Experimental and theoretical studies of gold nanoparticle decorated zinc oxide nanoflakes with exposed {1 0 1¯ 0} facets for butylamine sensing

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Highlights

  • A facile method was developed for loading gold particles on zinc oxide nanoflakes.

  • The ZnO/Au nanoflakes show excellent response and selectivity to toxic n-butylamine.

  • DFT simulation was used to study the n-butylamine-sensing mechanism of ZnO sensor.

Abstract

The exposed surface facets play an important role in determining the gas-sensing performance of nanostructured materials. This study reports the facile hydrothermal synthesis of zinc oxide nanoflakes with exposed {1 0 1¯ 0} facets, as confirmed by the high resolution transmission electron microscopy (HRTEM) and the corresponding selected area electron diffraction (SAED) analysis. The gas-sensing properties of the ZnO nanoflake sensor were investigated toward toxic n-butylamine, an important marker compound in food and medical industries. The pure ZnO nanoflake sensor exhibits a response of 23.9–50 ppm of n-butylamine at an optimum operating temperature of 300 °C. Density Functional Theory (DFT) simulations were used to study the adsorption behavior of n-butylamine on the ZnO(1 0 1¯ 0) surface. The results show that n-butylamine chemically adsorb on the ZnO(1 0 1¯ 0) surface through the formation of a bond between the nitrogen atom of the n-butylamine (C4H11N) and the surface Zn atom of ZnO. To further improve the gas-sensing properties, the as-prepared ZnO nanoflakes were subsequently loaded with three different quantities of Au (1.37, 2.82, and 5.41 wt% Au). The gas-sensing measurements indicate that the Au nanoparticle-decorated ZnO nanoflakes display superior sensing performance to non-modified ZnO nanoflakes by exhibiting 4–6 times higher response and an improved selectivity toward n-butylamine gas, along a decreased optimum operating temperature of 240 °C. Moreover, the response and recovery properties of the ZnO nanoflake sensor are improved by a factor of 1.5–2.5 depending on the Au loading. The enhanced sensing performance of the Au nanoparticle-decorated ZnO nanoflakes to n-butylamine gas can be attributed to the excellent catalytic activity of Au nanoparticles (NPs) which promotes a greater adsorption of oxygen molecules on the surface of ZnO and the presence of multiple electron depletion layers, specifically at the surface of ZnO and at the ZnO/Au interface, which greatly increases their conductivity upon exposure to the gas.

Introduction

Detection and monitoring of hazardous gases in food, chemical and manufacturing industries have received increasing attention recently due to the growing awareness about public health safety [1]. Many studies have been carried out to develop high-performance gas sensors with excellent response, selectivity and stability. Among the various forms of gas sensors, semiconducting (chemiresistive) gas sensors are particularly attractive for the sensing of harmful gases, because of the low manufacturing cost, easy processing, and reliable performance. Semiconducting gas sensors usually consist of metal oxide materials, such as zinc oxide (ZnO), tin dioxide (SnO2), iron oxide (Fe2O3), tungsten oxide (WO3), and indium oxide (In2O3) [2].

Among these oxides, ZnO, is an important n-type semiconductor with a band gap of Eg = 3.3 eV, and has been widely applied for different applications including gas sensors [3], [4], photocatalysts [5], [6] solar cells [7], [8], and lithium-ion batteries [9]. ZnO exhibits excellent thermal and chemical stability and high electrical conductivity, which makes it highly attractive for gas-sensing application [10]. However, pure ZnO sensors typically suffer from low selectivity and high optimum operating temperatures of ≥300 °C, which may impede their broader applications [11], [12]. To reduce such problems, many efforts have been devoted in this area aiming to controlling particle size and morphology [13], maximizing the exposure of active crystal facets [14], [15], and introducing transition metal dopants [11], [15], [16], [17], [18], [19], metal oxide additives [1], [17] or noble metal sensitizers [11], [17], [20], [21], [22], [23].

Organic amines (e.g., n-butylamine) are important marker compounds for quality control in food industries and medical diagnosis [24]. n-Butylamine is also frequently used as a vulcanizing accelerator and reaction initiator in the polymer industries, a chemical intermediate in the production of emulsifying agents, rubber chemicals, tanning agents and special soaps. Furthermore, it is also utilized in the manufacture of pharmaceuticals, dyes, insecticides and textiles. n-Butylamine is toxic and is easily absorbed through skin. Direct exposure of n-butylamine vapor can cause eye, skin and upper respiratory tract irritation [25]. Therefore, it is necessary to develop sensor material(s) to detect n-butylamine gas with excellent response, selectivity, and stability. To date, atmospheric n-butylamine has been quantified by isotachophoresis, and by high pressure liquid chromatography (HPLC), however, these techniques require expensive equipments [26]. To date, there were few limited reports on the use of solid state semiconductor nano-sensors for detection of n-butylamine, with only sensors based on WO3, V2O5 and AgVxOy nanostructures have been reported so far [27], [28], [29]. The reported responses of these sensors to n-butylamine, however, are relatively low and require further improvement.

A clear understanding of the sensing mechanism of ZnO nano-sensors is crucial for ensuring the successful application of these sensors in real life monitoring applications. Theoretical simulation studies have been carried out to investigate the adsorption of oxidizing (e.g., NOx, SOx) and reducing gases (CO, NH3, ethanol) on different ZnO surfaces [30], [31], [32], [33]. The DFT study by Breedon et al. [30], [34] found that NO2 and NO interacted weakly with ZnO(1 0 1¯ 0) and (2 1¯ 1¯ 0) crystal without generating any significant surface distortions. Prades et al. [32] expanded the study further by examining the adsorption of NO2 on ZnO(1 0 1¯ 0) and (1 1 2¯ 0) crystal planes with 12.5% O vacancy and found that NO2 was strongly adsorbed on the surface Zn atoms, in the presence of oxygen vacancies. Moreover, the DFT investigation by An et al. on the adsorption of several reducing and oxidizing gases on defect-free, single-walled ZnO nanotube reveal that O2 and H2 molecules were physisorbed on the sidewall of the ZnO nanotube while CO, NH3, and NO2 were molecularly chemisorbed. Despite some successes, theoretical simulation studies of the adsorption of organic amine (e.g., n-butylamine) on the ZnO surface were not yet reported. Hence, there exists a lack of clear understanding of the atomic scale interactions that occur between organic amine gas molecules and ZnO surface during the gas-sensing process, particularly in terms of: (i) the effect of the interaction on the bond length and bond angle of the amine gas molecules, and (ii) the adsorption mechanism of the amine gas molecules on the ZnO surface.

This study reports the facile hydrothermal synthesis of ZnO nanoflakes with {1 0 1¯ 0} facets under mild hydrothermal conditions and the subsequent modification with different Au contents (1.37, 2.82, and 5.41 wt% Au) for gas-sensing application. The morphology and composition of the as-prepared ZnO/Au nanocomposites were characterized by various analytical techniques, including transmission electron microscopy (TEM), high resolution TEM (HRTEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and inductively coupled plasma atomic emission spectroscopy (ICP-AES). The effect of the different Au loading on the sensing properties of ZnO nanoflakes toward n-butylamine and other gases and the important parameters such as response, selectivity and stability were evaluated. Additionally, density functional theory (DFT) simulations were also conducted for achieving a clear atomic-scale understanding of the interaction between n-butylamine gas (along with other gases e.g. ethanol, n-butanol, and acetone) and ZnO(1 0 1¯ 0) surface of the nanoflakes during the gas-sensing process.

Section snippets

Chemicals

Zinc chloride (ZnCl2, 99%), sodium hydroxide (NaOH, 99%), gold(III) chloride trihydrate (HAuCl4.3H2O, 99.99%), sodium borohydride (NaBH4), n-butylamine (C4H11N, 99%) acetone (C3H6O, 99.9%), n-butanol (C4H10O, 99.8%), ethanol (C2H6O, 99.8%), methanol (CH4O, 99.9%), and heptane (C7H14, 99%) were purchased from Sigma–Aldrich and used as received without further purification. All the chemicals were of analytical grade. Ultra-pure water was used throughout the experiments.

Synthesis of porous ZnO nanoflakes

The porous ZnO nanoflakes

Morphology and composition

The overall quantity of Au in the three Au-decorated ZnO nanoflake samples (labeled as ZnO NF/Au (1), ZnO NF/Au (2) and ZnO NF/Au (3)) were initially determined using ICP-AES. As shown in Table 1, the Au contents in samples ZnO NF/Au (1), (2), and (3) are 1.37, 2.82 and 5.41 wt%, respectively. The phase composition of the ZnO nanoflakes with or without metallic decoration was characterized using XRD. Fig. 1 displays the XRD patterns of the pure and Au-decorated ZnO products. All of the

Conclusions

This study has demonstrated the facile synthesis of ZnO nanoflakes with exposed {1 0 1 0} facets through a hydrothermal method and their subsequent modifications with Au NPs. The effects of the different Au loading on the crystal structure, surface properties, and amine sensing performance of the ZnO nanoflakes have been investigated. Several main findings of this study are summarized below:

  • The pure ZnO nanoflakes exhibit pores with sizes of 2–10 nm and the top/bottom surface of these nanoflakes

Acknowledgments

We gratefully acknowledge the financial support of the Australian Research Council (ARC DP1096185, FT0990942) projects. The authors also acknowledge access to the UNSW node of the Australian Microscopy and Microanalysis Research Facilities (AMMRF). The authors would like to thank Dr. Rabeya Akter for the ICP analysis, Dr. Jason Scott for the BET analysis, and Dr. Bill Gong for the XPS measurements.

Dr. Yusuf Valentino Kaneti has obtained his Ph.D. degree in July 2014 from the University of New South Wales (UNSW), Australia. He is currently working as a postdoctoral fellow in the School of Materials Science & Engineering at UNSW, focusing on the synthesis and characterization of semiconductor metal oxide nanostructures/nanocomposites for energy and environmental applications. He has published 14 journal articles and 1 book chapter, with a H-index of 8.

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    Dr. Yusuf Valentino Kaneti has obtained his Ph.D. degree in July 2014 from the University of New South Wales (UNSW), Australia. He is currently working as a postdoctoral fellow in the School of Materials Science & Engineering at UNSW, focusing on the synthesis and characterization of semiconductor metal oxide nanostructures/nanocomposites for energy and environmental applications. He has published 14 journal articles and 1 book chapter, with a H-index of 8.

    Mr. Xiao Zhang received his undergraduate degree in materials science and engineering from the University of New South Wales, Australia. His research interest is on the synthesis and functional application of metal oxide nanostructures for gas-sensing applications.

    Mr. Minsu Liu is a Ph.D. candidate studying materials science and engineering at Monash University, Australia. His research interest is on the synthesis of metal oxide (especially for vanadium oxides) nanostructures for energy and environmental applications.

    Mr. David Yu is an M.Sc. candidate studying materials science and engineering at Monash University, Australia. His research interest is on the synthesis and application of magnetic nanoparticles for environmental and biomedical applications.

    Ms. Yuan Yuan is a Ph.D. candidate studying materials science and engineering at the University of New South Wales, Australia. Her research interest is on the synthesis of shape-controlled noble metal nanostructures for catalytic applications.

    Dr. Leigh Aldous is an ARC DECRA fellow and he works as a lecturer/researcher in the School of Chemistry at the University of New South Wales. Dr. Aldous’ research focuses upon electrochemistry in ionic liquids (for biomass conversion, hydrogen storage and unique physical characteristics), as well as electroanalysis using nanocarbons. Dr. Aldous has published more than 67 journal articles and 2 patents in the past 6 years with SCI citations over 1800 times, leading to a H-index of 26.

    Prof. Xuchuan Jiang has fully devoted to the study on synthesis and self-assembly of nanoparticles for energy and environmental applications since the award of his Ph.D. in 2001. His research focuses on developing new techniques for the synthesis of noble metals, metal oxides and their nanocomposites for energy, environmental and biomedical applications. He has published over 100 papers with SCI citations of over 4000 times, leading to a H-index of 29.

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