Influence of Precursor Chemistry on Morphology and

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Influence of Precursor Chemistry on Morphology and Composition of CVD-Grown SnO2 Nanowires Ralf Müller,† Francisco Hernandez-Ramirez,‡ Hao Shen,† Hongchu Du,§ Werner Mader,§ and Sanjay Mathur*,† †

Institute of Inorganic Chemistry, University of Cologne, D-50939 Cologne, Germany Department of Electronics, University of Barcelona, E-08028 Barcelona, Spain § Institute of Inorganic Chemistry, University of Bonn, D-53117 Bonn Downloaded via NATL TAIPEI UNIV OF TECHNOLOGY on October 18, 2018 at 06:55:08 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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S Supporting Information *

ABSTRACT: Tin oxide (SnO2) nanowires (NWs) were synthesized via the goldcatalyzed chemical vapor deposition of tin(IV) and tin(II) precursors, namely, [Sn(OtBu)4] (1) and [Sn(OtBu)2]2 (2). Nanowires were deposited on goldcoated Si(001) substrates, following the vapor−liquid−solid mechanism. Energydispersive X-ray (EDX) analysis and high-resolution transmission electron microscopy (HR-TEM) measurements on individual nanostructures showed that the change in tin valence from +IV to +II has significant influence on the morphology and composition of the resulting NWs. Whereas 1 led directly to the growth of SnO2 nanowires, 2 underwent a disproportionation reaction whereby the elemental phase (Sn0) reacted with Au nanoparticles to form a Au−Sn intermetallic catalyst. Comparative analysis of gas-sensing behaviors of nanowires grown from 1 and 2 illustrated that crystallographic imperfections, such as oxygen deficiency and a change in the oxidation states of the cations, are subject to the precursor configurations (Sn:O ratio in 1 and 2) and can significantly alter the surface properties, such as transduction behavior and electronic transport, that are responsible for their sensitivity toward analyte gases. KEYWORDS: molecular precursor, chemical vapor deposition, tin alkoxide, Sn2+ disproportionation, VLS growth mechanism, gas sensing

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INTRODUCTION The sustaining interest in the growth, characterization, and application of one-dimensional (1D) metal oxide nanostructures is attributed to fundamentally new surface phenomena and bulk transport properties manifested in high-performance devices such as nanowire-based photovoltaics, lithium-ion batteries, or biological and chemical sensors.1 A wide variety of wide-band-gap semiconductors such as SnO2,2,3 ZnO,4,5 TiO2,6 WO3,7 Fe2O3,8 and In2O3,9 as well as their functionalized forms,10 have been intensively studied for chemicalsensing applications. In particular, tin oxide (SnO2) represents one of the most widely studied materials used for gas-sensing applications, because of its ability to detect both oxidizing, as well as reducing, gaseous species.11 The sensing mechanism of SnO2 is based on the chemisorption of analyte on the metal oxide surface, which modulates the density of charge carriers and, thus, the width of the conduction channel, thereby enabling the detection of oxidizing (NOx, O2) or reducing (CO, H2) gases as the change in resistivity of the material. Consequently, the sensing performance of the material is strongly influenced by the density of oxygen vacancies and nonstoichiometry created by doping of cations or anions in the lattice.12 Nanostructured materials offer a large number of © 2012 American Chemical Society

available sites that result in high sensitivity. However, controlling the density of oxygen vacancies in nanostructures in a reproducible manner is difficult to achieve just by varying the processing parameters.13−15 Previous reports have shown that the tuning of surface defects of SnO2 nanostructures can be achieved by post-thermal or plasma treatment (Table 1). Pal et al. have observed that the cathodoluminescence (CL) intensity of the defect bands is reduced both by prolonged hydrothermal treatment and air annealing at high temperatures, indicating a net decrease of defect content on thermal treatments.16 In a similar work, Prades et al. used a combined process of CL measurements and ab initio calculations to study the influence of surface oxygen vacancies in SnO2 nanowires (NWs) synthesized with different source materials.17 Several studies have demonstrated that the sensing response of the SnO2 nanowires was greatly enhanced by Ar/O2 plasma treatment resulting in higher surface defects.18,19 However, the influence of a corrugated or zigzag-shaped surface morphology on the Received: March 23, 2012 Revised: October 4, 2012 Published: October 14, 2012 4028

dx.doi.org/10.1021/cm300913h | Chem. Mater. 2012, 24, 4028−4035

Chemistry of Materials

Article

Table 1. Various Approaches To Tailor the Surface Defects of Metal Oxide Nanowires Material

Method of Treatment

Impact on Properties

ref

SnO2

SnO2

post-thermal annealing of SnO2 NWs in O2 atmosphere at 700 °C

SnO2

varying of process parameters, i.e., volume ratio of mixed solvent system (ethylenediamine and water) low-temperature growth (70 °C) of SnO2 nanoblades in aqueous solution measurements of photocurrent of SnO2 NWs in air and vacuum, respectively surface etching of ZnO NWs by isopropyl alcohol electronic transport characteristics of corrugated and smooth ZnO nanowires are compared H2(@400 °C) and Ar(@550 °C) treatment of as-grown NWs

shift in CL spectra indicates two different types of oxygen vacancies due to different process temperatures PL measurements show strong emissions originating from defect electronic states, formed by oxygen vacancies PL measurements indicate that the oxygen vacancies decrease with annealing control of surface defects and oxygen vacancies due to dependence of the NW morphology on solvent system PL spectra exhibits strong blue luminescence at 445 nm originating from oxygen-related defects optical gain was found to be 1 order of magnitude higher in vacuum conditions than in air, due to defect induced space charges surface treatment led to change of operation mode in ZnO NW-FETs tunability between depletion-mode and enhancement-mode behavior of ZnO NW-FETs NO2 sensing enhancement after Ar treatment due to surface defects

40

SnO2

SnO2 nanowires (NWs) were fabricated from two different source materials (SnO and SnO2) at different temperatures thermal annealing of SnO2 nanowires in vacuum and O2, respectively

SnO2 SnO2 ZnO ZnO ZnO

41 42 43 44 45 46 47 48

Figure 1. Molecular structures and in situ recorded mass spectra during the gas-phase decomposition of (a) [Sn(OtBu)4] and (b) [Sn(OtBu)2]2.

[Sn(OtBu)4] and [Sn(OtBu)2]2 and evaluate their gas-sensing properties.

gas-sensing behavior of nanowires has not been discussed in earlier reports.20 In our previous studies, we were able to show that the use of metal−organic precursors, such as the tin alkoxide, [Sn(OtBu)4], offer distinct advantages in the growth of SnO2 nanowires compared to other 1D growth approaches including lower deposition temperatures, higher morphological control, or simplified precursor delivery.21 In an attempt to precisely modify the chemical composition of SnO2 nanowires, we have employed a metastable divalent tin(II) alkoxide precursor, which decomposes under chemical vapor deposition (CVD) conditions to produce SnO2 NWs with highly irregular surface structures. We report herein the CVD growth of NWs from

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EXPERIMENTAL DETAILS

The alkoxide precursors [Sn(OtBu)4] and [Sn(OtBu)2]2 were synthesized following reported procedures.22,23 All experimental manipulations were performed in a modified Stock-type vacuum assembly, taking stringent precautions against atmospheric moisture. The synthesis of tin oxide nanowires was performed in a horizontal CVD reactor in which a high-frequency field was used to inductively heat the Si substrates by placing them on a graphite susceptor (650− 800 °C). The molecular precursors were introduced in the reactor through a glass flange by applying a dynamic vacuum (10−4 Torr) and heating the precursor reservoir to the desired temperature (25−60 °C). The precursor flux was regulated following the feedback of the 4029

dx.doi.org/10.1021/cm300913h | Chem. Mater. 2012, 24, 4028−4035

Chemistry of Materials

Article

Figure 2. Scanning electron microscopy (SEM) micrograph and X-ray diffraction (XRD) pattern of SnO2 nanowires on Si(001) obtained from (a) [Sn(OtBu)4] and (b) [Sn(OtBu)2]2. lab-class gases contained