Enhancement of gas-sensing characteristics of hydrothermally synthesized WO3 nanorods by surface decoration with Pd nanoparticles
Accepted Manuscript Title: Enhancement of Gas-sensing Characteristics of Hydrothermally Synthesized WO3 Nanorods by Surface Decoration with Pd Nanopar...
Accepted Manuscript Title: Enhancement of Gas-sensing Characteristics of Hydrothermally Synthesized WO3 Nanorods by Surface Decoration with Pd Nanoparticles Author: Pham Van Tong Nguyen Duc Hoa Nguyen Van Duy Dang Thi Thanh Le Nguyen Van Hieu PII: DOI: Reference:
Please cite this article as: P. Van Tong, N.D. Hoa, N. Van Duy, D.T.T. Le, N. Van Hieu, Enhancement of Gas-sensing Characteristics of Hydrothermally Synthesized WO3 Nanorods by Surface Decoration with Pd Nanoparticles, Sensors and Actuators B: Chemical (2015), http://dx.doi.org/10.1016/j.snb.2015.09.108 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Enhancement of Gas-sensing Characteristics of Hydrothermally Synthesized WO3 Nanorods by Surface Decoration with Pd Nanoparticles
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Pham Van Tong, Nguyen Duc Hoa*, Nguyen Van Duy, Dang Thi Thanh Le,
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Nguyen Van Hieu*
International Training Institute for Materials Science (ITIMS)
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Hanoi University of Science and Technology (HUST), No. 1, Dai Co Viet, Hanoi, Vietnam
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Corresponding authors
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* Nguyen Duc Hoa, Ph.D.
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* Nguyen Van Hieu, Ph.D. Associate Professor
International Training Institute for Materials Science (ITIMS), Hanoi University of Science and Technology (HUST) No. 1, Dai Co Viet Road, Hanoi, Vietnam Phone:
Abstract: Enhancement of the gas-sensing characteristics of nanostructured metal oxides by
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surface decoration with noble metal nanoparticles has recently received significant attention as a powerful technique for improving gas-sensing performance. Herein, we introduce a facile
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method of decorating tungsten oxide (WO3) nanorods by palladium (Pd) nanoparticles to achieve enhanced NH3 gas-sensing performance. The WO3 nanorods were synthesized by a scalable
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hydrothermal method followed by thermal calcination to activate surface bonding for Pd
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decoration. Decoration of Pd nanoparticles on the surface of WO3 was achieved through reduction of the complex Na2PdCl4 using Pluronic as both surfactant and reducing agent. The
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materials obtained were characterized by SEM, EDS, HRTEM, and XRD. The gas-sensing characteristics of bare WO3 and Pd–WO3 nanorods were tested for NH3, CO, H2, CO2, and CH4
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detection at different temperatures. Results revealed that decoration of Pd nanoparticles on the
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surface of WO3 nanorods significantly enhances the NH3 gas-sensing characteristics of the metal
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oxide. The transient stability of the sensor determined after 10 on/off cycles of switching from air to gas demonstrated the effective reusability of the fabricated device. Keywords: WO3 nanorods, Hydrothermal, Pd nanoparticles, Gas sensors
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1. Introduction Recent studies on gas sensing have focused on enhancements of sensor characteristics targeting
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lower detection limits and higher sensitivity for application in different fields, such as
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environment monitoring, disease diagnosis, military mission, security check, and industrial processing control [1–11]. The most attractive research topic in the field of resistive-type gas
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sensors is the use of nanostructured metal-oxide semiconductors, such as WO3, SnO2, ZnO, and
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In2O3, as sensing layers; researchers believe that interactions between analytes and the surface of these sensing materials will lead to significant variations in electrical resistivity [3–5]. The first
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study to use tungsten oxide (WO3) as a sensing layer revealed that this material is highly sensitive to NO and NO2 [5] but relatively insensitive to some reducing gases, such as NH3, CO,
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and H2 [1]. Decoration of the surface of tungsten oxide materials with noble metal nanoparticles
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has been reported by several researchers [6–11]. For instance, decoration of the surface of WO3
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with PdO or PtOx nanoparticles by decomposition of Pd(acac)2 and Pt(acac)2 precursors significantly enhances its H2-sensing performance with high sensitivity and good selectivity [6]. While decoration of WO3 with Pt nanoparticles enables detection of hydrogen gas at low concentrations with high sensitivity [7–9], decoration with Au or RuO2 enhances the NO2sensing characteristics of the oxide [10,11]. Decoration of the surface of metal oxide semiconductors with noble metal nanoparticles can enhance gas-sensing performance through (i) formation of a depletion layer at the interface between the metal and semiconductor and (ii) improvement through the catalytic activity of the noble metal, which accelerates the interaction between the analyte gas and the sensing layer [12–14]. Formation of a depletion layer in the metal–semiconductor junction modulates the conductive channel and enhances sensing
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performance; the catalytic activity of the noble metal further accelerates oxygen dissociation from the analyte through the spillover mechanism and results in increments in interactions between the analyte and the sensing layer [15,16]. However, till now, none of the general
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mechanism has been proposed to point out which metal /oxide materials are more advantageous over other materials to a specific gas. For instance, in the report by Liu et al. [17], they
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functionalized WO3 nanorods by Pt NPs, and they pointed out an enhancement of about to two-
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fold to Ethanol, and methanol, where the sensing mechanism was discussed under the light of catalytic activity of Pt to promote the ion sorption of oxygen species (O2-, O-, and O2-), the so
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called spillover effect. Vuong et al. [18] reported on the decoration of Au NPs on the surface of WO3 nanowires for enhanced response to H2S and CH4 gases, in which the enhancement of
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sensing characteristics were explained based on the surface catalytic activity of Au, which
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accelerates the dissociation reactions of O2, CH4, and H2S gases, thereby enhancing the oxygen
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ion sorption reaction and the gas-sensing reactions. The decoration of Pd nanoparticles on the surface of WO3 nanorods for enhanced NH3 gas sensing performance has not been systematically
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investigated yet.
Among the noble metals, Pd is one of the metals most commonly used as a catalyst or additive for decoration of WO3 to enhance gas-sensing characteristics [12,16,19]Error! Reference source not found.. Decoration of the surface of WO3 with Pd can be done through a number of methods. For instance, Boudiba et al. decorated the surface of tungsten oxide with Pd nanoparticles by dispersion of WO3 powders in a terpineol solution of palladium chloride salt followed by sonication and annealing at high temperature [20,21] to obtain Pd–WO3 nanocomposites. Liu et al. synthesized Pd nanoparticles on WO3 powders by Teflon-lined autoclave hydrothermal treatment using iodide ions as a strong adsorbate and poly(vinyl
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pyrrolidone) as the capping agent for PdCl2 reduction [22]. Fardindoost et al. prepared Pd-doped WO3 films by the sol–gel method for hydrogen sensing; the resultant sensors could monitor hydrogen gas at room temperature [23]. A thin film of WO3 decorated with Pd nanoparticles was
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also prepared by advanced reactive gas deposition for sensing applications [24]. While the Pd nanoparticles decorated the WO3 particles, both thick and thin films have fairly low sensitivity;
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thus, decoration of the surface of one-dimensional tungsten oxide, such as nanowire or nanorods,
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with Pd for enhanced gas-sensing characteristics has been recently reported [25–27]. Wisitsoorat et al. [25] reported decoration of the surface of WO3 nanorods with Pd nanoparticles by sputter
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deposition followed by thermal annealing at high temperature. Sputter deposition enables exact control of the thickness of the deposited Pd layer and, consequently, the density of Pd
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nanoparticles on the top surface of nanorods. Unfortunately, since sputter decoration is a high-
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vacuum and anisotropic process, it is unsuitable for mass production and decoration of the total
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surface of the dense sensing layer [26]. Chavez et al. [27] decorated the surface of WO3 nanowires with Pd nanoparticles by the drop casting method; however, this method was
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inefficient because the Pd nanoparticles tended to aggregate on the surface of WO3 nanowires. Aggregation or overlapping of Pd nanoparticles leads to less exposure of the WO3 nanowire surface to dissociated oxygen species and results in lower capture of electrons on the surface of the nanowires as well as lower sensing characteristics [27]. Thus, facile decoration of the surface of tungsten oxide with Pd nanoparticles for effective gas-sensing application through low-cost fabrication of large-scale highly sensitive gas sensors continues to challenge scholars today [6]. Reports dedicated to enhancement of the NH3 gas-sensing characteristics of tungsten oxide by surface decoration are also fairly limited despite the importance of such a gas sensor in environmental monitoring and disease diagnostic applications.
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Herein, we report the facile hydrothermal synthesis of tungsten oxide nanorods and their decoration with Pd nanoparticles to achieve enhanced NH3 gas-sensing characteristics. The synthesis procedure is based on in situ reduction of Pd salt on the surface of hydrothermally
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synthesized WO3 nanorods by using Pluronic (P123) as a reducing agent. Considering their exceptional robustness, superior characteristics, and simple solution process, WO3 nanorods
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decorated with Pd nanoparticles show excellent performance for NH3 gas sensing at low
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concentrations ranging from 100 ppm to 1000 ppm as well as fast response and recovery times. Decoration of the surface of WO3 nanorods with Pd nanoparticles not only enhances NH3
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sensitivity but also reduces the optimal working temperature of the sensor from to 400 °C to 300 °C. The effects of Pd loading amounts on the responses of the sensors to NH3, CO, H2, CO2, and
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CH4 were also studied and are reported in this work.
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2. Experimental
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WO3 nanorods were synthesized by a hydrothermal method as reported in a previous study [28].
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In a typical synthesis, sodium tungstate hydrate (1.5 g), sodium chloride (0.5 g), and P123 surfactant (0.5 g) were dissolved in 80 mL of deionized water; the pH of the solution was adjusted to 2.5 by addition of a sufficient amount of concentrated hydrochloric acid. This solution was poured into a Teflon-lined autoclave for hydrothermal treatment at 200 °C for 12 h. After cooling to room temperature, the precipitated products were collected by centrifugation at 4000 rpm for 10 min. The collected products were washed several times with deionized water and dried at 45 °C for 24 h prior to Pd decoration and characterization. After calcination at 400 °C for 2 h to activate the surface of WO3 nanorods, 300 mg of the collected powders were dispersed in a 2 mL mixed aqueous solution of PdCl2 and NaCl using ultrasonic vibration and magnetic stirring. Note that heat treatment of sample at temperature of about 400oC/2h is very
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important to activate the surface of WO3 nanorods by forming oxygen-containing groups for anchoring Pd nanoparticles. Without heat treatment, it is difficult to decorate Pd NPs on the surface of WO3 NRs as can be seen in Figure S8 (Supplementary Material). We varied the PdCl2
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content of 100 mg, 125 mg, and 150 mg to obtain different loading amounts of Pd NPs on the surface of WO3 nanorods. The samples were noted as Pd–WO3–100, Pd–WO3–125, and Pd–
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WO3–150 for PdCl2 content of 100 mg, 125 mg, and 150 mg, respectively. After stirring the
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mixture solution for several minutes, a solution of 1 g of Pluronic P123 dissolved in 40 mL of H2O was added to the above solution for reduction of Pd2+ into Pd nanocrystals. The reduction
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process was performed for 2 h in ambient atmosphere at room temperature. Finally, the products were collected and washed with ethanol through centrifugation at 4000 rpm. The synthesized
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materials were characterized through advanced techniques, such as field-emission scanning
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electron microscopy (FE-SEM, JEOL model 7400), and high resolution transmission electron
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microscopy (HRTEM, FEI Tecnai G2). Elemental analyses were performed through energydispersive X-ray spectroscopy (EDS) with an X-ray analyzer integrated into the FE-SEM
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instrument. The crystal structures of the materials were studied through wide-angle powder Xray diffraction (XRD) using CuKα X-radiation with a wavelength of 1.54178 Å. For sensor fabrication, 20 mg of the synthesized materials was gently dispersed in DMF solution using an ultrasonicator to obtain a colloidal solution of 4 mg/mL. Thereafter, the colloidal solution was dropped onto a thermally oxidized Si substrate equipped with a pair of interdigitated Pt electrodes. As-obtained sensors were dried at room temperature for 24 h and subsequently heat-treated at 600 °C for 2 h to stabilize sensor resistance. The gas sensors were measured by a flow-through technique with a standard flow rate of 400 sccm for both the dry air balance and analyte gases. Prior to the measurements, the sensors were aged at 400 °C for 2 h
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while dry air was flown through the sensing chamber until the resistance of the sensors had stabilized. Gas-sensing characteristics were measured at different temperatures ranging from 250 °C to 400 °C. During sensing measurement, the resistance of the sensors was continuously
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air balance and analyte gases were switched on/off in cycles [1,28].
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measured using a Keithley sourcemeter (Model 2602) interfaced with a computer while the dried
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3. Results and discussion 3.1. Materials characterization
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Morphological and elemental analysis results of bare WO3 nanorods after heat treatment are shown in Figures 1(A) and 1(B), respectively. The FE-SEM image shows that WO3 nanorods
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have an average diameter of about 160 nm and lengths of up to few micrometers. The surface of
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the WO3 nanorods was very smooth and clean and appeared to have been formed from a single
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crystal. The WO3 nanorod appeared to be formed by aggregation of smaller nanorods; thus, the surface of the nanorod showed a straight-line structure. As compared with the as-hydrothermal
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product the morphology of the WO3 nanorods almost maintained after heat treatment, where only slightly change in morphology at the ends of the nanorods can be observed [Figure 1S(A), Supplementary Material]. The as-hydrothermal products have a hexagonal crystal structure of a preferred growth direction of (200) [Figure 1S(B), Supplementary Material]. This indicates that the easy growth direction of the nanorod is along the c-axis. Elemental analysis by EDS indicated that the main composition of the synthesized materials is W and O; the ignoble quantity of Na detected is due to retention of the starting materials that had not been completely washed away. The Pd peak at energy of ~2.83 eV was not detected in this sample [Figure 1(B)].
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Morphological and elemental analysis results of Pd nanoparticle-decorated WO3 nanorods are shown in Figures 1(C) and 1(D). The FE-SEM image of the Pd–WO3–100 sample reveals that the decoration process did not damage the morphology of the WO3 nanorods [Figure 1(C)]. Tiny
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Pd nanoparticles with an average diameter of about 10 nm homogenously decorated the surface of WO3 nanorods. The density of Pd nanoparticles on the surface of WO3 nanorods was
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estimated to be about 25 particles or less per nanorod. This density can be controlled by tuning
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the concentration of PdCl2 in the solution, the reduction time, and the amount of WO3 treated. In the context of this work, we varied the PdCl2 content to obtain Pd-WWO3 nanorod samples of
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different loading amounts of Pd nanoparticles. EDS analysis of the Pd–WO3–100 sample confirmed the presence of O and W elements from the tungsten oxide and Pd from the
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nanoparticles. The Pd composition of this sample estimated from EDS analysis was about 2.8
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sensing characteristics.
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at.%. The presence of Pd on the surface of WO3 nanorods is important in enhancing their gas-
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Figure 1.
The XRD pattern of the synthesized WO3 nanorods shown in Figure S2(A) (Supplementary Material) indicates that the material has a monoclinic crystal structure (space group P21/n (14)) with lattice parameters of a = 0.729 nm, b = 0.7539 nm, and c = 0.7688 nm, = 90.91°. The pattern is consistent with a profile of the standard monoclinic tungsten oxide (JCPDS Card No. 43-1035). The result indicates that after heat treatment, the hexagonal structure of the ashydrothermal WO3 nanorods was transferred into monoclinic phase. The strong and sharp diffraction peaks of the synthesized product confirmed the high crystallinity of the WO3 nanorods. The XRD pattern of bare Pd nanoparticles synthesized in the absent of WO3 nanorods confirms the formation of cubic Pd crystals [Figure S2(B), Supplementary Material]. The XRD
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pattern of the Pd–WO3–100 nanorods exhibits typical diffraction peaks of the monoclinic crystal structure of WO3 but no phase corresponding to Pd (JCPDS, 46-1043) or PdO (JCPDS, 41-1107), likely because of the extremely low Pd amount added to the nanorods [Figure S2(C),
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Supplementary Material].
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TEM images of the Pd–WO3–100 nanorod sample are shown in Figure 2 (A), and (B). We can see that the nanorod is not a single rod but a bundle of the smaller nanowires of an average
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diameter of about 25 nm. HRTEM image (Figure 2(D)) reveals that the WO3 nanowire is a single
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crystal. The Pd NPs of homogenous size of about 10 nm decorated on the surface of WO3 nanorods. HRTEM image of a Pd NP shown in Figure 2(C) indicates the single crystallinity
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nature of the palladium. The interspace between two adjunction plans is d111=0.24 nm, corresponding to the gap between (111) plans of the cubic Pd (JSPDS, 46-1043). The SAED
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(Figure 2(D)) confirms the single crystallinity nature of the sample. However, the diffraction
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pots are not periodically arranged, suggesting the existence of two phases, one is WO3 and
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another is Pd. TEM images of the Pd–WO3–125, and Pd–WO3–150 nanorod samples are shown in Figure S9 (Supplementary Material). It is evident that the Pd NPs tend to aggregate together when the amount of PdCl2 increases, thus, we did not increase further amount of PdCl2 in the decoration of Pd on the surface of WO3 nanorods. Content of Pd in the Pd-WO3-100 sample calculated from the TEM image is about C(wt%)=mPd/mWO3=1/173 (see Figure S10, Supplementary Material).
Figure 2
3.2. Electrical properties and gas-sensing characteristics
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The electrical properties of bare WO3– and Pd–WO3 nanorod-based sensors were studied by measuring their I–V curves in air at different temperatures. Both sensors showed nonlinear asymmetrical I–V curves. As demonstrated in Figure 3, the I–V curves of the Pd–WO3 nanorod
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sensor revealed increases in electrical current as temperature increased from 150 °C to 400 °C. The nonlinear characteristics of the I–V curves can be explained by the difference between the
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work functions of WO3 and Pt. The work function of WO3 is about 4.8 eV, much lower than that
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of Pt (6.3 eV); this difference forms Schottky contacts at the interface of WO3 and Pt [29]. Schottky contacts formed between WO3 and Pt fingers mean the fabricated device has two
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Schottky diodes arranged oppositely in a circuit; thus, the device works as a bipolar transistor device under a reverse applied voltage. This configuration is advantageous because the gas-
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sensing characteristics of a Schottky device are higher under a reverse voltage than under a
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forward voltage [30].
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Figure 3
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The gas-sensing characteristics of the fabricated WO3 and Pd–WO3 sensors were tested using different concentrations of NH3 at different temperatures. The results of the bare WO3 and Pd– WO3 nanorod samples are shown in Figures 4 and 5, respectively. Figure 4(A) shows the transient resistance versus time of the bare WO3 nanorod sensor upon exposure to different concentrations of NH3. We can see that the base resistance decreases when the operating temperature increases from 300 to 450oC as a result of the thermal activation that excites electrons from valence band to conduction band to contribute on the conduction [18]. At all measured temperatures, the sensor resistance decreased rapidly upon exposure to NH3 and returned to their original values after the analyte gas supply was switched off. The response/recovery times of the sensor to 1000 ppm NH3 at temperatures of 300, 350, 400, and
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450 °C were 15 s/76 s, 13 s/23 s, 11 s/19 s, and 9 s/15 s, respectively (Figure S3(A), Supplementary Material). Response/recovery times decreased with increasing working temperature as a result of thermal acceleration of the adsorption and desorption processes. Figure
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S3(B) (Supplementary Material) shows the concentration dependent of the response and recovery times of the bare WO3 sensor measured at 300oC. The response and recovery times tend
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to increase with NH3 concentrations. The sensor required longer time for recovery compared
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with the response time. The response/recovery times of the sensor are dependent on the adsorption/desorption speed of analytic gas molecules on the surface of sensing material.
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According to Langmuir theory, the rates of adsorption and desorption of gas molecules on the
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surface of solid material can be expressed by the following equations [31]. (1)
des=B×exp(Ea(des)/kBT)
(2)
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ads=A×exp(Ea(ads)/kBT)
Where A, B are constants, kB is Bozeman constant, T is absolute temperature, Ea(ads), and Ea(des)
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are activation energy for analytic gas molecule adsorption and desorption, respectively. In our study, the sensor has a longer recovery time compared with the response time indicating the activation energy for adsorption is higher than the activation energy for desorption. The sensor responses, Rair/Rgas, to different concentrations of NH3 measured at different temperatures are shown in Figure 4(B). Sensor responses increased with increasing NH3 concentration at all measured temperatures. For a given concentration, the sensor response increased as the working temperature increased from 300 °C to 400 °C; however, further increases in working temperature led to a decrease in sensor responses. This result suggests that the optimal working temperature of the bare WO3 nanorod sensor for sensing NH3 is 400 °C. The bell-shape of the sensor
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response versus temperature can be explained in term of diffusion-reaction processes of analytic gas molecules inside the sensing layer [32]. With increase of working temperature from 300 to 400oC, the reaction rate of NH3 molecules and pre adsorbed oxygen increase, thus sensor
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response increase. However, further increase of working temperature above 400oC, the penetration of analytic gas to the sensing layer decrease, and results in decrease of sensor
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response. This result is similar to the report by Siciliano et al [33]. Sensor responses as a function
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of NH3 concentration measured at different temperatures are shown in Figure 4(C). The data reveal that the sensor shows the highest responses at a working temperature of 400 °C. The linear
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dependence of sensor responses on NH3 concentration within the measured temperature range is
the simple design of a read-out circuit [34].
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of critical importance in real-world applications, particularly because this relationship enables
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Figure 4.
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Figure 5 shows the NH3 gas-sensing characteristics of the Pd–WO3–100 nanorod sensor
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measured at different temperatures. Similar to the results of the bare WO3 nanorod sensor, the transient resistance versus time of the Pd–WO3–100 device upon exposure to different concentrations of NH3 measured at 200, 250, 300, 350 and 450 °C showed reversible response characteristics. The response/recovery times of the sensor to 1000 ppm NH3 were estimated to be about 9 s/107 s, 4 s/68 s, 2 s/19 s, 2 s/8 s, and 1 s/7 s at 250, 300, 350, 400 and 450 °C, respectively. Details of the calculation of the response and recovery times of the Pd–WO3–100 sensor may be found in Figure S4 (Supplementary Material). The Pd–WO3–100 sensor showed much faster response times than the bare WO3 device because of the high catalytic activity of Pd, which accelerates interactions between analyte molecules and the sensing material. Response and recovery times of the sensor as a function of NH3 concentration measured at 300oC are shown in
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Figure S4(B) (Supplementary Material). Response and recovery times increase with increasing of NH3 concentration. This result is similar to that of the report by Choi et al. [35]; where they investigated the gas-sensing characteristics of functional metal oxide nanowires. Herein, the
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response time of the sensor is very fast of about 3 to 6 sec, whereas recovery time is relatively longer of about 11 to 68 sec, depending on NH3 concentration. Responses of the Pd–WO3–100
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nanorod sensor to different NH3 concentrations measured at different temperatures are shown in
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Figure 5(B). In contrast to the bare WO3 nanorod, the Pd–WO3–100 nanorod sensor exhibited maximal responsivity at a working temperature of 300 °C, which indicates that surface
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decoration with Pd nanoparticles can reduce the working temperature of the WO3 nanorod sensor [Figure 5(B)]. Sensor responses as a function of NH3 concentrations measured at different
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temperatures are plotted in Figure 5(C); here, the linear dependence of responsivity on gas
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concentration within the measured temperature range may be observed. A detailed comparison of
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the sensing characteristics of the bare WO3 and Pd–WO3–100 nanorod sensors is provided in Figure S5 (Supplementary Material). The responsivity of the WO3 nanorod sensor to NH3 was
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improved by about three times compared with that of the bare sensor by surface decoration with Pd nanoparticles. In the report by Yoon et al. [36], an enhancement of about two times was obtained to 5 ppm NH3 at 350oC by doping 5% wt. of Pd on WO3 nanoparticles. Herein, the decoration of Pd nanoparticles on the surface of WO3 nanorods not only enhanced the sensitivity, but also reduced the working temperature of sensor. Enhancement of the responsivity of the sensor may also be expected to lower the detection limit of the device. Enhancement of responsivity is a very important consideration for environmental applications because the permissible concentration of NH3 in air for safety is only 25 ppm. Figure 5
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The selectivity of the Pd–WO3–100 sensor to reducing gases, such as CO, H2, and CO2, was also tested, and the data are presented in Figure S6 (Supplementary Material). The sensor showed detectable responses upon exposure to 10–200 ppm CO and 25–100 ppm H2 but very low
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responses to 25000 ppm CO2. The response of the sensor to 25000 ppm CO2 was only 1.23, whereas its responses to 100 ppm CO, 100 ppm H2, and 100 ppm NH3 were 1.93, 1.73, and 3.9,
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respectively. We also compared the response to CO of the bare WO3 and Pd–WO3–100 nanorod
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sensors, as shown in Figure S7 (Supplementary Material). The decoration of Pd nanoparticles also enhanced the response to CO at 400oC suggesting a possibility of using this sensor for dual
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monitoring of NH3 and CO gases. A summary of the responses of the sensor to different gases is provided in Figure 5(B); the highest response of the sensor may be observed upon exposure to
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NH3. As the reusability of the sensor must be taken into account for real-time monitoring of NH3,
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the transient resistance versus time of the Pd–WO3–100 sensor was detected in 500 ppm NH3 at
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temperatures of 350 and 400 °C. Figures 5(B) and 5(C) show identical response characteristics to NH3 gas over 10 cycles of measurement, which indicates good sensor reusability at both tested
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temperatures.
Figure 6
Effects of the Pd loading amounts on the sensitivity and selectivity of the WO3 nanorods are summarized in Figure 7. Details about the transient response to different gases of the Pd–WO3– 125, and Pd–WO3–150 samples can be seen in Figure S11, and Figure S12 (Supplementary Material). As we can see, at operating temperature of 300oC, the response of the prepared sensors to NH3 is highest, followed by H2, CO, and CO2. The prepared sensors showed ignorable response to 20.000 ppm CH4. The sensor response is highest for the Pd–WO3–150 sample, followed by Pd–WO3–125, and Pd–WO3–100 samples, respectively. This suggests that in the
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studied conditions, higher amount of Pd nanoparticles loaded on the surface of WO3 nanorods can results in higher sensitivity. However, we can see that the sensor response to 100 ppm NH3 increases only 4.38 % from 4.79 to 5, when the content of PdCl2 increases 20% from 125 mg to
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150 mg. This result is consistent with the observation by TEM images that with increase of PdCl2 content the Pd nanoparticles tend to aggregate together and lower the catalytic efficiency
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[37]. The decoration of Pd NPs on the surface of WO3 nanorods not only enhanced the response
100/SWO3)
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to NH3, but also to the CO and H2 significantly. The enhancement of sensor response (SPd-WO3to 100 ppm concentration of NH3, CO, and H2 at 300oC was about 3, 2, and 1.38 folds,
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respectively. The reason for why the highest enhancement of sensor response to NH3 is not clear yet, but this result is very interesting in fabrication of high sensitive NH3 gas sensors. Anyhow, it
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is effective that the response of the Pd-WO3 sensors to CO, H2, CO2, and CH4 is very low
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compared to NH3. Thus we believe that the fabricated sensor based on Pd-WO3 nanorods is
Figure 7
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highly selective for the monitoring of NH3.
A general mechanism for explaining the enhancement of gas sensing performance of metal oxide semiconducting materials by noble metal decoration/functionalization is still not completely understood. However, the chemical and electronic sensitization mechanisms are generally used to explain the enhancement of sensing performance [12–14]. When measured in air as reference, the oxygen adsorbed on the surface of WO3 in the form O 2 , O , or O 2 , according to the following equations depending on the operating temperatures [38]. O 2 ( gas ) e O 2 ( ads )
(3)
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O 2 ( gas ) e 2O ( ads )
(4)
2O ( ads ) e O 2
(5)
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Oxygen species ions adsorb on the surface of WO3, capture free electrons and form the electron depletion layer on the surface of WO3 nanorods. Upon exposure to NH3, the NH3 molecules
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interact with pre-adsorbed oxygen to form N2, H2O and released electrons back to the WO3 and
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reduced the thickness of electron depletion layer, thereby decrease the sensor resistance,
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according to the following equations.
(6) (7)
3 NH 3 (ads ) 3O (ads ) N 2 3H 2 O 3e
(8)
(9)
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2 NH 3 ( ads ) 2 / 2O 2 ( ads ) N 2 3 H 2 O 3e
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NH 3 ( gas) NH 3 (ads)
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2 NH 3 (ads) 3O 2 (ads) N 2 3H 2 O 6e
As can be seen in eq. (6-9), interaction of NH3 molecule with O2- releases more electrons to the crystal compared to others, leading to a higher sensitivity. In the pristine WO3, oxygen adsorb on the surface of WO3 in the form of O2- happened at high temperature, possibly at around 400oC. Thus temperature for maximum sensitivity of pristine WO3 is about 400oC. However, by decorating the Pd nanoparticles on the surface of WO3, the activation energy for ion adsorbed of O2- on the surface of WO3 decrease, possibly to 300oC, thus the sample showed maximum response at a lower temperature compared with the pristine counterpart. In addition to the catalytic based on spillover mechanism, the decoration of Pd nanoparticles on the surface of WO3 also modulate (decrease) the conductive channel of WO3 nanorod by generating a Schottky
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barrier at the interface between Pd and WO3 because the workfunction of WO3 (4.8 eV) is smaller than that of Pd (5.5 eV), thereby increase the sensitivity of the sensor [39].
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4. Conclusion
In conclusion, we have demonstrated the facile hydrothermal synthesis of tungsten oxide
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nanorods and their decoration with Pd nanoparticles for enhanced NH3 gas-sensing applications.
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The responsivity of the WO3 nanorod sensor was improved by about three times and its optimal working temperature was reduced considerably by surface decoration with Pd nanoparticles.
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Considering the robustness and simplicity of their synthesis process, the prepared Pd–WO3 nanorods are inexpensive and function as effective sensors that can achieve real-time
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measurement of NH3 gas at the ppm scale for environmental pollution monitoring.
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Acknowledgments
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This research was funded by the Vietnam National Foundation for Science and Technology
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Development (Nafosted Code: 103.02-2014.06). Hieu also acknowledges support from VLIRUOS under the Research Initiative Project (ZEIN2012RIP20). References
[1]
P. Van Tong, N.D. Hoa, D.D. Trung, N.D. Quang, N. Van Hieu, Tungsten oxide urchinflowers and nanobundles: Effect of synthesis conditions and heat treatment on assembly and gas-sensing characteristics, Sci. Adv. Mater. 6 (2014) 1081–1090. doi:10.1166/sam.2014.1854.
[2]
E. Comini, C. Baratto, G. Faglia, M. Ferroni, A. Vomiero, G. Sberveglieri, Quasi-one dimensional metal oxide semiconductors: Preparation, characterization and application as chemical sensors, Prog. Mater. Sci. 54 (2009) 1–67. doi:10.1016/j.pmatsci.2008.06.003.
[3]
A. Esfandiar, A. Irajizad, O. Akhavan, Pd-WO3/reduced graphene oxide hierarchical nanostructures as efficient hydrogen gas sensors, Int. J. Hydrogen Energy. 39 (2014) 8169–8179. doi:10.1016/j.ijhydene.2014.03.117.
18 Page 18 of 32
S. An, S. Park, H. Ko, C. Lee, Fabrication of WO3 nanotube sensors and their gas sensing properties, Ceram. Int. 40 (2014) 1423–1429. doi:10.1016/j.ceramint.2013.07.025.
[5]
M. Akiyama, J. Tamaki, N. Miura, N. Yamazoe, Tungsten oxide-based semiconductor sensor highly sensitive to NO and NO2, Chem. Lett. (1991) 1611–1614. doi:10.1246/cl.1991.1611.
[6]
J. Kukkola, M. Mohl, A. Leino, J. Mäklin, N. Halonen, A. Shchukarev, et al., Room temperature hydrogen sensors based on metal decorated WO3 nanowires, Sens. Actuators B. 186 (2013) 90–95. doi:10.1016/j.snb.2013.05.082.
[7]
J. Dai, M. Yang, Z. Yang, Z. Li, Y. Wang, G. Wang, et al., Performance of fiber Bragg grating hydrogen sensor coated with Pt-loaded WO3 coating, Sensors Actuators B. 190 (2014) 657–663. doi:10.1016/j.snb.2013.08.083.
[8]
T. Samerjai, C. Liewhiran, A. Wisitsoraat, A. Tuantranont, C. Khanta, S. Phanichphant, Highly selective hydrogen sensing of Pt-loaded WO3 synthesized by hydrothermal/impregnation methods, Int. J. Hydrogen Energy. 39 (2014) 6120–6128. doi:10.1016/j.ijhydene.2014.01.184.
[9]
T. Samerjai, N. Tamaekong, C. Liewhiran, A. Wisitsoraat, A. Tuantranont, S. Phanichphant, Selectivity towards H2 gas by flame-made Pt-loaded WO3 sensing films, Sensors Actuators B. 157 (2011) 290–297. doi:10.1016/j.snb.2011.03.065.
[10]
H. Xia, Y. Wang, F. Kong, S. Wang, B. Zhu, X. Guo, et al., Au-doped WO3-based sensor for NO2 detection at low operating temperature, Sens. Actuators B. 134 (2008) 133–139. doi:10.1016/j.snb.2008.04.018.
[11]
P. Thi Hong Van, N. Hoang Thanh, V. Van Quang, N. Van Duy, N. Duc Hoa, N. Van Hieu, Scalable fabrication of high-performance NO2 gas sensors based on tungsten oxide nanowires by on-chip growth and RuO2-functionalization, ACS Appl. Mater. Interfaces (2014). doi:10.1021/am5010078.
[12]
A. Labidi, E. Gillet, R. Delamare, M. Maaref, K. Aguir, Ethanol and ozone sensing characteristics of WO3 based sensors activated by Au and Pd, Sensors Actuators B. 120 (2006) 338–345. doi:10.1016/j.snb.2006.02.015.
[13]
D. Shin, T.M. Besmann, B.L. Armstrong, Phase stability of noble metal loaded WO3 for SO2 sensor applications, Sens. Actuators B 176 (2013) 75–80. doi:10.1016/j.snb.2012.08.084.
[14]
N.H. Kim, S.J. Choi, D.J. Yang, J. Bae, J. Park, I.D. Kim, Highly sensitive and selective hydrogen sulfide and toluene sensors using Pd functionalized WO3 nanofibers for potential diagnosis of halitosis and lung cancer, Sens. Actuators B 193 (2014) 574–581. doi:10.1016/j.snb.2013.12.011.
Ac ce p
te
d
M
an
us
cr
ip t
[4]
19 Page 19 of 32
A. Kolmakov, D.O. Klenov, Y. Lilach, S. Stemmer, M. Moskovitst, Enhanced gas sensing by individual SnO2 nanowires and nanobelts functionalized with Pd catalyst particles, Nano Lett. 5 (2005) 667–673. doi:10.1021/nl050082v.
[16]
M. Zhao, J.X. Huang, C.W. Ong, Diffusion-controlled H2 sensors composed of Pd-coated highly porous WO3 nanocluster films, Sens. Actuators B 191 (2014) 711–718. doi:10.1016/j.snb.2013.09.116.
[17]
X. Liu, J. Zhang, T. Yang, X. Guo, S. Wu, S. Wang, Synthesis of Pt nanoparticles functionalized WO3 nanorods and their gas sensing properties, Sensors Actuators B 156 (2011) 918–923. doi:10.1016/j.snb.2011.03.006.
[18]
N. Minh Vuong, D. Kim, H. Kim, Porous Au-embedded WO3 Nanowire Structure for Efficient Detection of CH4 and H2S, Sci. Rep. 5 (2015) 11040. doi:10.1038/srep11040.
[19]
Y.S. Shim, L. Zhang, D.H. Kim, Y.H. Kim, Y.R. Choi, S.H. Nahm, et al., Highly sensitive and selective H2 and NO2 gas sensors based on surface-decorated WO3 nanoigloos, Sens. Actuators B. 198 (2014) 294–301. doi:10.1016/j.snb.2014.03.073.
[20]
A. Boudiba, P. Roussel, C. Zhang, M.-G. Olivier, R. Snyders, M. Debliquy, Sensing mechanism of hydrogen sensors based on palladium-loaded tungsten oxide (Pd-WO3), Sens. Actuators B 187 (2012) 83–97. doi:10.1016/j.snb.2012.09.063.
[21]
A. Boudiba, C. Zhang, P. Umek, C. Bittencourt, R. Snyders, M.G. Olivier, et al., Sensitive and rapid hydrogen sensors based on Pd-WO3 thick films with different morphologies, Int. J. Hydrogen Energy. 38 (2013) 2565–2577. doi:10.1016/j.ijhydene.2012.11.040.
[22]
B. Liu, D. Cai, Y. Liu, D. Wang, L. Wang, Y. Wang, et al., Improved room-temperature hydrogen sensing performance of directly formed Pd/WO3 nanocomposite, Sens. Actuators B 193 (2014) 28–34. doi:10.1016/j.snb.2013.11.057.
[23]
S. Fardindoost, A. Iraji zad, F. Rahimi, R. Ghasempour, Pd doped WO3 films prepared by sol-gel process for hydrogen sensing, Int. J. Hydrogen Energy 35 (2010) 854–860. doi:10.1016/j.ijhydene.2009.11.033.
[24]
A. Hoel, L.F. Reyes, S. Saukko, P. Heszler, V. Lantto, C.G. Granqvist, Gas sensing with films of nanocrystalline WO3 and Pd made by advanced reactive gas deposition, Sens. Actuators B. 105 (2005) 283–289. doi:10.1016/j.snb.2004.06.011.
[25]
A. Wisitsoorat, M.Z. Ahmad, M.H. Yaacob, M. Horpratum, D. Phakaratkul, T. Lomas, et al., Optical H2 sensing properties of vertically aligned Pd/WO3 nanorods thin films deposited via glancing angle rf magnetron sputtering, Sens. Actuators B 182 (2013) 795– 801. doi:10.1016/j.snb.2013.03.091.
[26]
M. Horprathum, T. Srichaiyaperk, B. Samransuksamer, A. Wisitsoraat, P. Eiamchai, S. Limwichean, et al., Ultrasensitive hydrogen sensor based on Pt-decorated WO3 nanorods
Ac ce p
te
d
M
an
us
cr
ip t
[15]
20 Page 20 of 32
prepared by glancing-angle dc magnetron sputtering, ACS Appl. Mater. Interfaces 6 (2014) 22051–22060. doi:10.1021/am505127g. F. Chávez, G.F. Pérez-Sánchez, O. Goiz, P. Zaca-Morán, R. Peña-Sierra, A. MoralesAcevedo, et al., Sensing performance of palladium-functionalized WO3 nanowires by a drop-casting method, Appl. Surf. Sci. 275 (2013) 28–35. doi:10.1016/j.apsusc.2013.01.145.
[28]
P. Van Tong, N.D. Hoa, V. Van Quang, N. Van Duy, N. Van Hieu, Diameter controlled synthesis of tungsten oxide nanorod bundles for highly sensitive NO2 gas sensors, Sens. Actuators B. 183 (2013) 372–380. doi:10.1016/j.snb.2013.03.086.
[29]
Y. Liu, J. Yu, P.T. Lai, Investigation of WO3/ZnO thin-film heterojunction- based Schottky diodes for H2 gas sensing, Int. J. Hydrog. Energy. 39 (2014) 2–8. doi:10.1016/j.ijhydene.2014.04.155.
[30]
J. Yu, S.J. Ippolito, W. Wlodarski, M. Strano, K. Kalantar-zadeh, Nanorod based Schottky contact gas sensors in reversed bias condition, Nanotechnology 21 (2010) 265502. doi:10.1088/0957-4484/21/26/265502.
[31]
M. Gautam, A.H. Jayatissa, Ammonia gas sensing behavior of graphene surface decorated with gold nanoparticles, in: Solid. State. Electron., (2012) 159–165. doi:10.1016/j.sse.2012.05.059.
[32]
T.D. S. Ahlers, G. Müller, Factors Influencing the Gas Sensitivity of Metal Oxide Materials, in: M.V.P. C.A. Grimes, E.C. Dickey (Ed.), American Scientific, 2014: 413– 447.
[33]
T. Siciliano, A. Tepore, G. Micocci, A. Serra, D. Manno, E. Filippo, WO3 gas sensors prepared by thermal oxidization of tungsten, Sens. Actuators B 133 (2008) 321–326. doi:10.1016/j.snb.2008.02.028.
[34]
M. Grassi, P. Malcovati, A. Baschirotto, A high-precision wide-range front-end for resistive gas sensors arrays, Sens. Actuators B 111-112 (2005) 281–285. doi:10.1016/j.snb.2005.03.103.
[35]
S.-W. Choi, A. Katoch, J.-H. Kim, S.S. Kim, Striking sensing improvement of n-type oxide nanowires by electronic sensitization based on work function difference, J. Mater. Chem. C. 00 (2015) 1–7. doi:10.1039/C4TC02057J.
[36]
J.-H. Yoon, H.-J. Lee, J.-S. Kim, Ammonia Gas-Sensing Characteristics of Pd DopedWO3, Sens. Lett. 9 (2011) 46–50. doi:10.1166/sl.2011.1416.
[37]
I.S. Hwang, J.K. Choi, H.S. Woo, S.J. Kim, S.Y. Jung, T.Y. Seong, et al., Facile control of C2H5OH sensing characteristics by decorating discrete Ag nanoclusters on SnO2 nanowire networks, ACS Appl. Mater. Interfaces 3 (2011) 3140–3145. doi:10.1021/am200647f.
Ac ce p
te
d
M
an
us
cr
ip t
[27]
21 Page 21 of 32
H. Zhang, Z. Liu, J. Yang, W. Guo, L. Zhu, W. Zheng, Temperature and acidity effects on WO3 nanostructures and gas- sensing properties of WO3 nanoplates, Mater. Res. Bull. 57 (2014) 260–267. doi:10.1016/j.materresbull.2014.06.013.
[39]
A. Boudiba, P. Roussel, C. Zhang, M.-G. Olivier, R. Snyders, M. Debliquy, Sensing mechanism of hydrogen sensors based on palladium-loaded tungsten oxide (Pd-WO3), Sens. Actuators B. 187 (2012) 83-97. doi:10.1016/j.snb.2012.09.063.
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[38]
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Pham Van Tong received Bachelor degree in Physics at the Department of Physics, Hanoi
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University of Science E-learning (HUSE), Vietnam, in 2001. He received MSc degree in materials science at the International Training Institute for Materials Science (ITIMS), HUST, in
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Vietnam, in July, 2004. He is currently pursuing the PhD degree at ITIMS, where he is working
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on development of flammable and explosive gas detectors based on nanomaterials. Nguyen Duc Hoa obtained his PhD degree in materials science and engineering in 2009 at
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Chungnam National University in Korea. He awarded JSPS fellowship and conducted the
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research at National Institute for Materials Science (NIMS, Japan) from 2009 to 2011. His
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research activity has covered a wide range of nanostructured materials from synthesis, fundamental, and applications. Currently, he is a lecturer and scientist at Hanoi University of Science and Technology, Vietnam.
Nguyen Van Duy is currently working as a research lecture at International Training Institute for
Material Science (ITIMS), Hanoi University of Science and Technology (HUST). He received PhD degree from the Department of Electrical and Electronics Engineering at Sungkyunkwan University, South Korea, in 2011. His current research interests include nanomaterials, nanofabrications, characterizations, and applications to electronic devices, gas sensors, and biosensors. Dang Thi Thanh Le obtained her MSc and PhD degrees in Materials Science from
22 Page 22 of 32
International Training Institute for Materials Science (ITIMS) – Hanoi University of Science and Technology (HUST), Hanoi, Vietnam in 2001 and 2011. She worked as
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a visiting postdoc at The Angstrom Laboratory – Uppsala University, Sweden in the academic year of 2011–2012. She is working as a researcher/lecturer at ITIMS. Her
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current interests include synthesis, characterization and application of nanomate-rials for
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gas-sensing and bio-sensing.
Nguyen Van Hieu joined the International Training Institute for Material Science (ITIMS) at
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Hanoi University of Science and Technology (HUST) in 2004, where he is currently an Associate Professor. He received his PhD degree from the Faculty of Electrical Engineering at
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University of Twente in The Netherlands in 2004. He worked as a post-doctoral fellow at the
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Korea University from 2006-2007. His current research interests include functional
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nanostructures, gas sensors, and biosensors.
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Figure 1. (A) SEM image and (B) EDS analysis of the bare WO3 nanorods; (C) SEM
image and (D) EDS analysis of the WO3 nanorods decorated with Pd nanoparticles (Pd–
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WO3–100). (C) Inset: SEM image of the fabricated sensor.
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Figure 2. (A-C) TEM images, and (D) SAED of the Pd–WO3–100 nanorods:
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Figure 3. I–V curves of the Pd–WO3–100 nanorod sensor measured at different
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temperatures. Inset: Photo of the fabricated sensors.
Figure 4. NH3 gas-sensing characteristics of bare WO3 nanorods: (A) Transient resistance
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vs. time upon exposure to different concentrations of NH3 measured at temperatures
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ranging from 200 °C to 450 °C; (B) responses to 100, 250, 500, and 1000 ppm NH3 measured at different temperatures; (C) response as a function of NH3 concentration
resistance vs. time upon exposure to different concentrations of NH3 measured at temperatures ranging from 200 °C to 450 °C; (B) responses to 100, 250, 500, and 1000 ppm NH3 measured at different temperatures; (C) response as a function of NH3 concentration measured at different temperatures.
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Figure 6. Selectivity and stability of the Pd–WO3–100 nanorod sensor: (A) Responses of
the sensor to various concentrations of different gases; (B, C) short term stability of the
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sensor measured at 350 and 400 °C. Figure 7. Response to different gases of the Pd-WO3 nanorod samples decorated with
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various amount of PdCl2.
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Figure(s)
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(C)
(A)
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Figure 1. (A) SEM image and (B) EDS analysis of the bare WO3 nanorods; (C) SEM image and (D) EDS analysis of the WO3 nanorods decorated with Pd nanoparticles (Pd–WO3–100). (C) Inset: SEM image of the fabricated sensor.
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Pd NPs
ed
200 nm
(D)
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ce
pt
(B)
200 nm
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ce
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ed
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Figure 2. (A-C) TEM images, and (D) SAED of the Pd–WO3– 100 nanorods:
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Figure 3. I–V curves of the Pd–WO3–100 nanorod sensor measured at different temperatures. Inset: Photo of the fabricated sensors.
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40.0µ
ed
0.0
ce
-40.0µ
o
150 C o 200 C o 250 C o 300 C o 350 C o 400 C
pt
-20.0µ
-4
Ac
Current (A)
20.0µ
-3
-2
-1
0
1
2
3
4
Applied voltage (V)
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1500 1000 500 15k
1000 ppm 500 ppm
(A) 250 ppm 100 ppm
400
450
(B)
1000 ppm 500 ppm 250 ppm 100 ppm
4 3
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10k
T (o C )
350
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2
o
5k 10k
NH3@ 300 C
1
ed
10k
pt
5k
(C)
o
@300 C o @350 C o @400 C o @450 C
o
NH3@ 350 C
4
3
o
NH3@ 400 C
ce
10k 5k
Ac
Resistance()
5k
Rair/Rgas
Conc. (ppm)
300
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cr
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Figure 4. NH3 gas-sensing characteristics of pure WO3 nanorods: (A) Transient resistance vs. time upon exposure to different concentrations of NH3 measured at temperatures ranging from 200 °C to 450 °C; (B) responses to 100, 250, 500, and 1000 ppm NH3 measured at different temperatures; (C) response as a function of NH3 concentration measured at different temperatures.
0
100
200
300
2 o
NH3@ 450 C
400
Time (sec.)
500
1 0
200
400
600
800
1000
NH3 conc. (ppm)
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Figure 5. NH3 gas-sensing characteristics of Pd–WO3–100 nanorod sensor: (A) Transient resistance vs. time upon exposure to different concentrations of NH3 measured at temperatures ranging from 200 °C to 450 °C; (B) responses to 100, 250, 500, and 1000 ppm NH3 measured at different temperatures; (C) response as a function of NH3 concentration measured at different temperatures. T (oC)
500 ppm
250 ppm
100 ppm
NH3
10k @ 250oC
450
1000ppm 500ppm 250ppm 100ppm
ed
250oC 300oC 350oC 400oC 450oC
(C)
10 8 6
pt
20k 10k
8
2
@ 350oC
10k 5k
0
100
ce
@ 400oC
200
300
Ac
Resistance()
10k
10
4
@ 300oC
1k
12
6
M
1k 10k
(B)
400
Rair/Rgas
(A)
1000 ppm
350
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1500 1000 500
300
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Conc. (ppm)
250
400
Time (sec.)
4 2
@ 450oC
500
600 0
200
400 600 800 NH3 conc. (ppm)
1000
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Figure 6. Selectivity and stability of the Pd–WO3–100 nanorod sensor: (A) Responses of the sensor to to various concentrations of different gases; (B, C) short term stability of the sensor measured at 350 and 400 °C. o
10
cr
CO@400 C o H2@400 C o
an
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Rair/Rgas
NH3@300 C
(A)
1 100
M
10
1000
Gas (ppm)
o
Pd-WO3 @500 ppm NH3& 350 C
ed
(B)
pt
10k
(C)
o
ce
Pd-WO3 @500 ppm NH3& 400 C
10k
Ac
Resistance ()
100k
1k
200
400
600
800
1000
Time (sec.)
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Figure 7. Response to different gases of the Pd-WO3 nanorod samples decorated with various amount of PdCl2.
