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Utilizing ZnO Nanorods for CO gas detection by SPR technique
Optics Communications 463 (2020) 125490
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Utilizing ZnO Nanorods for CO gas detection by SPR technique H. fallah a,b , T. Asadishad a , M. Shafiei a , B. Shokri a , S. Javadianaghezi c , W.S. Mohammed c , S.M. Hamidi a ,∗ a
Laser and Plasma Institute, Shahid Beheshti University, Tehran, Iran Laser Application in Medical Sciences Research Center, Shahid Beheshti University, Tehran, Iran c Bangkok University, Center of Research in Optoelectronics, Communication and Control Systems (BU-CROCCS), School of Engineering, PathumThani, Thailand b
ABSTRACT Detection of Carbon Monoxide gas is very vital issue in the environment. This study demonstrates a simple technique to detect Carbon Monoxide gas by Plasmon resonance peak shift in gold nanoparticles (NPs) coated on ZnO nanorods that are grown on the SiO2 surface. The structure is used to sense very low concentrations of CO gas (From 1 ppm to 15 ppm). Coating the NPs on to Zno Nanorods increases the surface-to-volume ratio and hence enhances the device sensitivity. This result allows to make a simple and precise sensor with quick and high response.
1. Introduction Monitoring the environment to detect pollution level is very vital and complicated issue that a threat to the environment and can occur in the gaseous, liquid or solid phases with concentrations as low as a few parts per trillion (ppt) [1]. Commonly cost effective and rapid techniques for identification and quantification of pollutants in composite environmental are becoming very important. Metal oxide semiconductors such as ZnO, SnO2 , TiO2 , and WO3 are widely used as gas sensors. The idea of using metal oxide semiconductors for gas sensing application is started since 1953 by working on electronic properties of Germanium at Bell laboratory. Metal oxide sensors are based on change in the electrical resistance in the present of toxic and flammable gases or reactive gases [2–12]. Nanotechnology has recently demonstrated the ability to produce accurate, inexpensive and sensitive sensors based on the unique properties of the nanostructures [13]. One-dimensional metal oxide nanostructures, such as nanowires and nanorods, provide improved sensitivity and stability of the gas sensors [14]. Recently, many attempt have been done to improve the gas sensors parameters based on ZnO semiconductors. In 1962, Seiyama was the first to propose the idea of gas sensing using zinc oxide (ZnO) thin films [15]. In the same year Taguchi [16] proposed a gas sensor using tin dioxide (SnO2 ) as a sensing material [17]. Nanomaterial metal oxides (NMOs) have exceptional optical and electrical properties due to electron and phonon confinement, high surface-to-volume ratios, modified surface work function, high surface reaction activity, high catalytic efficiency and strong adsorption ability [18]. Nanostructured ZnO materials have received considerable interest from scientists due to their remarkable performance in electronics and photonics [19]. ZnO is a significant technological material due to the absence of a center of symmetry in its Wurtzite structure, along with
large electromechanical coupling, results in strong piezoelectric and pyroelectric properties. ZnO is therefore widely used in mechanical actuators and piezoelectric sensors [20]. In addition, ZnO is a wide band-gap (3.37 eV) compound semiconductor that is appropriate for short wavelength optoelectronic applications. ZnO nanocrystals can achieve higher sensitivity and efficiency due to the increase of surface area to volume ratio, which are beneficial to light absorption/emission or electron transfer [21]. ZnO nanostructures of various morphologies can be synthesized using simple methods making them ideal for sensing applications [22]. ZnO nanostructure were grown by Sol-Gel technique for gas sensing by Hojiri et al. at 2013. Another technique to growth ZnO nanostructure is hydrothermal method. This method is one the most potential and applicable technique for growth nanostructure due to its simplicity, low cost, and high efficiency [23]. The hydrothermal growth of ZnO nanostructures is attractive as the growth conditions are mild with growth temperatures below the boiling point of water and low pressure [24]. The function of optical gas sensors is mainly based on changing the optical properties of the environment due to the presence of the target gas or due to the change in optical properties of a metal oxide surface due to the adsorption or desorption of the target gas molecules. For example, the medium containing the target gas, absorbs selectively a photon of a particular wavelength and the amount of this absorption depends on the concentration of the target gas in the medium [25]. One of the main advantages of these sensors is their function at room temperature and non-direct detecting interaction with gas molecules, which makes them suitable for long-term gas monitoring. In addition, these sensors are highly selective and are suitable for identifying specific molecules in a complex gas mixture [26].
∗ Corresponding author. E-mail address: [email protected] (S.M. Hamidi).
https://doi.org/10.1016/j.optcom.2020.125490 Received 15 September 2019; Received in revised form 14 January 2020; Accepted 10 February 2020 Available online xxxx 0030-4018/© 2020 Elsevier B.V. All rights reserved.
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Optics Communications 463 (2020) 125490
In n-type semiconductors such as, ZnO, SnO2 , In2 O3 , and WO3 , electrons (e−) are the majority charge carrier, and the Fermi level is greater than that of the intrinsic semiconductor and lies closer to the conduction band than the valence band. Interaction with the reducing gas in this case increases the conductivity. In contrast, in ptype semiconductors such as positive holes are the majority charges and the Fermi level is below the intrinsic Fermi level and lies closer to the valence band than the conduction band. The interaction with the oxidizing gas increases the conductivity [41]. In n-type semiconductor material, when the concentration of O2 vacancies is high, the concentration of electron is high on the surface and leads to increased conductivity [42]. In p-type semiconductors, the adsorption of O2 molecules on the surface of these semiconductors causes ionization and yields positive holes. A positive hole reacts with reducing gas such as CO to form CO2 or is removed through interaction with each other and decreases the conductivity. This effect can be modeled using response model [43]. The model assumes that the interaction with the target gas, the adsorbed species on the surface of metal oxide is O2 . The sensor microstructure is included in this model as well. In this model, the change in resistance or conductivity is set to be proportional to the gas concentration.
Combustion of natural gases, coals, and oil release the Carbon monoxide (CO) gas. Carbon monoxide (CO) is a colorless, odorless, and poisonous gas that can be detected by metal oxide gas sensors. Detecting the concentration of CO gas is important because it irreversibly binds to their center of hemoglobin, which prevents the adsorption of O2 that can cause death at high concentration ‘‘around 35 ppm over 8 h period’’. 7, 8 CO gas sensors can be used for different applications in home safety, measuring atmospheric concentrations, exhaust of cars, and monitoring in industries [27]. 2. Proposed scheme In the study the shift in Plasmonic peak of gold NPs on ZnO nanorods is used to detect the presence of CO gas. Surface Plasmon resonance (SPR) is a very common form of Plasmonic shift at the interface of metal film, which was first introduced in the early 1990s [28]. SPR mainly measures the changes in the refractive index (RI) near a thin metal surface in response to molecular interactions. The RI is directly correlated to the concentration of substances near the surface (in the medium). The detection system typically consists of a monochromatic, linearly polarized light source, a thin metal film in contact with the base of a prism, and a photodetector [29]. This technique utilizes the extension of the evanescent wave on the metal film surface (Au or Ag) to detect chemical changes occurring at the surface [30]. A number of SPR systems based on various sensing membranes, such as colloidal Au [31], Ag [32], magnetically microbeads and TiO2 film were applied [33,34]. Recently metal nanoparticle on the surface of semiconductor nanostructure have gained interest recently because of the improved optical properties of semiconductors The coupling of metal and semiconductor are expected to change the optical and electronic properties. It has been reported that the electron transfer from the metal surface to the surrounding semiconductor material modifies the optical properties of the metal and the semiconductors [30]. Several studies reported that gold/zinc oxide (Au/ZnO) nanocomposite films have been effectively employed to enhance the performance of SPR for the detection molecules. For the sensor chip, a gold film has been widely utilized as a sensing substrate due to its good performance for excitation of the SPR response. Au nanoparticle has been used widely in owing to its excellent properties, such as easy reductive preparation, water-solubility, high chemical stability, significant biocompatibility and affinity Also, they can be simply functionalized using thiol groups in self-assembled monolayers. They can be easily deposited using thermal evaporation process in most chip configurations [35]. The mechanism behind the change of the electrical and optical properties of the metal oxide when interacting with target gas is still debatable [36]. Some studies have shown that adsorption and desorption of gas on the surface of metal oxide are responsible for this reaction due to the change of conductivity [37]. This effect was first demonstrated using a ZnO thin film layer The negative charges trapped in the oxygen molecules cause an upward band bending, which in turn reduces the conductivity compared to the flat band case [38]. When O2 molecules are adsorbed on the surface of metal oxides, they extract electrons from the conduction band Ec and trap the electrons at the surface in the form of ions. This leads to a band bending and an electron-depleted region. The electron-depletion region is referred as the space-charge layer, of which thickness is the length of the band-bending region [39]. The reaction of these oxygen species with reducing gases or a competitive adsorption and replacement of the adsorbed oxygen by other molecules decreases. That can reverse the band bending, resulting in an increased conductivity [40] When the sensors are exposed to the reference gas with CO, CO is oxidized by O− and releases electrons to the bulk materials. Together with the decrease of the number of surface O−, the thickness of the space-charge layer decreases. The Schottky barrier between the two grains is then lowered. That makes it possible for electrons to conduct in sensing layers through different grains [36].
1 O ← O−2 + 2𝑝+ , 2 2 𝑅 = 1 + 𝐴[𝐶𝑂] 𝑅0
(1) (2)
In Eqs. (1) and (2), A is the sensitivity parameter, R is the resistance after exposure to the gas, R0 is the baseline resistance, and CO is the concentration of the target gas. Here, a linear response is assumed. The assumption is made that only the exposed area of the material exhibits a response to the target gas, where the gas can reside and interacts at the surface. The sensing semiconductor medium is divided into three regions: surface, bulk, and particle boundary. Based on these assumptions, the following response is deducted [42]: ] [ 1 (3) 𝐺𝑇 = 𝛾𝑃 𝐵 (1 + 𝐴 [𝐶𝑂]) + 𝑆 (1 + 𝐴 [𝐶𝑂]) , 𝛾 where GT is the response (GT 1∕4 RT∕R0), RT is the total sensor resistance, and R0 is the base line sensor resistance in clean and dry air, and each x 1∕4 Rx;0∕RT;0 gives (x donates as PB; particle boundary, B; bulk, or S; surface. So, is the ratio of the baseline of x to the total baseline of sensor resistance in clean and dry air, PB is the particle boundary for particle B and S is the surface [34,35]. Optically, the change of resistance can be indicated through its effect on the material permittivity. The resistance is reciprocally proportional to conductivity as R 1∕4 I∕𝜎 Ar , where Ar is the cross-sectional area and I is the width of the material. The material permittivity can be defined in terms of conductivity as: 𝜀 = 𝜀0 𝜀𝑟 + 𝑖
𝜎 = 𝜀0 (𝑛 + 𝑖𝑘)2⋮ 𝜔
(4)
Far from resonance, we can assume small dependence of the real part on the imaginary part of the permittivity. In this limit, the refractive index of the bulk oxide can be expressed in terms of conductivity as Eqs. (5), (6): √ 𝜀 1 𝜎 ) (5) 𝑛2 = 𝑟 + 𝜀2𝑟 + ( 2 2 𝜀0 𝜔 √ 𝜀 1 𝜎 𝑘2 = − 𝑟 + 𝜀2𝑟 + ( ) (6) 2 2 𝜀0 𝜔 Hence, the flow of CO gas interaction at the surface of the ZnO nanorods alters them conductivity [Eq. (3)] that directly influences the complex refractive index and hence both forward and backward scattering properties are changed. This change can be measured by the increase or reduction of the measured total reflected power. 2
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Optics Communications 463 (2020) 125490
using the hydrothermal process where an aqueous solution containing 10 mM zinc nitrate hexahydrate [Zn(NO3)2 , 6H2 O, Aldrich, 99%] and 10 mM hexamethylenetetramine (CH12 N4 , Carlo Erba, 99.5%) was used as precursor solution in the beaker and heated up at 95 ◦ C. The hydrothermal reaction time was varied from 5 to 20 h, and to maintain a constant growth rate of the nanorods, the old precursor solution was replenished with new solution every 5 h until the end of hydrothermal process. Finally, the ZnO nanorod-coated was retracted from the precursor solution and rinsed thoroughly with DI water several times. Then dried at 90 ◦ C at oven and annelid then at 250 ◦ C at furnace for 2 h. 3.4. Coating ZnO NRs by gold nanoparticles Prepared sample were used for gold (Au) coating by Phase vapor deposition (PVD) method. A thin layer of about 30 nm of Au are coated on the surface of ZnO nanorods on the SiO2 substrates.
Fig. 1. Schematic diagram of the substrate treatment with plasma discharge barrier.
3. Experimental methods 3.5. Effect of gas flow 3.1. Sample preparation In the final step, we attempt to study the effect of gas flow (particularly CO) on the optical properties of the nanorods toward low-cost gas-sensing device as demonstrated in Fig. 2 this optical setup is including a broadband halogen fiber optic illuminator, collimator, Lenses, Glan-Taylor calcite Polarizer (GT10-A), aperture, sample chamber rotation stage, Ocean spectrometer, syringe pump and Mass flow controller (MFC). In order to supply the required gas with low specific concentration form (1 ppm to 20 ppm) this setup is prepared there are two gas capsules, N2 as carried gas and CO gas. To dilute the concentration of CO gas, and adjust the rate of flowing gas through system, the syringe pump and MFC is used respectively. The flow rate and the rate of gas injection through Syringe pump and MFC is fixed about 1 lit/min (1LPM). Afterwards, the reflection spectrum is recorded for every 10 s To calculate the precise rate of injected gas, this equation is used
Preparation of substrates have two different steps: the first step is cleansing and the second step is surface modification of SiO2 substrates by Dielectric Barrier Discharge plasma. The substrates are washed with water and soap and then keep inside ultrasonic bath for 10 min. then rinse the substrates with DI water and keep for 15 min in the ultrasonic bath. The ethylic alcohol 95% is used for substrates for 15 min inside ultrasonic bath and then each of the substrates was completely dried with pure nitrogen gas. 3.2. Surface medication of SiO𝟐 by dielectric barrier discharge plasma Fig. 1 depicts a schematic of the dielectric barrier discharge plasma for surface modification of the substrate. The copper-made upper electrode was covered with quartz of 1 mm thickness to be used as the dielectric barrier. The second electrode was a stainless steel grounded mesh which was in contact with the quartz in order to generate homogeneous plasma. Plasma is an ionized gas of electrically charged particles and extremely reactive neutral particles. All of these will react with the surface of the material either by direct reaction or by elastic collision. the plasma is created in a tub in which a flow of compressed air crosses a high frequency, high voltage electrical discharge. Cleaning surface sterilization of materials such as plastic, metal, glass, textiles and recycled or composite materials. Cleaning by atmospheric plasma also allows chemically stable, highly adhesive substances to be removed and is applied without problem to practically all shapes of parts [44]. Preparation A grounded sample holder was located in a distance of 2 mm from the grounded mesh. The power supply applied to electrodes to generate the plasma is a 6 kHz DC-pulsed high voltage which is variable from 0–14 keV. Glass substrates were cut into pieces of 1 × 1 cm2 and then these samples were put on a sample holder in order to be treated with the DBD plasma, in This study has been done constant input voltage 11 keV and 0.51 W/cm2 discharge power which determined by lissajous curves for 5 min [45].
C1 V1 = C2 (V2 + V1 )
(7)
C1 is the concentration of the CO gas. V1 is the volume of injection, C2 is the required concentration and V2 is the volume of the carrier gas. All samples are exposed to the gas flow for almost 180 s The data are then analyzed showing an error range of ±1 ppm. To observe the effect of the concentration on the response of the ZnO nanorods, different concentrations of CO gas from 1 ppm to 15 ppm were flown through the gas chamber separately for 10–60 s. To ensure proper gas concentration, the flow of the desired gas and N2 was controlled. In order to get optimum results, two groups of samples (ZnO + NRs + PDMS polymer + Au and ZnO NRs + Au) were prepared with above mentioned procedures. The gas chamber and optical set up which are explained, with the concentration calculation scheme above, were used to measure the change of the reflection from NR’S. The measurements focused on the visible range from 400 nm to 800 nm. In the experiment, the spectrum is recorded every 10 s where zero second indicates no gas present. 4. Results and discussion
3.3. Growth optimization of ZnO nanorods on SiO𝟐 surface
The normalized changing SPR intensity at different CO concentrations from 1 ppm to 15 ppm. The 𝜓 delta and 𝜓 confections graph are calculated from the measured spectrum for every 60 s for each sample as defined by the following equation and depicted in Fig. 2a and b. To get better sense about the effect of gas flow, we introduce new normalized parameters as [46]:
ZnO NRs were grown on glass substrates (3 × 1 cm2 ) by using hydrothermal process. In order to synthesize colloidal solution of ZnO NPs,1 mM zinc acetate (ZnC4 H6 O4 , Merk, 99% purity) solution prepared in 20 ml ethanol (C2 H6 O, Carlo Erba, 99.7% purity) under vigorous condition at 60 ◦ C for 1 h (h). In order to deposit the seeding layer, the cleansed glass samples were placed on a hot plate while maintaining the temperature at 60 ◦ C. Then, 100 μl of the zinc acetate solution was dropped. The dropping process was repeated 8–10 times. After that the samples kept at 350 ◦ C for 1 h. Nanorods were grown by
𝛥𝑒𝑓 𝑓 = 𝜓𝑒𝑓 𝑓 =
3
𝛥𝑥𝑝𝑝𝑚 − 𝛥0𝑝𝑝𝑚 𝛥𝑥𝑝𝑝𝑚 + 𝛥0𝑝𝑝𝑚 𝜓𝑥𝑝𝑝𝑚 − 𝜓0𝑝𝑝𝑚 𝜓𝑥𝑝𝑝𝑚 + 𝜓0𝑝𝑝𝑚
(8) (9)
H. fallah, T. Asadishad, M. Shafiei et al.
Optics Communications 463 (2020) 125490
Fig. 2. Schematic of the optical set up to flow gas through sample and detect the different changes in the spectrum.
Fig. 3. 𝜓 delta for different gas flow (a), 𝜓𝑒𝑓 𝑓 efficient for different gas flow (b).
in which x introduce the amount of gas flow in each state. One way to know that the delta parameter shows the positive or negative phase shift in each incidence polarizations [39] and by effective parameter, we have the main difference between each gas flow states. These graphs show blue shift in the delta parameter which indicates the positive phase shift between p and s polarization (Fig. 3). When ZnO NRs expose gas. The refractive index is changed as it expressed through Eqs. (5), (6) and yield to this phase sensitive answer in the delta parameter. The results show that the rapid response is for samples with ZnO NRS and a thin layer of 30 nm gold. The samples were then prepared with ZnO NRs and Au nanoparticles coated on a SiO2 substrate with dimensions of 0.5 * 0.5 cm. The SEM images of ZnO nanorods on the substrates is shown in Fig. 5a.b. The images show that ZnO nanorods are grown in a well aligned and uniform manner. The XRD pattern in Fig. 5c shows that the ZnO nanorods are highly crystalline and exhibit hexagonal wurtzite structure verified from the powder diffraction standards (JCPDS) no.36 1451. The maximum XRD peak intensity was found at double the diffraction angle(2𝛩) of 34.42◦ corresponding to the (002) plane of ZnO, indicating that the as grown ZnO nanorods are well oriented in their 𝑐-axis and the prefer entail growth of the ZnO nanorods is along the [2] direction. The changing reflection intensity at Fig. 4 for CO gas at four different concentrations clearly indicate increasing the intensity by increasing concentration of CO. The rising of the signal hence indicates the intensity increase due to increase in the permittivity with the
Fig. 4. The sensitivity of chip to various concentration of gas.
increase of conductivity. Comparing the four graphs to the case of no CO, the response for CO was saturated after first 10 s for all the used concentrations: 1, 5, 10, and 15 ppm. The setup was able to measure 4
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Optics Communications 463 (2020) 125490
Fig. 5. Top View SEM Images of ZnO NR’s on SiO2 Surface (a), Cross Section (b), XRD pattern (c).
Fig. 6. The changing intensity at Fig. 6 for CO gas at four different concentrations at different time, 10 s (a), after 20 s (b), 30 s (c).
CRediT authorship contribution statement
responses as low as 1 ppm CO. This technique allows to measure this poisonous gas at very low concentration which is usually is difficult to detect at this level. The response for different concentration was recorded for every 10 s until 60 s to compare without CO as shown in Fig. 6 it is shown that the first 10 s response is more pronounced and after that the response seems to not change. This is merely due to saturations of the samples. The results were repeated for different samples with same growth condition and equal measurement conditions. Despite the differences, to a good extend, all samples show similar trend. Samples showed higher increasing in reflection compared in first 10 s with other times. however, the response for CO seems to saturate after 10 s, it can prove that, preparation of lower concentrations of CO gas was possible in the present setup due to the chamber structure and flowing method through set up. To detect CO for lower concentration of CO the siring pump and flow meter were used and calculation technique which is explained in previous section. Based on the obtained results, it is evident that the proposed configuration, and SPR allows the detection of the effect of the gas flow on the optical signal recorded from reflection of ZnO nanorods.
H. fallah: Writing - original draft, Data curation. T. Asadishad: Software. M. Shafiei: Formal analysis. B. Shokri: Adviser. S. Javadianaghezi: Methodology of plasma curing. W.S. Mohammed: Edit manuscript. S.M. Hamidi: Supervision. References [1] S. Baruah, C. Thanachayanont, J. Dutta, Growth of ZnO nanowires on nonwoven polyethylene fibers, Sci. Technol. Adv. Mater. 9 (2) (2008) 025009. [2] Yamazoe Noboru, Go Sakai, Kengo Shimanoe, Oxide semiconductor gas sensors, Catalysis Surv. Asia 7 (1) (2003) 63–75. [3] A.B. Kashyout, H.M.A. Soliman, H.S. Hassan, A.M. Abousehly, Fabrication of ZnO and ZnO: Sb nanoparticles for gas sensor applications, J. Nanomater. 2010 (20) (2010). [4] I. Yao, P. Lin, T.Y. Tseng, Hydrogen gas sensors using ZnO–SnO2 core–shell nanostructure, Adv. Sci. Lett. 3 (4) (2010) 548–553. [5] J. Chen, J. Li, J. Li, G. Xiao, X. Yang, Large-scale syntheses of uniform ZnO nanorods and ethanol gas sensors application, J. Alloys Compd. 509 (3) (2011) 740–743. [6] D. Barreca, D. Bekermann, E. Comini, A. Devi, R.A. Fischer, A. Gasparotto, et al., Urchin-like ZnO nanorod arrays for gas sensing applications, CrystEngComm 12 (11) (2010) 3419–3421. [7] X. Yang, G. Xiao, Y. Lu, G. Li, Narrow plasmonic surface lattice resonances with preference to asymmetric dielectric environment, Opt. Express 27 (18) (2019) 25384–25394. [8] M.R. Mohammadi, D.J. Fray, Nanostructured TiO2 –CeO2 mixed oxides by an aqueous sol–gel process: effect of Ce: Ti molar ratio on physical and sensing properties, Sensors Actuators B 150 (2) (2010) 631–640. [9] J. Moon, J.A. Park, S.J. Lee, T. Zyung, I.D. Kim, Pd-doped TiO2 nanofiber networks for gas sensor applications, Sensors Actuators B 149 (1) (2010) 301–305. [10] D.W. Choi, S.J. Kim, J.H. Lee, K.B. Chung, J.S. Park, A study of thin film encapsulation on polymer substrate using low temperature hybrid ZnO/Al2 O3 layers atomic layer deposition, Curr. Appl. Phys. 12 (2012) S19–S23.
5. Conclusion Utilizing the reflection of ZnO nanorods on the SiO2 substrate was applied to build a simple and low-cost gas sensor to detect CO. The ZnO nanorods were grown on the surface of SiO2 Substrate by hydrothermal method. A thin layer of Au nanoparticles coated the ZnO NRs for SPR effect. The different concentrations CO sent through system. The results demonstrated that the sensor has a higher response at lower concentrations. The CO gas has the highest response after 10 s for 15 ppm 15 ppm, after that, it is saturated. 5
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Utilizing ZnO Nanorods for CO gas detection by SPR technique H. fallah a,b , T. Asadishad a , M. Shafiei a , B. Shokri a , S. Javadianaghezi c , W.S. Mohammed c , S.M. Hamidi a ,∗ a
Laser and Plasma Institute, Shahid Beheshti University, Tehran, Iran Laser Application in Medical Sciences Research Center, Shahid Beheshti University, Tehran, Iran c Bangkok University, Center of Research in Optoelectronics, Communication and Control Systems (BU-CROCCS), School of Engineering, PathumThani, Thailand b
ABSTRACT Detection of Carbon Monoxide gas is very vital issue in the environment. This study demonstrates a simple technique to detect Carbon Monoxide gas by Plasmon resonance peak shift in gold nanoparticles (NPs) coated on ZnO nanorods that are grown on the SiO2 surface. The structure is used to sense very low concentrations of CO gas (From 1 ppm to 15 ppm). Coating the NPs on to Zno Nanorods increases the surface-to-volume ratio and hence enhances the device sensitivity. This result allows to make a simple and precise sensor with quick and high response.
1. Introduction Monitoring the environment to detect pollution level is very vital and complicated issue that a threat to the environment and can occur in the gaseous, liquid or solid phases with concentrations as low as a few parts per trillion (ppt) [1]. Commonly cost effective and rapid techniques for identification and quantification of pollutants in composite environmental are becoming very important. Metal oxide semiconductors such as ZnO, SnO2 , TiO2 , and WO3 are widely used as gas sensors. The idea of using metal oxide semiconductors for gas sensing application is started since 1953 by working on electronic properties of Germanium at Bell laboratory. Metal oxide sensors are based on change in the electrical resistance in the present of toxic and flammable gases or reactive gases [2–12]. Nanotechnology has recently demonstrated the ability to produce accurate, inexpensive and sensitive sensors based on the unique properties of the nanostructures [13]. One-dimensional metal oxide nanostructures, such as nanowires and nanorods, provide improved sensitivity and stability of the gas sensors [14]. Recently, many attempt have been done to improve the gas sensors parameters based on ZnO semiconductors. In 1962, Seiyama was the first to propose the idea of gas sensing using zinc oxide (ZnO) thin films [15]. In the same year Taguchi [16] proposed a gas sensor using tin dioxide (SnO2 ) as a sensing material [17]. Nanomaterial metal oxides (NMOs) have exceptional optical and electrical properties due to electron and phonon confinement, high surface-to-volume ratios, modified surface work function, high surface reaction activity, high catalytic efficiency and strong adsorption ability [18]. Nanostructured ZnO materials have received considerable interest from scientists due to their remarkable performance in electronics and photonics [19]. ZnO is a significant technological material due to the absence of a center of symmetry in its Wurtzite structure, along with
large electromechanical coupling, results in strong piezoelectric and pyroelectric properties. ZnO is therefore widely used in mechanical actuators and piezoelectric sensors [20]. In addition, ZnO is a wide band-gap (3.37 eV) compound semiconductor that is appropriate for short wavelength optoelectronic applications. ZnO nanocrystals can achieve higher sensitivity and efficiency due to the increase of surface area to volume ratio, which are beneficial to light absorption/emission or electron transfer [21]. ZnO nanostructures of various morphologies can be synthesized using simple methods making them ideal for sensing applications [22]. ZnO nanostructure were grown by Sol-Gel technique for gas sensing by Hojiri et al. at 2013. Another technique to growth ZnO nanostructure is hydrothermal method. This method is one the most potential and applicable technique for growth nanostructure due to its simplicity, low cost, and high efficiency [23]. The hydrothermal growth of ZnO nanostructures is attractive as the growth conditions are mild with growth temperatures below the boiling point of water and low pressure [24]. The function of optical gas sensors is mainly based on changing the optical properties of the environment due to the presence of the target gas or due to the change in optical properties of a metal oxide surface due to the adsorption or desorption of the target gas molecules. For example, the medium containing the target gas, absorbs selectively a photon of a particular wavelength and the amount of this absorption depends on the concentration of the target gas in the medium [25]. One of the main advantages of these sensors is their function at room temperature and non-direct detecting interaction with gas molecules, which makes them suitable for long-term gas monitoring. In addition, these sensors are highly selective and are suitable for identifying specific molecules in a complex gas mixture [26].
∗ Corresponding author. E-mail address: [email protected] (S.M. Hamidi).
https://doi.org/10.1016/j.optcom.2020.125490 Received 15 September 2019; Received in revised form 14 January 2020; Accepted 10 February 2020 Available online xxxx 0030-4018/© 2020 Elsevier B.V. All rights reserved.
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In n-type semiconductors such as, ZnO, SnO2 , In2 O3 , and WO3 , electrons (e−) are the majority charge carrier, and the Fermi level is greater than that of the intrinsic semiconductor and lies closer to the conduction band than the valence band. Interaction with the reducing gas in this case increases the conductivity. In contrast, in ptype semiconductors such as positive holes are the majority charges and the Fermi level is below the intrinsic Fermi level and lies closer to the valence band than the conduction band. The interaction with the oxidizing gas increases the conductivity [41]. In n-type semiconductor material, when the concentration of O2 vacancies is high, the concentration of electron is high on the surface and leads to increased conductivity [42]. In p-type semiconductors, the adsorption of O2 molecules on the surface of these semiconductors causes ionization and yields positive holes. A positive hole reacts with reducing gas such as CO to form CO2 or is removed through interaction with each other and decreases the conductivity. This effect can be modeled using response model [43]. The model assumes that the interaction with the target gas, the adsorbed species on the surface of metal oxide is O2 . The sensor microstructure is included in this model as well. In this model, the change in resistance or conductivity is set to be proportional to the gas concentration.
Combustion of natural gases, coals, and oil release the Carbon monoxide (CO) gas. Carbon monoxide (CO) is a colorless, odorless, and poisonous gas that can be detected by metal oxide gas sensors. Detecting the concentration of CO gas is important because it irreversibly binds to their center of hemoglobin, which prevents the adsorption of O2 that can cause death at high concentration ‘‘around 35 ppm over 8 h period’’. 7, 8 CO gas sensors can be used for different applications in home safety, measuring atmospheric concentrations, exhaust of cars, and monitoring in industries [27]. 2. Proposed scheme In the study the shift in Plasmonic peak of gold NPs on ZnO nanorods is used to detect the presence of CO gas. Surface Plasmon resonance (SPR) is a very common form of Plasmonic shift at the interface of metal film, which was first introduced in the early 1990s [28]. SPR mainly measures the changes in the refractive index (RI) near a thin metal surface in response to molecular interactions. The RI is directly correlated to the concentration of substances near the surface (in the medium). The detection system typically consists of a monochromatic, linearly polarized light source, a thin metal film in contact with the base of a prism, and a photodetector [29]. This technique utilizes the extension of the evanescent wave on the metal film surface (Au or Ag) to detect chemical changes occurring at the surface [30]. A number of SPR systems based on various sensing membranes, such as colloidal Au [31], Ag [32], magnetically microbeads and TiO2 film were applied [33,34]. Recently metal nanoparticle on the surface of semiconductor nanostructure have gained interest recently because of the improved optical properties of semiconductors The coupling of metal and semiconductor are expected to change the optical and electronic properties. It has been reported that the electron transfer from the metal surface to the surrounding semiconductor material modifies the optical properties of the metal and the semiconductors [30]. Several studies reported that gold/zinc oxide (Au/ZnO) nanocomposite films have been effectively employed to enhance the performance of SPR for the detection molecules. For the sensor chip, a gold film has been widely utilized as a sensing substrate due to its good performance for excitation of the SPR response. Au nanoparticle has been used widely in owing to its excellent properties, such as easy reductive preparation, water-solubility, high chemical stability, significant biocompatibility and affinity Also, they can be simply functionalized using thiol groups in self-assembled monolayers. They can be easily deposited using thermal evaporation process in most chip configurations [35]. The mechanism behind the change of the electrical and optical properties of the metal oxide when interacting with target gas is still debatable [36]. Some studies have shown that adsorption and desorption of gas on the surface of metal oxide are responsible for this reaction due to the change of conductivity [37]. This effect was first demonstrated using a ZnO thin film layer The negative charges trapped in the oxygen molecules cause an upward band bending, which in turn reduces the conductivity compared to the flat band case [38]. When O2 molecules are adsorbed on the surface of metal oxides, they extract electrons from the conduction band Ec and trap the electrons at the surface in the form of ions. This leads to a band bending and an electron-depleted region. The electron-depletion region is referred as the space-charge layer, of which thickness is the length of the band-bending region [39]. The reaction of these oxygen species with reducing gases or a competitive adsorption and replacement of the adsorbed oxygen by other molecules decreases. That can reverse the band bending, resulting in an increased conductivity [40] When the sensors are exposed to the reference gas with CO, CO is oxidized by O− and releases electrons to the bulk materials. Together with the decrease of the number of surface O−, the thickness of the space-charge layer decreases. The Schottky barrier between the two grains is then lowered. That makes it possible for electrons to conduct in sensing layers through different grains [36].
1 O ← O−2 + 2𝑝+ , 2 2 𝑅 = 1 + 𝐴[𝐶𝑂] 𝑅0
(1) (2)
In Eqs. (1) and (2), A is the sensitivity parameter, R is the resistance after exposure to the gas, R0 is the baseline resistance, and CO is the concentration of the target gas. Here, a linear response is assumed. The assumption is made that only the exposed area of the material exhibits a response to the target gas, where the gas can reside and interacts at the surface. The sensing semiconductor medium is divided into three regions: surface, bulk, and particle boundary. Based on these assumptions, the following response is deducted [42]: ] [ 1 (3) 𝐺𝑇 = 𝛾𝑃 𝐵 (1 + 𝐴 [𝐶𝑂]) + 𝑆 (1 + 𝐴 [𝐶𝑂]) , 𝛾 where GT is the response (GT 1∕4 RT∕R0), RT is the total sensor resistance, and R0 is the base line sensor resistance in clean and dry air, and each x 1∕4 Rx;0∕RT;0 gives (x donates as PB; particle boundary, B; bulk, or S; surface. So, is the ratio of the baseline of x to the total baseline of sensor resistance in clean and dry air, PB is the particle boundary for particle B and S is the surface [34,35]. Optically, the change of resistance can be indicated through its effect on the material permittivity. The resistance is reciprocally proportional to conductivity as R 1∕4 I∕𝜎 Ar , where Ar is the cross-sectional area and I is the width of the material. The material permittivity can be defined in terms of conductivity as: 𝜀 = 𝜀0 𝜀𝑟 + 𝑖
𝜎 = 𝜀0 (𝑛 + 𝑖𝑘)2⋮ 𝜔
(4)
Far from resonance, we can assume small dependence of the real part on the imaginary part of the permittivity. In this limit, the refractive index of the bulk oxide can be expressed in terms of conductivity as Eqs. (5), (6): √ 𝜀 1 𝜎 ) (5) 𝑛2 = 𝑟 + 𝜀2𝑟 + ( 2 2 𝜀0 𝜔 √ 𝜀 1 𝜎 𝑘2 = − 𝑟 + 𝜀2𝑟 + ( ) (6) 2 2 𝜀0 𝜔 Hence, the flow of CO gas interaction at the surface of the ZnO nanorods alters them conductivity [Eq. (3)] that directly influences the complex refractive index and hence both forward and backward scattering properties are changed. This change can be measured by the increase or reduction of the measured total reflected power. 2
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using the hydrothermal process where an aqueous solution containing 10 mM zinc nitrate hexahydrate [Zn(NO3)2 , 6H2 O, Aldrich, 99%] and 10 mM hexamethylenetetramine (CH12 N4 , Carlo Erba, 99.5%) was used as precursor solution in the beaker and heated up at 95 ◦ C. The hydrothermal reaction time was varied from 5 to 20 h, and to maintain a constant growth rate of the nanorods, the old precursor solution was replenished with new solution every 5 h until the end of hydrothermal process. Finally, the ZnO nanorod-coated was retracted from the precursor solution and rinsed thoroughly with DI water several times. Then dried at 90 ◦ C at oven and annelid then at 250 ◦ C at furnace for 2 h. 3.4. Coating ZnO NRs by gold nanoparticles Prepared sample were used for gold (Au) coating by Phase vapor deposition (PVD) method. A thin layer of about 30 nm of Au are coated on the surface of ZnO nanorods on the SiO2 substrates.
Fig. 1. Schematic diagram of the substrate treatment with plasma discharge barrier.
3. Experimental methods 3.5. Effect of gas flow 3.1. Sample preparation In the final step, we attempt to study the effect of gas flow (particularly CO) on the optical properties of the nanorods toward low-cost gas-sensing device as demonstrated in Fig. 2 this optical setup is including a broadband halogen fiber optic illuminator, collimator, Lenses, Glan-Taylor calcite Polarizer (GT10-A), aperture, sample chamber rotation stage, Ocean spectrometer, syringe pump and Mass flow controller (MFC). In order to supply the required gas with low specific concentration form (1 ppm to 20 ppm) this setup is prepared there are two gas capsules, N2 as carried gas and CO gas. To dilute the concentration of CO gas, and adjust the rate of flowing gas through system, the syringe pump and MFC is used respectively. The flow rate and the rate of gas injection through Syringe pump and MFC is fixed about 1 lit/min (1LPM). Afterwards, the reflection spectrum is recorded for every 10 s To calculate the precise rate of injected gas, this equation is used
Preparation of substrates have two different steps: the first step is cleansing and the second step is surface modification of SiO2 substrates by Dielectric Barrier Discharge plasma. The substrates are washed with water and soap and then keep inside ultrasonic bath for 10 min. then rinse the substrates with DI water and keep for 15 min in the ultrasonic bath. The ethylic alcohol 95% is used for substrates for 15 min inside ultrasonic bath and then each of the substrates was completely dried with pure nitrogen gas. 3.2. Surface medication of SiO𝟐 by dielectric barrier discharge plasma Fig. 1 depicts a schematic of the dielectric barrier discharge plasma for surface modification of the substrate. The copper-made upper electrode was covered with quartz of 1 mm thickness to be used as the dielectric barrier. The second electrode was a stainless steel grounded mesh which was in contact with the quartz in order to generate homogeneous plasma. Plasma is an ionized gas of electrically charged particles and extremely reactive neutral particles. All of these will react with the surface of the material either by direct reaction or by elastic collision. the plasma is created in a tub in which a flow of compressed air crosses a high frequency, high voltage electrical discharge. Cleaning surface sterilization of materials such as plastic, metal, glass, textiles and recycled or composite materials. Cleaning by atmospheric plasma also allows chemically stable, highly adhesive substances to be removed and is applied without problem to practically all shapes of parts [44]. Preparation A grounded sample holder was located in a distance of 2 mm from the grounded mesh. The power supply applied to electrodes to generate the plasma is a 6 kHz DC-pulsed high voltage which is variable from 0–14 keV. Glass substrates were cut into pieces of 1 × 1 cm2 and then these samples were put on a sample holder in order to be treated with the DBD plasma, in This study has been done constant input voltage 11 keV and 0.51 W/cm2 discharge power which determined by lissajous curves for 5 min [45].
C1 V1 = C2 (V2 + V1 )
(7)
C1 is the concentration of the CO gas. V1 is the volume of injection, C2 is the required concentration and V2 is the volume of the carrier gas. All samples are exposed to the gas flow for almost 180 s The data are then analyzed showing an error range of ±1 ppm. To observe the effect of the concentration on the response of the ZnO nanorods, different concentrations of CO gas from 1 ppm to 15 ppm were flown through the gas chamber separately for 10–60 s. To ensure proper gas concentration, the flow of the desired gas and N2 was controlled. In order to get optimum results, two groups of samples (ZnO + NRs + PDMS polymer + Au and ZnO NRs + Au) were prepared with above mentioned procedures. The gas chamber and optical set up which are explained, with the concentration calculation scheme above, were used to measure the change of the reflection from NR’S. The measurements focused on the visible range from 400 nm to 800 nm. In the experiment, the spectrum is recorded every 10 s where zero second indicates no gas present. 4. Results and discussion
3.3. Growth optimization of ZnO nanorods on SiO𝟐 surface
The normalized changing SPR intensity at different CO concentrations from 1 ppm to 15 ppm. The 𝜓 delta and 𝜓 confections graph are calculated from the measured spectrum for every 60 s for each sample as defined by the following equation and depicted in Fig. 2a and b. To get better sense about the effect of gas flow, we introduce new normalized parameters as [46]:
ZnO NRs were grown on glass substrates (3 × 1 cm2 ) by using hydrothermal process. In order to synthesize colloidal solution of ZnO NPs,1 mM zinc acetate (ZnC4 H6 O4 , Merk, 99% purity) solution prepared in 20 ml ethanol (C2 H6 O, Carlo Erba, 99.7% purity) under vigorous condition at 60 ◦ C for 1 h (h). In order to deposit the seeding layer, the cleansed glass samples were placed on a hot plate while maintaining the temperature at 60 ◦ C. Then, 100 μl of the zinc acetate solution was dropped. The dropping process was repeated 8–10 times. After that the samples kept at 350 ◦ C for 1 h. Nanorods were grown by
𝛥𝑒𝑓 𝑓 = 𝜓𝑒𝑓 𝑓 =
3
𝛥𝑥𝑝𝑝𝑚 − 𝛥0𝑝𝑝𝑚 𝛥𝑥𝑝𝑝𝑚 + 𝛥0𝑝𝑝𝑚 𝜓𝑥𝑝𝑝𝑚 − 𝜓0𝑝𝑝𝑚 𝜓𝑥𝑝𝑝𝑚 + 𝜓0𝑝𝑝𝑚
(8) (9)
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Optics Communications 463 (2020) 125490
Fig. 2. Schematic of the optical set up to flow gas through sample and detect the different changes in the spectrum.
Fig. 3. 𝜓 delta for different gas flow (a), 𝜓𝑒𝑓 𝑓 efficient for different gas flow (b).
in which x introduce the amount of gas flow in each state. One way to know that the delta parameter shows the positive or negative phase shift in each incidence polarizations [39] and by effective parameter, we have the main difference between each gas flow states. These graphs show blue shift in the delta parameter which indicates the positive phase shift between p and s polarization (Fig. 3). When ZnO NRs expose gas. The refractive index is changed as it expressed through Eqs. (5), (6) and yield to this phase sensitive answer in the delta parameter. The results show that the rapid response is for samples with ZnO NRS and a thin layer of 30 nm gold. The samples were then prepared with ZnO NRs and Au nanoparticles coated on a SiO2 substrate with dimensions of 0.5 * 0.5 cm. The SEM images of ZnO nanorods on the substrates is shown in Fig. 5a.b. The images show that ZnO nanorods are grown in a well aligned and uniform manner. The XRD pattern in Fig. 5c shows that the ZnO nanorods are highly crystalline and exhibit hexagonal wurtzite structure verified from the powder diffraction standards (JCPDS) no.36 1451. The maximum XRD peak intensity was found at double the diffraction angle(2𝛩) of 34.42◦ corresponding to the (002) plane of ZnO, indicating that the as grown ZnO nanorods are well oriented in their 𝑐-axis and the prefer entail growth of the ZnO nanorods is along the [2] direction. The changing reflection intensity at Fig. 4 for CO gas at four different concentrations clearly indicate increasing the intensity by increasing concentration of CO. The rising of the signal hence indicates the intensity increase due to increase in the permittivity with the
Fig. 4. The sensitivity of chip to various concentration of gas.
increase of conductivity. Comparing the four graphs to the case of no CO, the response for CO was saturated after first 10 s for all the used concentrations: 1, 5, 10, and 15 ppm. The setup was able to measure 4
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Optics Communications 463 (2020) 125490
Fig. 5. Top View SEM Images of ZnO NR’s on SiO2 Surface (a), Cross Section (b), XRD pattern (c).
Fig. 6. The changing intensity at Fig. 6 for CO gas at four different concentrations at different time, 10 s (a), after 20 s (b), 30 s (c).
CRediT authorship contribution statement
responses as low as 1 ppm CO. This technique allows to measure this poisonous gas at very low concentration which is usually is difficult to detect at this level. The response for different concentration was recorded for every 10 s until 60 s to compare without CO as shown in Fig. 6 it is shown that the first 10 s response is more pronounced and after that the response seems to not change. This is merely due to saturations of the samples. The results were repeated for different samples with same growth condition and equal measurement conditions. Despite the differences, to a good extend, all samples show similar trend. Samples showed higher increasing in reflection compared in first 10 s with other times. however, the response for CO seems to saturate after 10 s, it can prove that, preparation of lower concentrations of CO gas was possible in the present setup due to the chamber structure and flowing method through set up. To detect CO for lower concentration of CO the siring pump and flow meter were used and calculation technique which is explained in previous section. Based on the obtained results, it is evident that the proposed configuration, and SPR allows the detection of the effect of the gas flow on the optical signal recorded from reflection of ZnO nanorods.
H. fallah: Writing - original draft, Data curation. T. Asadishad: Software. M. Shafiei: Formal analysis. B. Shokri: Adviser. S. Javadianaghezi: Methodology of plasma curing. W.S. Mohammed: Edit manuscript. S.M. Hamidi: Supervision. References [1] S. Baruah, C. Thanachayanont, J. Dutta, Growth of ZnO nanowires on nonwoven polyethylene fibers, Sci. Technol. Adv. Mater. 9 (2) (2008) 025009. [2] Yamazoe Noboru, Go Sakai, Kengo Shimanoe, Oxide semiconductor gas sensors, Catalysis Surv. Asia 7 (1) (2003) 63–75. [3] A.B. Kashyout, H.M.A. Soliman, H.S. Hassan, A.M. Abousehly, Fabrication of ZnO and ZnO: Sb nanoparticles for gas sensor applications, J. Nanomater. 2010 (20) (2010). [4] I. Yao, P. Lin, T.Y. Tseng, Hydrogen gas sensors using ZnO–SnO2 core–shell nanostructure, Adv. Sci. Lett. 3 (4) (2010) 548–553. [5] J. Chen, J. Li, J. Li, G. Xiao, X. Yang, Large-scale syntheses of uniform ZnO nanorods and ethanol gas sensors application, J. Alloys Compd. 509 (3) (2011) 740–743. [6] D. Barreca, D. Bekermann, E. Comini, A. Devi, R.A. Fischer, A. Gasparotto, et al., Urchin-like ZnO nanorod arrays for gas sensing applications, CrystEngComm 12 (11) (2010) 3419–3421. [7] X. Yang, G. Xiao, Y. Lu, G. Li, Narrow plasmonic surface lattice resonances with preference to asymmetric dielectric environment, Opt. Express 27 (18) (2019) 25384–25394. [8] M.R. Mohammadi, D.J. Fray, Nanostructured TiO2 –CeO2 mixed oxides by an aqueous sol–gel process: effect of Ce: Ti molar ratio on physical and sensing properties, Sensors Actuators B 150 (2) (2010) 631–640. [9] J. Moon, J.A. Park, S.J. Lee, T. Zyung, I.D. Kim, Pd-doped TiO2 nanofiber networks for gas sensor applications, Sensors Actuators B 149 (1) (2010) 301–305. [10] D.W. Choi, S.J. Kim, J.H. Lee, K.B. Chung, J.S. Park, A study of thin film encapsulation on polymer substrate using low temperature hybrid ZnO/Al2 O3 layers atomic layer deposition, Curr. Appl. Phys. 12 (2012) S19–S23.
5. Conclusion Utilizing the reflection of ZnO nanorods on the SiO2 substrate was applied to build a simple and low-cost gas sensor to detect CO. The ZnO nanorods were grown on the surface of SiO2 Substrate by hydrothermal method. A thin layer of Au nanoparticles coated the ZnO NRs for SPR effect. The different concentrations CO sent through system. The results demonstrated that the sensor has a higher response at lower concentrations. The CO gas has the highest response after 10 s for 15 ppm 15 ppm, after that, it is saturated. 5
H. fallah, T. Asadishad, M. Shafiei et al.
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