The role of gold catalyst on the sensing behavior of ZnO nanorods for CO and NO2 gases

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Abstract

A facile one-pot strategy was developed for the assembly of gold nanoparticles (Au NPs) onto single crystalline ZnO nanorods using cetyltrimethylammonium bromide (CTAB) as a capping agent. Zinc oxide nanorods were synthesized by hydrothermal method whereas the Au NPs (below 5 nm) were deposited on the surface of ZnO nanorods by the solution growth method. Gas sensing properties of Au/ZnO nanorods were studied at various temperatures for various concentrations of reducing (CO) and oxidizing (NO2) gases in synthetic air and compared with pristine ZnO nanorods. Sensor fabricated by Au/ZnO nanorods showed significantly enhanced sensing performances for CO gas while opposite was the case with NO2 gas as compared to pristine ZnO nanorods. The highest response of Au/ZnO nanorods for CO gas was 12 at 150 °C while for ZnO nanorods, it was 6.12 at 400 °C. Whereas the highest response of Au/ZnO nanorods for NO2 gas was 4.14 while for ZnO nanorods, it was 10 at 300 °C. It was found that Au NPs acted as promoter for CO gas while inhibiter for NO2 gas sensing due to their different sensing mechanisms. This study suggested that noble metals decoration of ZnO nanorods can be used for selectivity issue between CO and NO2 gases.

Introduction

Chemical sensors have a profound influence on personal safety, medical diagnosis, detection of pollutants and toxins, and the transportation industries [1], [2]. Metal-oxide semiconductors (MOS) such as SnO2, ZnO, and In2O3 have been extensively investigated as sensing materials in the last ten years, due to their high sensitivity to the target gases and simplicity in preparation. Recently, MOS with one-dimensional (1D) nanostructures such as nanowires, nanorods, and nanofibers have been demonstrated to be promising candidates for ultrasensitive sensors because of their single crystalline nanostructure, high surface to volume ratios and special physical and chemical properties [2]. Among various types of MOS, ZnO is an important semiconductor material with extensive applications in electronics, photoelectronics, sensors, and optical devices. It is well known for its wide band gap of 3.37 eV, large exciton binding energy of 60 meV at room temperature, high electron mobility, and excellent chemical and thermal stability. In nanocrystalline ZnO films, for example, using elongated, rod-like ZnO crystals instead of spherical crystals increases the overall electron mobility by a factor of 50 [3]. Therefore, ZnO nanowire sensors, for example, have been widely used to detect chemicals such as ethanol (C2H5OH), hydrogen (H2) and ammonia (NH3) [4], [5], [6], [7]. In recent years, research attention in the area of chemical sensors has been focused on further increasing the sensor sensitivity. A very efficient strategy is to load noble metals such as Au, Pt and Pd onto an oxide matrix as sensitizer or promoter to improve the sensing reactions [8], [9], [10]. Many approaches have been developed to assemble noble metals onto semiconductors [11], [12], [13], [14], [15], [16]. Recently, Zhang et al. developed a PVP-assisted method for attaching Pd NPs onto ZnO nanowires and the as-obtained hybrid material showed enhanced sensing performances towards detecting H2S gas [13]. A more recent report from Joshi and coworkers demonstrated the synthesis of Au decorated ZnO nanowires for CO sensing [15]. While these studies are unique and effective, the synthetic processes are not facile and convenient due to their multiple steps, where the noble metal nanoparticles are chemically pre-formed and subsequently attached onto support materials. Furthermore, metal nanoparticles prepared by most of these methods were above 5 nm. It is well known that noble metals, below 5 nm which is also called as mitohedrical region, show better catalytic activity, because in this region pronounced nanosized effect is found [17].

Furthermore, enhanced gas sensing performances using noble metal is generally explained by “electronic mechanism” and “chemical mechanism”. The “electronic mechanism” proposes that the noble metal acts as an electron acceptor on semiconductor oxide surfaces, which contributes to the increase of the depletion layer [18]. Therefore, as compared with the pristine oxide, the change in resistance is larger, leading to the increase in response. The “chemical mechanism” proposes that the noble metal catalytically activate the dissociation of molecular oxygen, whose atomic products then adsorb on the metal oxide support resulting in a greater degree of electron withdrawal from the metal oxide than for the pristine metal oxide [19]. Thus, noble metal helps in the improvement of sensing performance. Recently, in our laboratory, the effect of Au NPs on the CO sensing property has been examined using Au NPs core and SnO2 shell strategy rather deposition of Au NPs on the surface of SnO2 nanoparticles. It has been found that Au NPs remarkably enhanced the response for CO gas in Au@SnO2 nanostructure in low test temperature below 150 °C as compared to pristine SnO2 which shows its catalytic activity towards CO gas [20]. However, in most of the cases the catalytic effect of noble metal is examined mostly for reducing gases and rarely investigated for oxidizing gases [21], [22], [23], [24].

In spite of numerous papers reporting on the benefits in sensor performances due to presence of noble metal, little information exist about the origin of these improvements. Since, the sensing mechanism for reducing and oxidizing gases is different hence it will be interesting to know the effect of noble metal on the sensing performance of oxidizing and reducing gases. In this paper, a facile one-pot strategy to synthesize and simultaneous deposition of Au NPs onto single crystalline ZnO nanorods is reported. Au NPs below 5 nm are formed on the surface of ZnO using CTAB as a capping agent. This approach is convenient, time-saving and requires no need for pre-synthesis of noble metals. Furthermore, gas sensor based on the synthesized ZnO and Au/ZnO nanorods materials is fabricated. A systematic sensing test is performed using mainly CO and NO2 as the probe gas to evaluate the effect of noble metal on the sensing property of the ZnO sensor. The Au/ZnO nanorods sensor shows significantly enhanced response for CO gas in comparison to pristine ZnO materials while opposite is the case with NO2 where ZnO nanorods show high response as compared to Au/ZnO nanorods. Our results demonstrate that the functionalization of ZnO nanorods with noble metals can be used to solve the selectivity problem between reducing and oxidizing gases.

Section snippets

Chemicals

All the chemicals (Zn(NO3)2·6H2O (Reagent Grade, 98% Sigma–Aldrich), cetyltrimethylammonium bromide (CTAB; Aldrich), HAuCl4·4H2O (99% Showa Chemicals) and trisodium citrate (99.5% Showa Chemicals) were analytical grade and used as such.

Synthesis of ZnO nanorods

In the first step ZnO nanorods were grown by a simple hydrothermal reaction as reported in previous work [25].

Synthesis of Au/ZnO nanorods

In this step ZnO nanorods (0.1 g) were dispersed in 50 ml water and sonicated for 1 h. It was then transferred in round bottom flask and stir at 80 °C for 1 h

Structural analysis

The phase and structural analysis of Au/ZnO nanorods is carried out by XRD and shown in Fig. 2. All the peaks are indexed to typical wurtzite ZnO consistent with the standard values for bulk ZnO (JCPDS card No. 36-1451). The sharp diffraction peaks indicate the good crystallinity of the prepared crystals and no peaks for other impurities were detected in the profile. In addition to the peaks of ZnO, three weak diffraction peaks, attributing to Au (1 1 1), Au (2 0 0) and Au (2 2 2) planes, are also

Growth mechanism

The synthetic process for the assembly of Au NPs on ZnO nanorods is illustrated in Scheme 1. This is a quasi one-pot approach, where Au NPs are formed in the presence of CTAB and simultaneously locked on the surface of ZnO nanorods. Here it should be noted that no reducing agent is used for the synthesis of Au NPs and CTAB may played both role. CTAB is a cationic surfactant having small hydrophilic head and long hydrophobic tail. It is well known surfactant for the shape selective synthesis of

Conclusions

Au/ZnO nanorods are synthesized by solution method using CTAB as a capping agent while no reducing agent is used. Au NPs below 5 nm are uniformly distributed on the surface of ZnO nanorods. Presence of Au NPs on the surface of ZnO nanorods enhanced the response and lowered the work temperature for CO gas while same thing is achieved by pristine ZnO nanorods in case of NO2 gas. It is found that electronic as well as chemical effect of Au NPs both favored for CO sensing while chemical effect

Acknowledgements

This work was supported by (a) Post-BK21 program from Ministry of Education and Human-Resource Development. (b) National Research Foundation (NRF) grant funded by the Korea government (MEST) (NRF- 2010-0019626, 2010-0028802).

Prabhakar Rai is currently pursuing his Ph.D. degree in Department of Information and Electronics Materials Engineering, Chonbuk National University. He got his Master degree in Inorganic chemistry in 2006 at Allahabad University, India. His research interests are synthesis of nanoscale metals and metal oxides for gas sensor application.

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    Prabhakar Rai is currently pursuing his Ph.D. degree in Department of Information and Electronics Materials Engineering, Chonbuk National University. He got his Master degree in Inorganic chemistry in 2006 at Allahabad University, India. His research interests are synthesis of nanoscale metals and metal oxides for gas sensor application.

    Yun-Su Kim is doing his study for Master degree in Department of Information and Electronics Materials Engineering, Chonbuk National University and his research interest is mainly about the synthesis and application of Au@TiO2, Ag@TiO2, Pt@TiO2 core-shell nanostructure for gas sensor and photocatalysis application.

    Hyeon-Min song is pursuing his Master degree in Department of Information and Electronics Materials Engineering, Chonbuk National University and currently interested in the synthesis of core-shell nanostructures such as Au@SnO2, Pt@SnO2, Ag@SnO2 and their applications for gas sensor.

    Min-Kyung Song continues her study for Master degree in Department of Information and Electronics Materials Engineering, Chonbuk National University and carrying out research in the field of dye sensitized solar cell and gas sensor.

    Yeon-Tae Yu received his engineering diploma in department of metallurgy engineering in 1988 from Chonbuk National University, Korea and a Doctoral degree (1993) from Tohoku University, Japan. From 1993 until 2003, he was a senior researcher for Korea Institute of Geoscience & Mineral Resources. In 2003, he moved to Division of Material Science Engineering of Chonbuk National University. His main research interests are synthesis of core-shell structure composite nanoparticles and their application for gas sensors.

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