Sensors and Actuators B 145 (2010) 114–119 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb Photoluminescence investigation on the gas sensing property of ZnO nanorods prepared by plasma-enhanced CVD method Ning Han a,b , Peng Hu a , Ahui Zuo a,b , Dangwen Zhang a,b , Yajun Tian a , Yunfa Chen a,∗ a State Key Laboratory of Multi-phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China b Graduate University of Chinese Academy of Sciences, Beijing 100049, PR China article info Article history: Received 31 July 2009 Received in revised form 29 October 2009 Accepted 18 November 2009 Available online 24 November 2009 Keywords: ZnO nanorod Intrinsic defects Photoluminescence Peak decomposition Gas sensor abstract Gas sensing property of ZnO nanorods prepared by plasma-enhanced chemical vapor deposition (CVD) method is studied using formaldehyde as the probe gas, and the intrinsic defects are investigated by photoluminescence (PL). The results show that high ratio of visible to ultra-violet luminescence cannot account for high gas response. The PL spectra are Gaussian decomposed to subpeaks according to their origination, which are separated into donor- (DL) and acceptor-related (AL) ones. A conclusion is derived that where the content of DL is high and that of AL is low, the gas response is high. This conclusion is further confirmed by tuning the PL spectra and gas sensing property through annealing in different atmospheres. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Since the first application in gas sensor [1], ZnO has found its way in explosive gas alerting and toxic gas detection for decades, with various morphologies and different dopants contributing much to the development of resistance-based ZnO gas sensors [2,3]. But the key factor determining the gas sensing property of ZnO is still under debate. The most prevalent model is founded using SnO 2 by Xu et al. [4] that compares the particle diameter (D) and depth of sur- face charge layer (L): if D is comparable to or less than 2L, the gas response is expected to be high. However, just comparing D and L leads to dilemma in some studies [5–7], because L is hard either to be measured or to be calculated. As the electronic property of ZnO mainly depends on its intrin- sic defects [8], the gas response defined as the ratio of resistance in air and in detectant (R a /R g ) is therefore closely correlated to the intrinsic defects [9,10]. Photoluminescence (PL) spectra is the lumi- nescences originated from the photo-induced electron/hole and/or the intrinsic defects in ZnO [8]. Therefore, there are several defect- related luminescences as well as the photo-induced near band edge excitation in ZnO PL spectra. Thus some researchers used the ∗ Corresponding author at: State Key Laboratory of Multi-phase Complex Sys- tems, Institute of Process Engineering, Chinese Academy of Sciences, Haidian District, Beijing 100190, PR China. Tel.: +86 10 82627057; fax: +86 10 62542803. E-mail addresses: nhan@home.ipe.ac.cn (N. Han), pengh@home.ipe.ac.cn (P. Hu), ahzuo@home.ipe.ac.cn (A. Zuo), dwzhang@home.ipe.ac.cn (D. Zhang), yjtian@home.ipe.ac.cn (Y. Tian), yfchen@home.ipe.ac.cn (Y. Chen). intensity ratio of visible luminescence to ultra-violet luminescence (I VL /I UL ) to evaluate the crystallinity of ZnO crystal: the higher the ratio, the more the intrinsic defects [10,11]. However, Shi et al. [12] pointed out that the ratio of I VL /I UL is affected by sample type as well as excitation density, and could not be simply used to assess the crystal defects. Therefore, we tried to use decomposed PL spectra to distinguish donor-related (DL) and acceptor-related lumines- cences (AL) and further to investigate the relationship between the intrinsic defects and the gas sensing property of ZnO. The ZnO nanorods with different lengths used here are pre- pared in seconds by plasma-enhanced chemical vapor deposition (CVD) method under high temperature and thus possess lots of intrinsic defects [13,14]. Furthermore, the defects were tailored by annealing in different atmospheres and the relationship between the intrinsic defects measured by PL and the gas sensing property of ZnO was confirmed. It should be noted that the donors and accep- tors in ZnO crystal play different roles in electron transport, and they affect the gas sensing property of ZnO in opposite way. 2. Experimental 2.1. ZnO nanorods preparation The ZnO nanorods were prepared by RF thermal plasma- enhanced CVD method as we reported earlier [13]. Briefly, zinc powder and oxygen were introduced into the Argon plasma (30 kW, 4 MHz), after a vapor-solid (VS) growth process and with a two- directional growth mechanism, the ZnO nanorods were gained. The 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.11.042