Sensors and Actuators B 160 (2011) 364–370 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical j o ur nal homep a ge: www.elsevier.com/locate/snb High sensitive and selective formaldehyde sensors based on nanoparticle-assembled ZnO micro-octahedrons synthesized by homogeneous precipitation method Lexi Zhang a,b , Jianghong Zhao a,∗ , Haiqiang Lu a,b , Liming Gong a,b , Li Li a , Jianfeng Zheng a , Hui Li a,b , Zhenping Zhu a,∗ a State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taoyuan South Road 27, Taiyuan 030001, PR China b Graduate University of Chinese Academy of Sciences, Beijing 100039, PR China a r t i c l e i n f o Article history: Received 31 May 2011 Received in revised form 18 July 2011 Accepted 28 July 2011 Available online 4 August 2011 Keywords: Homogeneous precipitation ZnO micro-octahedron Gas sensor Formaldehyde Defect Oxygen species a b s t r a c t Nanoparticle-assembled ZnO micro-octahedrons were synthesized by a facile homogeneous precipitation method. The ZnO micro-octahedrons are hexagonal wurtzite with high crystallinity. Abundant structure defects were confirmed on ZnO surface by photoluminescence. Gas sensors based on the ZnO micro- octahedrons exhibited high response, selectivity and stability to 1–1000 ppm formaldehyde at 400 ◦ C. Especially, even 1 ppm formaldehyde could be detected with high response (S = 22.7). It is of interest to point out that formaldehyde could be easily distinguished from ethanol or acetaldehyde with a selectivity of about 3. The high formaldehyde response is mainly attributed to the synergistic effect of high contents of electron donor defects (Zn i and V O ) and highly active oxygen species (O 2− ) on the ZnO surface. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Metal oxide semiconductor (MOS) gas sensors, including SnO 2 , ZnO, In 2 O 3 , WO 3 , and so on, have attracted considerable atten- tion owing to their ability of detecting trace gases [1]. As one of the key wide bandgap (∼3.4 eV at 1.2 K) [2] semiconductors, ZnO has been proved to be an excellent gas-sensing material for mea- suring both oxidative and reductive target gases at ppm (parts per million) level and above [3]. Taking advantages of the small size, large surface-to-volume ratios and high density of surface active sites compared to their bulk counterparts, great interest has been focused on performance-enhanced gas sensors based on ZnO nanostructures, such as nanoparticles [4], nanorods [5], nanobelts [6], nanotubes [7] and nanosheets [8]. Recently, hierar- chical structures constructed by low-dimensional nanomaterials, for example, nanoparticle-organized hollow spheres [9], nanorod- combined flower-like structures [10], nanosheet-assembled 3D architectures [11,12], have began to catch much of researchers’ attention, because they exhibited enhanced gas-sensing perfor- mances which originated from the improvement in exposing more ∗ Corresponding authors. Tel.: +86 351 4048715; fax: +86 351 4041153. E-mail addresses: zjh sx@sxicc.ac.cn (J. Zhao), zpzhu@sxicc.ac.cn (Z. Zhu). available surface, facilitating gas diffusion and transportation, and so forth. However, great efforts are still needed to further develop their synthesis processes, since hard templates, surfactants or rel- ative high temperature is usually necessary for fabricating these hierarchical structures. Accordingly, it is significantly important to develop template-free, facile and low temperature methods to synthesize novel hierarchical nanostructures, and to carry out in- depth research on their gas sensing properties. Compared with other methods, homogeneous precipitation is a more economic (no need for special apparatus) and environment-friendly (no need for surfactants or organic solvents) method to prepare metal oxides for sensor applications [13] in large scale at low temperature. In spite of its advantages in preparation, based on this method, it is usually difficult to controlled synthesize metal oxide nanostructures [14] (except for nanoparticles), to say nothing of nano-building block assembled hierarchical architectures. As an important industrial chemical, formaldehyde has been widely used to manufacture plastics, medicine, synthetic fibers and household products. Regrettably, formaldehyde is very harm- ful to human health because of its volatility, irritability and toxicity, thus is considered as one of the main indoor air pollutants in res- idential and industrial occupational environments. It is of great practical importance to detect formaldehyde rapidly and accu- rately in the atmosphere. Till now, significant progress has been 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.07.062