of GAC and ZNRs show a core-shell structure (where the GAC is coated on the surface of ZNRs), is exposed by the systematic material analysis. The as-prepared samples were then fabricated as dual-functional sensors and, most fascinatingly, the as-fabricated core shell structure exhibits better UV and H2 sensing properties than those of as-fabricated ZNRs and GAC. Thus, the present core-shell structure based H2 sensor exhibits a fast response of 11% (10 ppm) and 23.2% (200 ppm) with ultra-fast response and recovery. While the UV sensor offers an ultrahigh photo-responsivity of 57.9 AW-1, which is superior to as-grown ZNRs (0.6 AW-1). Aside from this, switching photoresponse of GAC/ZNRs core-shell structure exhibits higher switching ratio (between dark and photo current) of 1585 with ultra-fast response and recovery than as-grown ZNRs (40). Due to the fast adsorption ability of GAC, it was observed that the finest distribution of GAC on ZNRs form rapid electron transportation between the conduction bands of GAC and ZNRs while sensing H2 and UV. Furthermore, the present core-shell structure based UV and H2 sensors also well retained with excellent sensitivity, repeatability and long-term stability. The salient feature of this combination is thus it provides a dual functional sensor with bio-waste cocoon and ZnO, which is ecological and inexpensive.

Keywords. H2, UV, dual-functional sensor, granular activated carbon, ZnO-cocoon.

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1. INTRODUCTION Activated carbon (AC) synthesized from bio-wastes are easy to prepare, cost-effective and eco-friendly that exhibits excellent biological, electrical and optical properties.13 AC possesses considerable interest among researchers due to their significant enhancement in a wide range of fields such as biosensor, supercapacitors, ion detection, energy storage and various nanoelectronic devices.4,5 Numerous methods have been used for synthesizing AC from various kind of economical and renewable sources such as industrial and agricultural bio-wastes.68 On the other hand, bombyx-mori cocoons are natural silk compounds, have unique structure and multifunctional properties. The silkworm cocoons are major source of silk products and composed of two components; inner shell fibroin covered with outer shell sericin, which contains several amino acids, proteins and carboxyl elements.9-11 During silk production, large amount of dry silkworm cocoons are turn into bio-waste materials. In particular, most of the futile and dirty silkworm cocoons are discarded as industrial wastes in many countries. Apart from mining silk fiber, silkworm cocoons are admirably applied in different areas and several researchers revealed the unique multifunctional features such as photo-protection, temperature regulation, electrical membrane, water-proofing and UV protection. It is also revealed that silk worm cocoon contains various materials such as Na, Cl, K, S, Ca, Mg, Cu, Zn, which helps to generate electricity and progress of ionic charge carriers.1216 Aside from this, in the contest of finding unique and low cost material, silk wastes can be a potential alternative to synthesis biowaste based AC materials. On the other hand, ZnO is a well-studied and economic material that possesses wide bandgap of 3.37 eV with exciton binding energies. ZnO related materials effectively utilized in multifunctional device applications such as field emitters, logic circuits, solar cells, sensors, and photodetectors.17 Fortunately, wide research efforts remain to develop the novel ZnO-based


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heterostructures or core-shell structures for further enhancement.18, 19 Among ZnO nanostructures, the ZnO nanorods (ZNRs) have unique electrical and optical properties with large surface-tovolume ratio with high oxygen vacancies and defects. Up to now, few studies were focused on ZNRs based UV photodetectors and gas sensors due to their exceptional optical properties, however the stable response of ZNRs based gas or UV sensors at room temperature is limited.20 In particular, carbon based ZnO composites attained great attention in various application such as gas sensors, UV photodetectors, photocatalytic applications and so on. Because when the carbon materials combined with ZnO, the sorption nature of ZnO increases which is highly desired for sensor applications. 21, 22 UV photodetectors have wide scientific and technical appeal since they can effectively monitor UV exposures of humans and their surroundings; such substantial applications include medical diagnostics, flame detection, environmental security, and satellite communications.2325 As an example of a practical application, UV photodetectors can be used to monitor the level of UV rays flowing on human skin. Similarly, hydrogen (H2) gas has been identified as future resource for fuel cell and other commercial usage.26 However, it is dangerous when H2 explodes into atmosphere about 4 vol%, therefore, highly effective and sensitive H2 gas detector is essential to fabricate with unique materials. Hitherto, hybrid ZnO based gas sensors and UV photodetectors have shown good sensor response, however the as-grown ZnO nanostructures exhibits poor H2 performance with limited stability as reported in the previous literatures.27,28 In this context, ZNRs possesses large surface area with oxygen vacancies, enable to absorb more electrons; aside, AC materials exhibit unpredictable electrical properties and there is less focus on these devices among researchers. Thus, the silkworm cocoons derived AC-coated ZNRs nanostructure is expected to achieve enhanced optoelectronic properties such as electrical properties, effective surface



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passivation, enhanced crystallinity, and improved adhesion. Nevertheless, there is no effort have been devoted to fabricate ZnO nanorods and silkworm cocoons derived AC based devices with superior H2 and UV sensing properties. In this study, we successfully synthesized bio-waste silkworm cocoons derived AC by one step physical activation and subsequently coated into ZNRs using simple hydrothermal method. The fabricated AC-coated ZNRs core-shell structure possess high UV photo-responsivity and excellent H2 sensing properties. The systematic observations reveal that the present fabricating strategy is a promising route to improve the performance of ZnO materials based UV and H2 sensing properties. 2. EXPERIMENTAL METHODS Synthesis of AC/ZNRs The abundant bio-waste silk-worm cocoons were received from the Danee Silk International Company in Taiwan for preparation of AC material. Prior to the preparation of AC, cocoon wastes were washed by hot water and dried in oven at 100 °C. Followed by that, cleaned cocoons were cut into many pieces and burned into carbon particles. The carbonaceous materials were then activated at 800 °C for 2 h in tubular-furnace, where the N2 gas was used as triggering component to prepare AC materials. The prepared AC materials were added with zinc acetate (ZnAc, Zn (CH3COO2)·2H2O) and hexamethylenetetramine (HMT, C6H12N4) for the preparation of AC-coated ZNR. The simple hydrothermal method was used to prepare the AC-coated ZnO nanorods. In this process, different weight percentage (0.05 g, 0.1g, 0.15g and 0.2 g) of AC powder particles were added into the 40 mM precursor solution of zinc acetate and HMT. The prepared solution was then ultra- sonicated for 10 min and subsequently grown at 90 °C for 5 h. Prior to ZNRs growth, the ZnO seed layer 4 ACS Paragon Plus Environment

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was prepared on Si/SiO2 substrates via radio frequency (RF) sputtering process, which develops the adhesion and uniform growth of well-aligned ZNRs. Thus, Figure 1 explains the systematic growth process and fabrication of AC-coated ZNRs. Characterization of AC/ZNRs The surface morphology of AC-coated ZNRs were characterized by field emission scanning electron microscopy (FESEM, (JSM-6500F)). The detailed microstructure investigations were performed using transmission and high resolution transmission electron microscopy (TEM and HRTEM, Joel 2100F). X-ray powder diffraction spectra (XRD) were performed on BRUKER (D2 PHASER-XRD) with CuKα1 radiation (λ = 1.54056 Å). The bonding nature and material compositions of AC-coated ZNRs were observed using Raman Spectroscopy with an excitation wavelength of 514 nm. Electrochemical impedance spectroscopy (EIS) was performed in 0.1 M KCl solution to find the resistivity of the samples, contains the electrolyte solution of 5.0 mM [Fe(CN)6]3-/4-. Sensor fabrications and Measurements The Pt interdigitated electrodes were fabricated with multi-fingers configuration using sputtering to form AC/ZNRs based photodetector and H2 sensor devices. Photosensing measurements were examined in dark chamber using He-Cd laser (Kimmon, 1K) with a wavelength of 365 nm-UV region at laser power intensity of 1 mW. The H2 sensing properties were tested in a vacuum chamber with H2 flow of 99.99% (diluted with dry air) via a mass flow regulator at room temperature. The H2 sensing studies were measured systematically using designed ppm flow setup with source measurement unit of Keithley 237. The sensor response is calculated by S (%) = Ra/Rg x 100, where Ra is the resistance measured in air and Rg is the gas



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resistance. For the comprehensive comparison, we also grown bare ZNRs on SiO2/Si substrates and fabricated as photodetectors and H2 sensors. 3. RESULTS AND DISCUSSION Morphological and Structural properties of AC/ZNRs The morphology of AC and AC-coated ZNRs are revealed by FESEM images. Figure 2a shows the morphology of AC particles, which suggests that annealing treatment strongly affect and carbonized bio-waste silkworm cocoons remarkably. From the FESEM images, it was observed that the present AC comprises large micro-particles with highly porous structure. Such AC is typically known as granular activated carbon (GAC) and greatly diffuse the adsorbate, which is markedly desired for gas/vapor sensing.29 Hereinafter, AC is named as GAC. It is worth noticing that there are lots of noticeable micropores distributed on the surface of GAC materials due to the gas activation. The high carbonization temperature at N2 atmosphere helps the formation of high pore volumes in the activated GAC samples. The high carbonization temperature significantly improves the morphology and contributes the formation of carbon framework due to the activated sites of silk cocoon and gas molecules. Figure 2b represents the morphology of as-grown ZNRs, which are well aligned and the inset shows the enlarged image of as-grown ZNRs. Initially, four different concentration of AC (0.05 g, 0.1g, 0.15g and 0.2 g) was utilized in the preparation of GAC/ZNRs (for each concentration, ZnO solution kept constant), where the FESEM images are shown in the Figure S1 and S2. Among that, proper doping level of GAC is given in Figure 2c, represents the surface structure of GAC coated ZNRs, which are highly uniform and well distributed. The inset of Figure 2c shows the enlarged images of GAC coated ZNRs, which reveals that GAC particles evenly deposited on ZNRs and expected to form core-shell structure which is further confirmed by TEM and HRTEM. 6 ACS Paragon Plus Environment

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FESEM images reveal the morphology of GAC coated ZNRs, however the detailed analysis and composite elements were investigated by HRTEM and EDX measurements. Figure 3a shows the few nanometers thick GAC was well-coated on the surface of ZNRs and form crowned-like shapes. As shown in the inset of Figure 3a, ring-shaped patterns were detected from the selected area electron diffraction (SAED) pattern of the GAC ZNRs core–shell. Accompanied by that, HRTEM images in Figure 3b and c illustrates the detailed morphology of GAC coated ZNRs, which clearly show a layer of GAC particles covered on ZNRs. Furthermore, the lattice fringes observed from Figure 3e apparently shows the values of interplanar spacing distance were 0.59 nm and 0.26 nm, which are in good consistent with the ZnO crystalline planes of (002) and (001), respectively. Besides, the amorphous layer coated on the ZnO lattice fringes markedly confirms the core-shell structure of GAC. The presence of GAC layer on the ZNR was confirmed by TEM surface EDX spectrum analysis as shown in Figure 3d. In addition, elemental mapping from TEM was also performed to investigate the elemental distribution in the core-shell structure of GAC coated ZNRs (Figure. 4a-d). The corresponding elemental maps to GAC/ZNRs unambiguously confirmed the homogeneous dispersion of Zn (K), Zn (L), O (K) and C (K) elements within the core-shell structure. In particular, one can see that the outer layers of each element in the GAC/ZNRs, where the diameter of the nanorods differ from each other. Comparatively, C (K) element in Figure 4d markedly shows the darker region in the outer layer which confirms the finest elemental coating of GAC on ZNRs. Comparison of the X-ray diffraction patterns of the as-prepared GAC and GAC/ZNRs coreshell structure help to recognize the structural information, crystallinity and purity of GAC coated ZNRs. Figure 5a exhibits the XRD spectra of GAC, where the (002) and (101) crystal phase are observable. Figure 5b represents the crystalline XRD spectra of GAC/ZNRs core-shell structure, 7 ACS Paragon Plus Environment


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which exhibits similar XRD patterns with main characteristic peaks at 2θ values of 31.81, 34.41, 38.21, 44.21, and 56.51, corresponding to the (100), (002), (101), (102), and (110) planes of the wurtzite structure of pure ZnO lattice (JCPDF No. 36-1451) with lattice constant of a=3.249 Å , c = 5.205 Å . On the other side, prepared GAC and GAC/ZNRs core-shell structure exhibits the significant peaks of carbon, which located at 47.75⁰, 53.7⁰ and 81.45⁰ are in good accord with a hexagonal Carbon (JCPDS: 75-1621).30 Furthermore, the peak intensity diffractions corresponding to the z-axis orientation (002 and 101) are stronger rather than those of other diffractions, which is in good agreement with the results obtained from FESEM images of GAC/ZNRs core-shell structure. Figure 5c, d describes the bonding structure of as-prepared GAC and GAC/ZNRs coreshell structure, interestingly Figure 5c exhibit the Raman shifts around 1100 cm-1 to 1600 cm-1 are generally considered to be the carbonaceous materials, and those peaks are named as D band, G band and 2 D band, respectively. Moreover, Figure 5d exhibits the sharp peaks of D, G and 2D bands, indicating that GAC were well coated on ZNRs. Furthermore, the sharp peak of 1347.5 cm-1 (D-band) 1593.19 cm-1(G-band) are expected to be recognized to the existence of higher volume of carbon materials in the GAC/ZNRs core-shell structure.31,32 It is endorsing that the ZnO nanostructure has the Raman peaks were observed around 330 to 335 cm-1 and 430 to 440 cm-1, are the low and high frequency vibration modes E1 and E2, revealing the materialization of defects (or) O2 vacancies as present in Figure 5d. UV detection properties of GAC/ZNRs core-shell structures The measurements for photoresponse characteristics of different concentrations (0.05, 0.1, 0.15 and 0.2 g) of GAC on ZNRs were executed in dark and UV environments, which is given in Figure S3. According to the switching ratio results from Figure S3a, the 0.1 g of GAC coated on ZNRs is chosen for further UV detection studies. In general, the switching ratio of dark and UV


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current is stated as Iphoto/Idark and the photo-responsivity is defined as Iphoto-Idark/P-laser, where Iphoto, is the photocurrent measured under 365 nm UV illumination and Idark were measured in dark box, and P-laser is the laser power intensity. The current versus voltage (I-V) characteristics are shown in Figure 6a. Thus, the GAC /ZNRs attains a dark current value of 3.674×10-5 A at 5 V bias. In the UV illumination, the Iphoto dramatically increases to 5.825×10-2 GAC/ZNRs core-shell structure offers an ultra-high photo-responsivity of 57.9 AW-1, which is superior to as-grown ZNRs (0.6 AW-1). The GAC /ZNRs offers an excellent switching ratio of 1585.4 compared to that of the as grown ZNRs (40) and GAC (1.08), where the switch ratio curves of as-prepared GAC is given in Figure S3b. However, the bare ZNRs only exhibit dark current and photocurrent values of 1.683×10-6 A and 6.656×10-5 A. Whereas the dark current and photocurrent values of as-prepared GAC is 0.01089 A and 0.0118 A, respectively. The obtained photo-responsivity of GAC/ZNRs is superior to other semiconductor materials reported earlier that are listed in Table 1.33-43 Figure 6b and c shows the time dependent real time UV on-off response of as grown ZNRs and GAC/ZNRs device. The better response and reproducibility were observed from GAC/ZNRs photodetector at multiple on/off switching as shown in Figure 6c. The high photoresponse properties attained from GAC/ZNRs might be due to the laterally arranged multifingers that efficiently transport electron as a consequence of the presence of the conducting carbon coated layers. Similarly, it is well reported that the effective photo-carrier conversion mechanism among conducting electrodes through carbon materials in UV-detection studies.37 It is observed that the photoresponse properties of ZNRs is much lower than that of core-shell structured GAC/ZNRs. The detailed photoresponse properties are investigated using the observation of recovery time and response time. The core-shell structured GAC/ZNRs exhibit super-fast response and recovery time when exposed UV light. Interestingly, core-shell structured



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GAC/ZNRs provide the fast response within 54 s and effortlessly returned within 48 s as compared to as-grown ZNRs which is ~2 min. The significant UV behavior of core-shell structured GAC/ZNRs photodetector exhibit the super-fast UV fascinating ability i.e. absorption, photocurrent and photoresponse compared with as grown ZNRs. It is believed that the well aligned GAC/ZNRs core-shell structure increases the charge carriers and strong absorbing behavior that could be the key factor for excellent photoresponse properties.

In general, under the dark room atmosphere, ZNRs accumulate the free oxygen (O2) molecules on the surface of well oriented nanorods. The O2 absorbing behavior of decreases the carrier concentration of ZNRs and the depletion region is formed due to the mobility of inactive electrons, also the surface to volume ration become large and absorption of O2 considerably lower the conductivity of ZNRs, thus the observation of current at dark is much lower. In general, when UV light exposed on the ZNRs, the populated electrons and holes produces the pairs (e-h), which instantaneously increase the conductivity of ZNRs. Fascinatingly, the conducting nature of GAC/ZNRs generate more e-h pairs under UV illuminations. The conductivity of GAC/ZNRs further significantly increased by active photo-carriers. Due to the conducting carbon properties the electrons travel easily and pair with photo-generated hole, the UV current increases steadily up to the desorption of O2 on the nanorod surface, after UV exposure is stopped the current become lower due to hole density lower than the electron density in ZNRs. The GAC/ZNRs photodetector exhibit short recovery time due to the increment of O2 absorption and more e-h recombination due to the fast harvesting. Moreover, the conductivity is considerably increase due to the coating of AC material. It is found that the highly conductive GAC exhibit direct and easy transportation of electron charge carriers to the ZNRs. Therefore, the GAC is the key factor to optimize the UV


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photo-detection through its efficient transformation of electrons with higher density of e-h recombination. H2 gas detection properties of GAC/ZNRs core-shell structures The four different concentration of GAC on ZNRs were measured under H2 atmosphere, and observed that the different concentration (0.05, 0.1, 0.15 and 0.2 g) exhibit the sensitivity of 20.9, 23.2, 19.4 and 15.5 %, which are comparatively lower than 0.1 g of AC (23.2%). The least sensor response is observed from 0.2 g due to the weak adhesion of ZNRs on the substrate, which are given in the Figure S3c. Hence, the 0.1 g of GAC is taken for the preparation of GAC/ZNRs. Figure 7 presents comparison of H2 sensor response with time dependent, which is examined under the exposure of H2 concentrations from 10 ppm to 200 ppm. The ZNRs based H2 sensing mechanism was widely studied by several researchers. Generally, it is reported that the ZNRs easily expose with air atmosphere, resulting the creation of depletion region. As discussed in UV section, surface molecules of ZNRs play significant part in the performance of the device. Similarly, in the H2 sensing operation, the gas species captured by ZNRs strongly influence the resistance and charge carrier absorption that improve the H2 gas response. The as-prepared GAC materials (Figure 7a) possess inferior H2 sensing properties due to the high conducting nature, however, exhibit decent changes in the resistance when switch to gas and air atmosphere because of their gas/vapor adsorption ability. On the other hand, the as-grown ZNRs based sensor reveals the substantial response to H2 gas (Figure 7b). As shown in Figure 7b, the response and recovery of the as-grown ZNRs is imbalanced while switching different ppm and unstable. The GAC/ZNRs core-shell structure based sensor shows excellent response to H2 gas even at low ppm, Figure 7c thus shows the dynamic transient curves of GAC/ZNRs and the response is gradually increased while increase different H2 ppm. It should be noted that the GAC/ZNRs based sensor shows best



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stability compared to as-grown GAC and ZNRs, respectively. Thus, GAC/ZNRs gives highest H2 response of 23% at 200 ppm with excellent stability and repeatability, which is overwhelmingly better than as-grown-ZNRs (4%) and GAC (0.5%) based sensors. The response and recovery time of GAC/ZNRs core-shell structure is markedly increase as compared to as-grown ZNRs. Thus, the response and recovery time of the present core-shell structure is 18 s and 15 s, while as-grown ZNRs exhibits 72 s and 80 s, respectively. From the systematic investigation, it is noticed that the GAC/ZNRs based sensor shows best performances in H2 detection. It is because, the adsorptivity of GAC is highly diffuse the H2 molecules into H2 atom when undergoes H2 exposure, and obviously, the electron transportation between the conduction band of GAC and ZnO becomes very quick and exhibits excellent H2 sensing behavior. Utilizing the best performances of GAC/ZNR in H2 sensing, stability test was also conducted at 50 ppm, which is shown in Figure 8a. From the stability test, the sensor shows superb stability even at lower H2 concentration of 50 ppm. Apart from sensitivity and stability, selectivity is also a significant aspect of sensor devices. The as-fabricated ZNR, GAC and GAC/ZNR based sensors were evaluated at 100 ppm under different gases such as ammonia and acetone where the sensor response curves are indicated as bar diagram in Figure 8b. In addition, H2 sensor based on GAC/ZNRs shows superior performance compared to other related semiconductor and carbon based composite materials, which is listed in the Table 2.44-54 Possible H2 gas and UV detection mechanism of GAC/ZNRs core-shell structures Figure 9a shows the possible schematic model of ZNRs and GAC coated ZNRs. The gas and UV sensing mechanism of semiconducting materials such as ZNRs and GAC based core-shell structured sensors can be elucidated in terms of the modulation of space charge region by oxygen adsorption.55 Thus, the possible band model of AC coated ZNRs in air/dark environment is given


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in Figure 9b. In particular, the band gap of pristine AC is 3.6 eV while ZNRs has the band gap of 3.28 eV according to the UV-Vis spectra of GAC and ZNRs, which are in good accord with the previous studies.56-57 Whereas the measurement of UV-Vis spectra and their corresponding Tauc plot is given in the Figure S4. Under the air atmosphere, the GAC/ZNRs adsorbs oxygen by extracting more electrons from the conduction band and converted into chemisorbed ions in the surface. Meanwhile, space charge region layer is increased due to the extraction of electrons from the conduction to form chemisorbed species, thereby the resistance of GAC/ZNRs based sensor is also increased. Thus, the reaction is O2 (g) → O2 (ad.) O2 (ads.) + e- → O2- (ads.) In contrast, Figure 9c and d demonstrates the band model of GAC/ZNRs in gas/UV environment. When the GAC/ZNRs based sensor is exposed to H2 gas, the chemisorbed oxygen ions in the surface reacts with H2 leads to desorption of H2O and release huge electrons to the conduction band. Thereby, the accumulation layer is created and the space charge region is decreased by decreasing the sensor resistance. H2 + O-(ads.) → H2O + eThus, the sensor response is highly enhanced in GAC/ZNRs based sensor compared to ZNR based sensor. Correspondingly, when the sensors are exposed to dark and UV light, the size of space charge region is changed. In the dark light, the space charge region is increased by trapping huge electrons from the conduction band, which cause low conductivity. O2 (g) + e- → O2-(ads.) Contrarily, when the sensors under high energy light (UV), the electron and holes are photogenerated and recombined. Then the holes are migrated with the oxygen ions in the surface produce 13 ACS Paragon Plus Environment


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oxygen molecules with electrons, which forms the accumulation layer. Accompanied by that, the size of space charge region is reduced by increase the conductivity of the sensor, where the reaction as follows hʋ → e- + h+ h+ + O2-(ad.) → O2 (gas.) The activated carbon shell layer not only offer a larger surface region than as grown ZNRs to achieve additional H2 gas ions, but also provide more free electrons that can be easily injected from the conduction band of GAC to ZNRs. In the GAC coated ZNRs structure, the carbon particles from GAC provides additional electrons to ZNR. Thus, the adsorption capability of ZNR is increased after coated with GAC and makes sufficient active sites in air and gas/UV atmosphere. However, the UV and gas sensing performances of as-prepared ZNRs and GAC is inferior due to their poor surface active sites while reacted with gas molecules and photons. The EIS analysis is significant technique to investigate the electrical conductivity or resistance of composite material by measuring complex impedance. It is believed that the GAC/ZNRs core-shell structure exhibit superb plane conductivity and lower resistance. To confirm this phenomenon, we utilize the EIS technique to measure impedance characteristics of GAC/ZNRs core-shell structure. In general, the GAC/ZNRs core-shell structures on substrate can be demonstrated via series combination of substrate material resistance (𝑅𝑆𝑖 ) or charge transfer resistance, GAC/ ZNRs core-shell structures resistance (𝑅𝐺𝐴𝐶/𝑍𝑁𝑅𝑠 ). The model illustrates using lump component circuit diagram containing of interfacial resistance ( 𝑅𝑙𝑐 ) and interfacial capacitance (𝐶𝑙𝑐 ). The Cole-Cole plots were used to illustrate the resistance of GAC/ZNRs coreshell structure and as grown ZNRs materials. In the Cole-Cole plot, in which the imaginary part


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(( 𝑖𝑧′′(𝜔)) of impedance is schemed against real part (( 𝑧′(𝜔)) of impedance at different quantifying frequencies. 𝑧 ∗ (𝜔) =z′(𝜔) − 𝑖𝑧′′(𝜔) Basically, the semicircle in Cole-Cole plot is corresponding to the frequency response of 𝑅𝑙𝑐 and 𝐶𝑙𝑐 . In the low frequency region, reactance of 𝑅𝑙𝑐 is smaller than 𝐶𝑙𝑐 and the intersection of the semicircle represents interfacial resistance and conductivity. On the other hand, at the high frequency region, the reactance of 𝐶𝑙𝑐 is smaller than 𝑅𝑙𝑐 - 𝐶𝑙𝑐 lump circuit. Henceforth the semicircle in the Cole-Cole plot obtained at low frequency region on the real axis confirm the resistance of the materials.58,59 Notably, Figure 10 exhibits the Cole-Cole plot of as grown ZNRs and GAC/ZNRs core-shell structure. The as grown ZNRs possess relatively large resistance of 20,078 Ω, interestingly the GAC/ZNRs core-shell structures exhibit remarkably lower resistance of 6726 Ω (curve II). The appropriate resistance is obtained due to the synergistic association within ZnO and activated carbon materials that help the transport of active electrons. The EIS technique is demonstrated that the GAC/ZNRs core-shell structures not only reveals the smaller resistance and also enhanced in-plane conductivity that help the electron transportation in UV and H2 sensing studies. 4. CONCLUSIONS In summary, we reported a novel, inexpensive and ecological method to synthesis GAC from the natural bio-waste cocoons with gas treatment, and subsequently coated on ZNRs by simple hydrothermal route. The essential material analysis such as SEM, Raman and TEM results reveal that the GAC nanoparticles were coated and formed core-shell structure on the surface of ZNRs. More interestingly, the as-fabricated GAC/ZNRs core-shell structures exhibit superb gas and UV sensing performances, which is higher than that of the as-grown ZNRs. Thus, the combined impact 15 ACS Paragon Plus Environment


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of highly porous GAC and ZNRs exhibit remarkable dual functional properties due to their rapid sorption process under air/H2 and dark/UV atmosphere. Therefore, it is expected that a simple and practical approach to synthesis GAC based novel material will stimulate the rational synthesis of defect-free core-shell structures for high performance multifunctional applications.

ASSOCIATED CONTENT Supporting Information SEM images, H2 and UV sensing properties, UV-Vis spectra. 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 106-2221-E-011-036. REFERENCES [1] Madhu, R.; Sankar, K. V.; Chen, S. M.; Selvan, R. K. Ecofriendly Synthesis of Activated Carbon from Dead Mango Leaves for the Ultrahigh Sensitive Detection of Toxic Heavy Metal Ions and Energy Storage Applications. RSC Adv. 2014, 4, 1225−1233. [2] Ni, Y.; Xu, J.; Liang, Q.; Shao, S. Enzyme-free Glucose Sensor based on HeteroatomEnriched Activated Carbon (HAC) Decorated with Hedgehog-like NiO nanostructures. Sens. Actuators, B 2017, 250, 491–498. [3] Abioye, A. M.; Ani, F. N. Recent Development in the Production of Activated Carbon Electrodes from Agricultural Waste Biomass for Supercapacitors: A Review. Renew. Sustainable Energy Rev. 2015, 52, 1282–1293. [4] Madhu, R.; Veeramani, V.; Chen, S. M.; Manikandan, A.; Lo, A.Y.; and Chueh, Y. L. Honeycomb-like Porous Carbon−Cobalt Oxide Nanocomposite for High-Performance


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TABLES Table 1. Comparison table of photo-responsivity values for the present study showing significant enhancement compared to ZnO based similar results UV photodetectors

responsivity (AW-1) 1.5 0.009 0.002 0.5 2.1 0.14 2.1 6 0.18 0.03 0.25 0.6 57.9

ZnO33 ZnO/Cu2O34 ZnO/WS235 Zn2SnO4–SnO236 ZnO nanourchins-CNT37 ZNR-SWCNT38 ZNR/diamond39 ZNRs-graphene foam40 ZNR- hybrid polymer41 ZNR/porous Si42 ZnO/Zn2GeO4 porous43 ZNRS this study GAC-ZNRs this study

Light intensity (mW) -6 0.18 0.01 -1 1 1.3 1 4 -1 1

Table 2. Comparison on hydrogen response of various ZnO hybrid room temperature hydrogen sensors. H2 sensors

Multiple ZNRs44 Single ZNR45 ZnO tubes46 ZnO-Graphene47 ZNT-Graphene48 ZnO-MWCNT49 ZnO-RGO50 ZNR/In51 ZnO/Carbon fiber52 ZNR/Textile53 ZNR/Silk Protein54 ZNRsthis study GAC-ZNRsthis study

Sensor response (%)

tres (s)

trec (s)

5.3 4 16.2 2.3 28.1 1.5 1 20.5 10 5.9 17.8 4 23.2

---22 30 ---8 -60 72 18

---90 38 25 --12 -60 80 15

Operating temperature (°C) 200 RT RT 150 RT RT 190 RT 280 RT RT RT RT


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Figure Captions: Figure 1: The schematic illustration of preparation of silk worm granular activated carbon (GAC) and ZNRs core-shell structure. Figure 2: The FESEM images of GAC and ZNRs core-shell structure (a) as-prepared GAC (b) as grown ZNRs with inset shows the enlarged image (c) GAC coated ZNRs with inset of enlarged image. Figure 3: (a) TEM, (b) and (c) HRTEM, (d) EDX images and (e) lattice fringes of GAC/ZNRs core-shell structure. Figure 4: TEM elemental mapping images of GAC/ZNRs core-shell structure (a), Zn (K) (b) Zn (L) (c) O (K) and (d) C (K). Figure 5: XRD Spectra of (a) as-prepared GAC and (b) GAC/ZNRs core-shell structure; Raman spectra of (c) as-prepared GAC and (d) GAC/ZNRs core-shell structure. Figure 6: The UV sensing properties: (a) I-V curves of GAC/ZNRs core-shell structure, Real-time characteristics of (b) ZNRs and (c) GAC/ZNRs core-shell structure exposed under UV and dark light. Figure 7: The transient sensor response curves of (a) as-prepared GAC, (b) ZNRs and (c) GAC/ ZNRs core-shell structure with different H2 concentration (10 to 200 ppm) Figure 8: (a) Selectivity and (b) long-term stability (50 ppm) of GAC/ZNRs core-shell structure based sensor. Figure 9: (a) The schematic illustration and possible band models under (b) air (c) gas and (d) UV atmospheres of GAC/ZNRs core-shell structure. Figure 10: Impedance spectra of (i) as-grown ZNRs and (ii) GAC/ZNRs core-shell structure.



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Figure 1


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Figure 2



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Figure 3


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Figure 5


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Figure 7


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Figure 9


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Graphical Abstract

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Natural Biowaste-Cocoon-Derived Granular Activated Carbon-Coated

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