Elsevier

Sensors and Actuators B: Chemical

Volume 202, 31 October 2014, Pages 1270-1280
Sensors and Actuators B: Chemical

Role of various interfaces of CuO/ZnO random nanowire networks in H2S sensing: An impedance and Kelvin probe analysis

https://doi.org/10.1016/j.snb.2014.06.072Get rights and content

Abstract

CuO-modified ZnO random nanowire networks have been demonstrated to enhance the sensitivity and selectivity towards H2S. CuO being p-type and ZnO being n-type semiconductors, modification with CuO results in the formation of random nano p–n junction distributed over the nanowire surface, thereby leading to depleted nanowires. The enhanced response has been attributed mainly to the interaction of CuO with H2S forming CuS, a degenerated semiconductor with a metallic conductance behaviour, causing a drastic change in the resistance. The governing sensing mechanism can be envisaged to have contributions from the different regions namely nanowires bulk (depleted), junctions among nanowires and the interface between sensor and Au contact electrode, respectively. To establish the governing sensing mechanism, it becomes critical to isolate the contribution arising from each of them. In the present work, we report the impedance and the Kelvin probe studies of CuO/ZnO random nanowire network sensor films. Impedance studies indicate that the contributions arising from the bulk and the nanowire–electrode contact is negligible. A drastic variation in the resistance of the sample arises mainly due to the band bending. The extent of band bending depends on the ambient oxygen and the interaction with the test gas. Temperature- and gas concentration-dependent studies clearly indicated that the CuS formation is the major cause for such bending. Work function measurements further corroborates the finding of impedance studies.

Introduction

In recent years, a great deal of research has been focused on the synthesis of metal oxide-based nanomaterials because of their superior and enhanced functional properties for realizing functional nanodevices. Among different nanostructures, nanowires (NWs), in particular, are looked upon as a favourable candidate for realizing next-generation gas sensors. They offer various advantages including high surface area-to-volume ratio, effective pathway for electron transfer (length of NWs), dimensions comparable to the extension of the surface charge region, enhanced and tunable surface reactivity implying possible room-temperature operation, faster response and recovery time, relatively simple preparation methods allowing large-scale production, convenient to use, ease of fabrication and manipulation, high integration density, and low power consumption [1]. In order to harness the complete advantage of nano-dimension, i.e., high surface area-to-volume ratio, it is desirable to use the single nanostructure. However, the problems associated with the use of single nanostructure, viz. sample to sample variation, complexity of the sensor fabrication approach, and the in-built issue of randomness raise a major concern over the important parameters, namely reproducibility and repeatability of the sensor. Use of NWs in thin-film form wherein the average properties of multiple NWs is measured circumvents the above-measured problems to a great extent. Herein NWs can be selectively grown between the predefined electrodes or electrical contacts can be provided by depositing the electrodes with known dimensions on the NW network itself [2]. Accordingly, NW-based sensors in thin form have been investigated widely for possible sensor device applications. Among these, ZnO NWs, in particular, provide the advantages of ease of synthesis using physical/chemical processes, wide bandgap, high thermal stability, and easy control over morphology [3]. Besides, the ability to manipulate the wide bandgap provides the opportunity to tailor the response of ZnO NWs towards different gases. Accordingly, it is often modified with sensitizers like In, Pd, Pt, Au, Cu, etc. and has demonstrated sensors with improved response kinetics towards gases like C2H5OH, CO, and H2S [4], [5], [6], [7].

H2S is one of the highly toxic and flammable gases used widely in various industrial applications. These include oil and gas industries, pulp and paper industries, wastewater treatment plants, and heavy water plants [8], [9]. It has a characteristic rotten egg odour and can easily be detected by human nose at concentrations >0.13 ppm. However, repeated exposure declines the ability to smell and thus it becomes crucial to detect H2S. Besides, it has adverse effect on the nervous system, cause loss of consciousness, and interacts with the enzymes in the bloodstream inhibiting cell respiration and accordingly, could be fatal. Its long-term (8 h) and short-term (10 min) exposure limits have been set to 10 and 15 ppm, respectively. We have recently shown that the response of ZnO NWs could be tailored towards different gases by a proper choice of sensitizer (Au, CuO) and control over its amount and distribution over the NW surface [10], [11], [12]. In particular, the optimum temperature of maximum sensitivity towards H2S was systematically brought down to room temperature using Au and CuO as sensitizers. Modification with CuO resulted in a H2S sensor with an enhanced response at 200 °C in comparison to that pure ZnO NWs that exhibited a maximum response at 300 °C. Additionally, modification with Au resulted in a H2S sensor with maximum response at room temperature. The enhanced and selective response in these cases were attributed to the formation of p–n junction (between p-CuO and n-type ZnO) and Schottky contacts (between Au and ZnO), respectively.

In the case of CuO-modified ZnO nanowires, exposure to H2S leads to the destruction of the p–n junction due to the formation of metallic CuS which causes a drastic variation in the sensor resistance. Apart from this, H2S can also interact with the adsorbed oxygen species present over the sensor surface and contribute towards sensing. Thus, in a typical sensor configuration, the sensing mechanism can be envisaged to have contributions arising from different regions of the samples, namely NW bulk, NW junction (junction among NWs and between NWs and CuO islands), and interface between the sensor and the contact electrode [13], [14], [15]. Modification with thin layer of CuO (10 nm) over the sensor surface results in the formation of random p–n junctions distributed uniformly over the sensor surface. The formation of p–n junctions causes depletion of NWs. The extent of depletion has a contribution from the ambient or the adsorbed oxygen species. Thus, in order to clearly understand and get a complete control over sensor performance, it becomes crucial and desirable to investigate and correlate the mechanism of gas interaction with each element.

In order to clearly understand the sensing mechanism involved, systematic impedance and work function investigations have been performed. Impedance spectroscopy is one of the powerful tools that have been widely employed for the identification of different elements of a complex device [16], [17]. We successfully demonstrate that the gas sensing is predominantly a surface phenomenon wherein bulk of NWs and sample–electrode interface do not undergo any changes on interaction with the gas molecules. The major role is played by the resistive and capacitance contribution coming from the high-resistance depletion layers between relatively conducting plates of bulk materials, i.e., ZnO NWs. A drastic variation in the resistance of the sample arises mainly due to the band bending. The extent of band bending depends on the ambient oxygen and the interaction with the test gas. Temperature- and gas concentration-dependent studies clearly indicated that the CuS formation is the major cause for such bending. Work function measurements performed in the presence and absence of H2S further corroborates the finding of impedance studies.

Section snippets

ZnO nanowire growth and modification with CuO

ZnO NW random networks were grown by hydrothermal method using ZnO NPs as the seed layer, as reported elsewhere [18]. In brief, ZnO NPs were synthesized via chemical route by adding 10 mM NaOH solution in methanol dropwise to the 30 mM zinc acetate solution in methanol at 60 °C under rigorous stirring. The stirring was continued further for ∼120 min. This led to the formation of NPs ranging from 5 to 15 nm. The solution containing the NPs was spin-casted onto Si/SiO2 (100 nm) substrates. Hydrothermal

Results and discussions

Using hydrothermal method, a uniform growth of vertically aligned NWs on substrate size as large as 2 in. is achieved [16]. In the present case, hydrothermal growth resulted in a random network of ZnO NWs distributed uniformly over the substrate of size 1 in. As shown in Fig. 1, ZnO NWs were having diameter of 50–200 nm and length between 6 and 10 μm [8]. The quasi-hexagonal ends of the ZnO NWs indicate that their main axis is preferentially oriented along the [0001]-direction, which is in

Conclusions

H2S-sensing mechanism of CuO/ZnO random nanowire network has been investigated using impedance and work function measurements. We successfully demonstrate that the gas sensing is predominantly a surface phenomenon wherein the bulk and the sample–electrode interface do not undergo any changes on interaction with the gas molecules. The major role is played by the resistive and capacitance contribution coming from the high-resistance depletion layers (interface between p-type CuO and n-type ZnO)

Acknowledgement

This work is partly supported by “DAE-SRC Outstanding Research Investigator Award” (2008/21/05-BRNS) granted to D.K.A.

Ms. Niyanta Datta has completed her B.Sc. (Hons) in Physics from Delhi University and M.Sc. in Physics from IIT Roorkee in 2008. She joined Bhabha Atomic Research Center in 2008 through 52nd batch of training school. Currently she is working as a scientific officer-D and her interests include study of charge transport and gas-sensing properties of various nanostructures.

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    Ms. Niyanta Datta has completed her B.Sc. (Hons) in Physics from Delhi University and M.Sc. in Physics from IIT Roorkee in 2008. She joined Bhabha Atomic Research Center in 2008 through 52nd batch of training school. Currently she is working as a scientific officer-D and her interests include study of charge transport and gas-sensing properties of various nanostructures.

    Dr. Niranjan S. Ramgir completed his Ph.D. (Physics) in 2006 from National Chemical Laboratory, Pune, India. After completing his Humboldt fellowship at Nanotechnology Group, University of Freiburg, Germany, he joined Bhabha Atomic Research Centre as Scientific Officer. His current research work is focused on applications of organic and inorganic semiconducting materials like polypyrrole, polyaniline, ZnO, WO3, CuO, and SnO2 based thin films and nanostructures for sensing, e-nose, and photovoltaic.

    Mr. Suresh Kumar is pursuing his M.Tech (Nanotechnology) at the University of Rajasthan, Jaipur. At present, he is working for his M.Tech project at Bhabha Atomic Research Centre, Mumbai on design and development of electronic nose based on metal oxide thin films and nanostructures.

    Mr. P. Veerender has completed his M.Tech (Chemical Engineering) from Indian Institute of Technology (IIT)-Kharagpur, India, in 2009. Presently, he is working as a graduate student at Homi Bhabha National Institute (HBNI), Bhabha Atomic Research Center. His current research work is focused on organic/nanophotovoltaics, nanotechnology, and sensors.

    Dr M. Kaur received her Ph.D. from Devi Ahilya Vishwavidyalaya, Indore, in 1998. Her thesis work involved effect of heavy ion irradiation on high-temperature superconductors. She joined Bhabha Atomic Research Center, Mumbai, in 1999 as a research associate. Her present interests include development of metal oxide thin films and nanomaterials for sensing toxic gases.

    Mrs. S. Kailasaganapathi completed her B.Sc. in Physics from Manonmanium Sundaranar University, Tirunelveli, Tamil Nadu, India, in 2003. She joined Bhabha Atomic Research Center in 2006 as a scientific assistant. Her research work involves development of gas sensors based on metal oxides.

    Dr. A.K. Debnath is presently working as scientific officer (F) at Technical Physics Division of BARC. He has extensively worked on oxide material-based gas sensor, particularly for H2S detection. His current research interest is to understand the charge transport and gas-sensing properties of ultra-thin films of organic semiconductors grown using MBE.

    Dr. D.K. Aswal joined Bhabha Atomic Research Center in 1986 through 30th Batch of Training School after completing M.Sc. (Physics) from Garhwal University and is presently Head of Thin Films Devices Section. His area of scientific interest is condensed matter physics, specializing in device-oriented research leading to hybrid molecule-on-Si nanoelectronics, thermoelectric devices, and gas sensors. He is a recipient of several international fellowships, including JSPS fellowship, Japan (1997–1999), IFCPAR fellowship, France (2004–2005), BMBF fellowship, Germany (2006), and CEA fellowship, France (2008). He is recipient of several awards, including “MRSI Medal 2010”, “Homi Bhabha Science and Technology Award-2007”, “DAE-SRC Outstanding Research Investigator Award-2008”, and “Paraj: Excellence in Science Award, 2000”.

    Dr. S.K. Gupta joined Bhabha Atomic Research Center in 1975 and is presently Head of Technical Physics Division. Over the years, he has worked on space quality silicon solar cells, high-temperature superconductor thin films and single crystals, gas sensors, and thermoelectric materials. He has carried out extensive studies on vortex dynamics in superconductors. He is a member of the National Academy of Sciences, India.

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