Impact of hydrogen concentrations on the impedance spectroscopic behavior of Pd-sensitized ZnO nanorods
© Kashif et al; licensee Springer. 2013
Received: 21 December 2012
Accepted: 31 January 2013
Published: 11 February 2013
ZnO nanorods were synthesized using a low-cost sol-gel spin coating technique. The synthesized nanorods were consisted of hexagonal phase having c-axis orientation. SEM images reflected perpendicular ZnO nanorods forming bridging network in some areas. The impact of different hydrogen concentrations on the Pd-sensitized ZnO nanorods was investigated using an impedance spectroscopy (IS). The grain boundary resistance (Rgb) significantly contributed to the sensing properties of hydrogen gas. The boundary resistance was decreased from 11.95 to 3.765 kΩ when the hydrogen concentration was increased from 40 to 360 ppm. IS gain curve showed a gain of 6.5 for 360 ppm of hydrogen at room temperature. Nyquist plot showed reduction in real part of impedance at low frequencies on exposure to different concentrations of hydrogen. Circuit equivalency was investigated by placing capacitors and resistors to identify the conduction mechanism according to complex impedance Nyquist plot. Variations in nanorod resistance and capacitance in response to the introduction of various concentrations of hydrogen gas were obtained from the alternating current impedance spectra.
While hydrogen gas has been increasingly used as a clean and green fuel in household and transportation appliances, the absence of color, odor, and taste has made it difficult to trace and detect hydrogen under complex matrices . Hydrogen is a light and diffusible gas (diffusion coefficient of 0.61 cm2/s in air)  with a wide ranging inflammability (4% to 75%) . Even 4.65% hydrogen in air is sufficient to cause explosion . Thus, the detection and leakage control of this gas is a challenging task, and there is an increasing demand in the development of methodology for the ultrasensitive detection of hydrogen.
Previously, selective H2 sensors were proposed for the detection of hydrogen leakage in solid-state fuel cells , proton exchange membrane fuel cells , hydrogen engines , and hydrogen storage devices . Bamsaoud et al.  used nanoparticulate tin oxide (SnO2)-based resistive films for the selective detection of hydrogen against relative humidity and CO2 at 265°C. Wang et al.  used mesostructured SnO2 for the selective detection of hydrogen against methane, butane, and CO at 300°C. Tianshu et al.  studied the effect of different Cd-doped SnO2-based sensors from 200°C to 450°C and selectively detected 1,000 ppm of hydrogen against 1,000 ppm of CO and 1,000 ppm of isobutane (i-C4H10) in the absence of ethanol vapor at a Cd to Sn ratio of 0.1. Lupan et al.  detected 10% H2 in N2 at 112°C using nanosensor based on zinc oxide (ZnO) nanorods. Garcia et al.  utilized Pd-decorated ZnO and tungsten oxide (WO3) nanowires for the selective detection of 4,500 ppmv H2/N2 at 100°C. Yamazoe et al.  studied the effect of different additives on SnO2 films and found that Ag-SnO2 film showed the highest sensitivity and selectively towards 0.8% hydrogen against 0.5% CH4, 0.2% C3H8, and 0.02% CO. Choi et al.  used electrospun Pd-doped SnO2 hollow nanofibers for the detection of hydrogen under ethanol background. Lupan et al.  studied the hydrogen selective response at room temperature using tetrapod ZnO sensor. Using an UV source of activation, they detected 100 ppm of hydrogen against 100 ppm of CO, isobutane, CH4, CO2, and SO2. However, in the forthgoing era, there is a requirement of hydrogen sensors having superior stability, sensitivity, and fast response time, along with low operating power and weight.
Recently, semiconductor metal oxides have been increasingly used in humidity, gas, and chemical sensing devices . This is probably because of their simple fabrication, low cost, size reduction, appreciable sensitivity, and fast response time . Catalytic metal-doped semiconductor metal oxides such as SnO2, titanium dioxide (TiO2) , ZnO , and WO3 have been used to develop hydrogen sensors. The addition of suitable quantity of appropriate metal catalyst enhances chemical reaction through the lowering of activation energy at the metal oxide thin film and target gas interfaces. The addition of metal as a catalyst also improves target response and selectivity at room temperature . ZnO nanorods and nanowires are particularly promising for these applications because of its large surface area, wide bandgap and exciton energy, fascinating sensitivity, biocompatibility, low weight, and resistance to rust formation . For hydrogen sensing applications, surface modifications of ZnO with metal additives such as Pt, Pd, and/or Au through various techniques have been under intensive investigations [19, 21, 22]. Several studies have demonstrated that Pd doping on ZnO nanowires and nanorods enhances room temperature hydrogen sensing through the catalytic dissociation of molecular hydrogen to atomic hydrogen at room temperature . The predominant methods documented to synthesize ZnO nanorods for this particular application are chemical vapor deposition (CVD) and molecular beam epitaxy (MBE) [21, 22]. However, both CVD and MBE methods involve high temperature growth and expensive instrumentations which are not available and affordable in ordinary laboratories. These techniques also need gold (Au) and/or other expensive metal coatings for the synthesis of ZnO nanorods and nanowires [10, 11]. Moreover, Pd doping on the synthesized zinc oxides requires RF sputtering which also demands expensive laboratory setup. Additionally, previous researchers used DC measurements [19, 21, 22] which cannot elucidate the contributing factors such as the grain, grain boundary, and electrodes that might influence the target response on the Pd-sensitized ZnO nanostructures.
Recently, sol-gel spin coating technique has received enormous attention because of its simplicity, affordable instrumentations, low cost, and controllable growth temperatures . In this paper, c-axis-aligned hexagonal ZnO nanorods with good crystalline properties were synthesized using a low-cost spin coating technique. Pd doping on the synthesized ZnO was performed using very simple instrumentations that require only micropipette and hot plate. However, to the best of our knowledge, such a method is not documented for the synthesis of Pd-sensitized ZnO nanorods for hydrogen detection applications. Room temperature hydrogen sensing was performed in a low-cost homemade gas chamber, and superior sensitivity and stability over the literature-reported Pd-sensitized ZnO nanorods were achieved. Potential contributors to sensor functionalities were elucidated through impedance study which is an AC measurement technique that can define contributions from grain, grain boundary, electrodes, and other associated elements. The simplicity and reproducibility of the method suggested its potential applications in the large-scale synthesis of Pd-sensitized ZnO nanorods for use in hydrogen, chemical, and other gas sensing devices that involved Pd-mediated catalysis.
An oxide layer of approximately 1-μm thickness was grown on a p-type silicon substrate of resistivity 1 to 50 Ω cm through a wet oxidation process. Prior to the oxide growth, the wafer was cleaned with RCA1 and RCA2 solutions followed by draining in dilute HF to remove the native oxide. An interdigitated electrode layer was deposited onto the oxide layer through Cr/Au evaporation using a hard mask and Auto 306 thermal evaporator (Edwards High Vacuum International, Wilmington, MA, USA). ZnO seed layer was deposited on the thermally oxidized silicon substrate using a spin coater rotating at 1,000 rpm for 10 s and then ramped up to 3,000 rpm for 45 s. After coating the seed layer, the film was dried at 250°C for 20 min. The coating and drying processes were repeated five times. After depositing five successive layers, the sample was incubated in a furnace to anneal the thin film at 450°C for 1 h under air atmosphere.
For the growth of ZnO nanorods, the prepared substrate was inserted inside a Teflon sample holder at the cut edges to keep the deposited side downward inside the growth solution. The growth solution was prepared by mixing zinc nitrate hexahydrate (99%; Sigma-Aldrich) and hexamethyltetramine (99%; Merck) in deionized (DI) water, and the final concentration of the solution was maintained at 25 mM. The beaker was placed inside a preheated oven, and the growth process was continued at 90°C for 6 h. The prepared ZnO nanorods were washed in IPA and DI water to remove the excess and contaminated salts. Subsequently, the synthesized ZnO nanorods were annealed at 450°C for 1 h under air environment. For Pd doping, 0.01 M solution of Pd was prepared by mixing the required amount of palladium chloride (PdCl2, 99.999%; Sigma-Aldrich) in ethanol. The solution was stirred overnight to completely dissolve the Pd particles. Five-microliter portion of the above solution was precisely transferred onto the synthesized ZnO nanorods using a micropipette, and the whole mixture was heated at 250°C for 5 min to dry out the residual chloride.
The structural properties of the Pd-sensitized ZnO nanorods were investigated using Bruker X-ray diffractometer (D8 Advance, Bruker AXS GMBH, Karlsruhe, Germany) with Cu Kα radiation at λ = 1.5406 Å. The X-ray diffraction (XRD) pattern was recorded in the range of 20° to 60° operating at a voltage of 40 kV and a current of 40 mA. The X-ray spectra peak analysis was carried out by Diffraction plus 2003 version of Eva 9.0 rev.0 software. The material composition was analyzed using X-ray photoelectron spectroscopy (XPS) (Omicron Dar400, Omicron, Erlangen, Germany). The chamber pressure was maintained at 2.4 e−10 Torr throughout the measurement. CasaXPS software was used for the XPS peak deconvolution. Morphological studies were performed using a scanning electron microscope (JEOL JSM-6460LA, Akishima, Tokyo, Japan). Gas sensing measurements were carried out in a homemade gas chamber of 3-L capacity. The base of the chamber was made up of stainless steel, and the upper area was covered with a high-vacuum glass dome. All the measurements were performed under atmospheric pressure. The chamber inlet was connected with the air pump and 1% H2 in balance N2 gas (Moxlinde, Malaysia). The flow of 1% H2 gas was regulated using a mass flow controller (GFC-17, 0 to 100 ml/min; AALBORG, Orangeburg, NY, USA), whereas the air flow was controlled using a mass flow meter. Impedance spectra were collected at room temperature (RT) in the frequency range of 1 Hz to 10 MHz using Novocontrol alpha high-frequency analyzer (Hundsangen, Germany) under the exposure of variable ppm levels of hydrogen.
Results and discussion
Modeled RC parameters for Pd-sensitized ZnO nanorods under different H 2 concentrations at room temperature
However, the variation in the capacitance values was not significant. This reflected that the hydrogen gas mainly affects the surface charge region of the grain boundaries of Pd-sensitized ZnO nanorods.
In 1 Hz- to 100-kHz range, the space charge region rules the conductivity process. There is a sharp decrement in the sensitivity with the increment of frequency and little variation in the gain values at frequency higher than 100 kHz, where the conductivity is mainly dependent on the surface charge of the grains. This revealed that a suitable selection of frequency could achieve maximum gain in sensitivity.
The weak bonding of Pd atoms with the oxygen gas results in the dissociation of the complex at relatively low temperature releasing atomic oxygen. The oxygen atoms migrate along the surface of the grains. This migration is induced by the Pd catalyst and is known as spillover of the gaseous ions . Thus, the oxygen atoms capture electrons from the surface layer forming an acceptor surface at the grain boundary. The presence of catalyst atoms activates the reaction between reducing gases and the adsorbed oxygen [39–41]. Thus, the Pd sensitization on the ZnO nanorod surface enabled the hydrogen sensing at relatively low operating temperature.
A hydrogen sensor was successfully developed using Pd-sensitized ZnO nanorods synthesized on oxidized silicon substrate using a sol-gel spin coating technique. The sensor detected ppm level hydrogen at room temperature with more sensitivity over the literature-reported values for the ZnO-based sensors. The variation in the resistance value of the grain boundary which was the basis of analyte detection mechanism was due to the sole variation in hydrogen concentration. Nyquist plot strongly supported the impedance findings.
MK acknowledges the financial support of the Malaysian Ministry of Higher Education (MOHE) through FRGS grant number 9003-00276 to Professor UH. The authors are thankful to the technical staff of the Institute of Nano Electronic Engineering and School of Microelectronic Engineering in Universiti Malaysia Perlis for their kind support in performing this research.
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