Sensors and Actuators B 185 (2013) 265–273 Contents lists available at SciVerse ScienceDirect Sensors and Actuators B: Chemical journal h om epage: www.elsevier.com/locat e/snb Hierarchical SnO/SnO 2 nanocomposites: Formation of in situ p–n junctions and enhanced H 2 sensing Arunkumar Shanmugasundaram, Pratyay Basak, L. Satyanarayana, Sunkara V. Manorama ∗ Nanomaterials Laboratory, Inorganic & Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology, Hyderabad 500007, Andhra Pradesh, India a r t i c l e i n f o Article history: Received 6 October 2012 Received in revised form 6 April 2013 Accepted 23 April 2013 Available online 7 May 2013 Keywords: SnO SnO2 p–n junction Hydrogen sensor Palladium Room temperature sensing a b s t r a c t Herein, we present a new approach to achieve well-defined SnO/SnO 2 composites with in situ formation of p–n heterojunctions. The materials synthesized by a simple one-pot hydrothermal method were char- acterized in detail using powder X-ray diffraction (XRD), thermogravimetry (TGA), micro-Raman, X-ray photo electron spectroscopy (XPS), and ultra violet-diffused reflectance spectroscopy (UV-DRS). Analysis confirms the presence of mixed phases and the findings are consistent. The morphological evaluations were carried out by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The investigations reveal the self-assembly of smaller nanoparticles into hierarchical structures resem- bling nanorods which aggregate further to form loose cube-like morphology finally transforming into dense micro-prisms. Selected area electron diffraction (SAED) establishes the phases of these nano- and microstructures. Synthesized materials also show improved electrical properties owing to the presence of SnO/SnO 2 multiple p–n heterojunctions in the bulk. The advantage is reflected in the results of gas sensing studies that indicate enhanced hydrogen gas sensing response. Significant improvement in sen- sor response, selectivity and sensitivity could be achieved further with incorporation of Pd. A plausible mechanism of gas sensing on the material surface based on the formation of heterojunctions is discussed. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Hydrogen has positioned itself as the next generation energy source for modern industries with particular relevance to the evolving fuel cell technologies. Nevertheless, serious apprehen- sions owing to its lower explosive limit (LEL approximately at 4% in normal air) demands addressing the associated safety norms to be in place. High performance, highly selective and sensi- tive sensing devices that can operate at lower temperatures (preferably at ambient) can provide effective deterrent to prevent accidents [1,2]. Among the several types of metal oxides based semiconductor gas sensor materials, by far tin oxide has promised the most poten- tial for gas sensing applications [3,4]. Its excellent characteristics, such as, high sensitivity, rapid response, reasonably good stability and reusability makes it the material of choice for the researchers [5]. Tin oxide exists primarily in two phases, SnO 2 and SnO, with oxidation states of +4 and +2, respectively. SnO 2 , the more sta- ble of the two phases, is a wide band gap semiconductor (∼3.6 eV at RT) that offers a balanced combination of physico-chemical and optoelectronic properties resulting in an impressive range of ∗ Corresponding author. Tel.: +91 40 27193225; fax: +91 40 27160921. E-mail address: manorama@iict.res.in (S.V. Manorama). applications [6–9]. Primarily, SnO 2 , an n-type semiconductor with its intrinsic oxygen vacancies has been widely studied and the prop- erties are well documented. SnO, on the other hand, shows high p-type conductivity due to the naturally occurring Sn vacancies and is an indirect band gap material (2.1–2.5 eV at RT). Literature reveals that detailed investigation on the SnO phase and its properties are few primarily because of its ease of transformation into SnO 2 . Several years of research on metal-oxide based gas sensors have provided key understanding on the control parameters and material tailoring techniques. Doping, alloying, surface functional- ization, size reduction, morphology and phase control, are some of the most popular approaches towards improving the sensor sensitivity, selectivity and reproducibility. Formation of p–n junc- tion produces significant changes at the interface, particularly, in the electrical and optical properties. Efforts have demonstrated that a combination of materials forming p–n junction can greatly enhance sensor performance. With a phenomenal increment of the material defect sites at the interface (surface/bulk), these are envisaged as efficient gas sensing materials. Yamazoe et al. and our own earlier studies have demonstrated that CuO with SnO 2 forms semiconductor-semiconductor p–n junctions with high sensing performance [10–12]. Recently, an observation on NiO/SnO 2 sys- tem is also reported by Zhang et al. [13]. Though, a similar possibility of p–n heterojunction formation exists for SnO/SnO 2 mixed phase system, the available literature effectively lacks any 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.04.097