Elsevier

Sensors and Actuators B: Chemical

Volume 208, 1 March 2015, Pages 379-388
Sensors and Actuators B: Chemical

Ar/O2 and H2O plasma surface modification of SnO2 nanomaterials to increase surface oxidation

https://doi.org/10.1016/j.snb.2014.11.049Get rights and content

Abstract

Tin oxide operates widely as a gas sensor for a variety of molecules via a mechanism that relies on interactions with adsorbed oxygen. To enhance these interactions by increasing surface oxygen vacancies, commercial SnO2 nanoparticles and CVD-grown SnO2 nanowires were plasma modified by Ar/O2 and H2O plasmas. Scanning electron microscopy revealed changes in nanomaterial morphology between pre- and post-plasma treatment of H2O treated materials but not Ar/O2 treated materials. Powder X-ray diffraction patterns of the bulk SnO2 showed the Sn4+ is reduced by H2O and not Ar/O2 plasma treatments. X-ray photoelectron spectroscopy indicated Ar/O2 treatment results in increasing oxygen adsorption with increasing plasma power and treatment time, without changing Sn oxidation. With the lowest plasma powers and treatment times, however, H2O plasma treatment results in nearly complete bulk Sn reduction. Although both plasma systems increased oxygen adsorption over the untreated materials, there were clear differences in the tin and oxygen as well as morphological variations upon plasma treatment.

Introduction

As a result of the increasing amounts of energy produced and consumed worldwide, excessive production of toxic gas species continues to cause detriment to human health and well-being [1], [2], [3]. Gases such as NOx, benzene, and formaldehyde are just a few examples that can result in immediate and long-term health problems [4], [5]. To monitor such toxic gases, a large body of research has focused on developing highly sensitive and selective gas sensors [1], [2], [3]. Nevertheless, there remains room for further improvement as current sensors have significant limitations. For example, most lack the sensitivity to detect analytes at concentrations below relevant toxicity levels, and they often require high temperatures to achieve high sensitivity with rapid response [1], [2], [3]. High operating temperatures limit the lifetime of many sensors anywhere from a few months to a couple of years because of the wear on the materials due to power consumption required for temperature maintenance [1], [2], [3]. Additionally, the accuracy of many of these sensors is affected by the presence of poisoning and interfering species, which greatly decreases the selectivity.

An extremely active area of sensor research lies with metal oxide materials, which have many benefits, such as their capabilities as semiconductors in electronics [2], [6]. Many semiconductor metal oxide-based sensors are designed from thin films because their conductivity and surface-gas interactions have created high performing gas sensors. Metal oxide sensors monitor gases by changes in resistance resulting from interactions between gases and oxygen adsorbed to the material surfaces [2], [6]. In particular, tin (IV) oxide (SnO2) has been used extensively because of tin's dual valency (i.e. Sn4+ and Sn2+). This property yields more diverse surface chemistry than monovalent metal oxides which allows for greater material tunability, often resulting in SnO2 being a more sensitive and selective gas sensor than other metal oxides [4], [7]. Markedly, it is necessary to monitor the tin oxidation state, as sensitivity and selectivity of particular gases can drop drastically with increased tin reduction. Additionally, unmodified SnO2 in its simplest thin film variation has several limitations, most notably its low surface area available for interaction with gases. Maximizing surface area is thus critical to improving SnO2 sensor sensitivity.

Recent work on SnO2 materials with greater surface area has explored doped and composite SnO2 thin films, nanoplates, nanoparticles, nanowires, and porous nanofoams [8], [9], [10]. Although these materials increase sensitivity over thin films, detection below toxicity levels has yet to be achieved for many sensors, especially at room temperature [9], [10], [11]. Aside from maximizing surface area, an additional method to increase surface–gas interactions is surface modification, which can alter surface chemistry as well as morphology. As a method with an expansive parameter space, plasma surface modification allows for significant control over the modification process [10], [11], [12], and in many instances, can tune surface properties with a high degree of accuracy.

Within the last 10 years, interest in plasma treatment of sensor materials has increased significantly and in particular, Ar/O2, H2, O2, and N2 plasmas have been used to modify SnO2 materials [10], [13], [14]. Interestingly, much work has centered on Ar/O2 plasma surface modification of SnO2 nanomaterials for gas sensing [10], [11], [15], [16]. Pan et al. [10] treated SnO2 nanowires with Ar/O2 plasmas (1:1, applied rf power (P) = 10–80 W for 4 min), creating a sensor that showed increased sensitivity to ethanol gas. Tin reduction was observed on the nanowire surfaces at the maximum power treatment. Mathur et al. [11] also modified SnO2 nanoplates with Ar/O2 plasmas (3:1, P = 25–125 W for 3 min). Treated SnO2 nanoplates showed similar increases in ethanol sensitivity relative to nanowires at low applied powers. For higher power treatments, however, changes in nanoplate morphology from nanoplates to nanoglobular particles were observed as a result of tin reduction throughout the nanoplates.

Although there are limited studies on alternative plasma modification systems, such as H2, O2, and N2 plasmas, these systems exhibit a similar etching effect as Ar/O2 has on SnO2 [13], [17]. Importantly, the limited plasma parameters and materials explored leave room for further investigation and optimization of plasma surface modification strategies. For example, an alternative, nominally etching plasma containing oxygen that has not, to our knowledge, been used for treatment of SnO2 nanomaterials is an H2O vapor plasma. In a related study, however, Tarlov et al. [18], [19] examined the effect of using extremely low power H2O plasmas (P = 1–5 W, 10–15 min) to remove surface carbon from SnO2 films. Interestingly, they found that the H2O plasma generally oxidized the surface of annealed SnO2 films, converting Sn2+ to Sn4+ at the surface. In the same work, the authors demonstrated that annealing H2O plasma treated SnO2 films resulted in formation of oxygen vacancies and that Ar+ bombardment of the SnO2 film served to reduce the Sn at the surface. Tarlov et al. did not explore higher power plasmas as their major goal with the H2O plasma treatment was to remove adventitious carbon while still preserving the hydroxylated surface of the SnO2 films.

With our study, we sought to explore whether water plasma treatments would hydroxylate nanostructured SnO2 materials similarly to the work of Tarlov et al. with thin films and if such treatments would provide an improved platform for ultimately creating more sensitive SnO2 gas sensors. The H2O plasma system is also a good comparison system for the Ar/O2 plasma as it should add oxygen to the SnO2 surface, but may have milder etching/oxidizing behavior [18], [19], [20]. Thus in this work, both SnO2 nanoparticles and SnO2 nanowires were plasma treated with Ar/O2 and H2O plasma systems to compare effects of the plasmas on surface composition, morphology, and bulk crystallinity of SnO2 materials with different morphologies and surface areas.

Section snippets

Chemical vapor deposition (CVD)

All SnO2 materials were supported on zirconia (ZrO2) wafers (50 nm ZrO2 on n-type 100 Si wafer, BioStar). Note that ZrO2 was used in place of SiO2 or Al2O3 as it can also withstand the elevated temperatures often necessary for sensing. Additionally, with a higher electrical resistivity and dielectric constant, ZrO2 may allow for observing smaller changes in resistance and thus contribute to developing a more sensitive gas sensor. ZrO2 wafers were sputter coated with 5 nm Au (Denton Vacuum Desk II

Results

SEM analysis provides information on relative particle sizes for the SnO2 nanoparticles, Fig. 2a, and nanowires, Fig. 2b and c, on ZrO2 substrates. For the nanoparticles, the sizes range from 10 to 50 nm with many on the smaller end of that range, whereas SnO2 nanowires have diameters ranging 10–50 nm with most diameters around 20 nm and lengths of several microns. Throughout the CVD process, color changes were observed on the substrate surface. Fig. 2d shows the substrate color change from gold

Discussion

Scant literature exists on plasma surface modification of metal oxide materials for gas sensing, specifically SnO2, providing limited information on effects of such processes on materials properties and composition. Thus, this study was designed to focus on more comprehensive material characterization and to explore the effects of different plasma systems on the chemical and morphological properties of SnO2 nanomaterials. This is especially important because the bulk and surface properties of a

Summary

Through an increase in surface oxygen vacancies created via plasma treatments, our SnO2 nanowires and nanoparticles exhibit increased oxygen adsorption, which can increase gas-surface interactions and ultimately affect sensor sensitivity. Thus, plasma modification is an effective method of surface modification for these materials. Notably, the 145 mTorr Ar/O2 plasma (P = 150 W, 30 min) used here effectively etched lattice oxygen from SnO2 nanowires and nanoparticles. This resulted in increased

Acknowledgments

Funding for this work was provided by the National Science Foundation (CHE-1152963). We also thank Mr. Christopher Miller for assistance with nanowire synthesis and plasma treatments.

Erin P. Stuckert is a doctoral student at Colorado State University. She received the B.S. degree from University of Wisconsin Eau Claire in 2011 in Chemistry and Mathematics. Her research interests include nanomaterials synthesis and modification for improved gas sensors. In addition she is using analytical characterization techniques to develop new sensor materials and architectures.

References (47)

  • M.J. Tarlov et al.

    Static SIMS and XPS study of water plasma exposed tin oxide films

    Appl. Surf. Sci.

    (1993)
  • F. Hernandez-Ramirez et al.

    High response and stability in CO and humidity measures using a single SnO2 nanowire

    Sens. Actuators B

    (2007)
  • M. Kwoka et al.

    XPS study of the surface chemistry of L-CVD SnO2 thin films after oxidation

    Thin Solid Films

    (2005)
  • J. Szuber et al.

    XPS study of the L-CVD deposited SnO2 thin films exposed to oxygen and hydrogen

    Thin Solid Films

    (2001)
  • N. Ramgir et al.

    Metal oxide nanowires for chemiresistive gas sensors: issues, challenges, and prospects

    Colloids Surf. A

    (2013)
  • H. Huang et al.

    Plasma treatment of SnO2 nanocolumn arrays deposited by liquid injection plasma-enhanced chemical vapor deposition for gas sensors

    Sens. Actuators B

    (2009)
  • W. Liu et al.

    Influence of O2/Ar flow ratio on the structure and optical properties of sputtered hafnium dioxide thin films

    Surf. Coatings Technol.

    (2010)
  • N.M.D. Brown et al.

    The etching of natural alpha-recoil tracks in mica with an argon RF-plasma discharge and their imaging via atomic force microscopy

    Appl. Surf. Sci.

    (1996)
  • A.K. Batra et al.

    Micro-and nano-structured metal oxides based chemical sensors: an overview

    J. Nanosci. Nanotechnol.

    (2014)
  • G. Eranna et al.

    Oxide materials for development of integrated gas sensors – a comprehensive review

    Crit. Rev. Solid State Mater. Sci.

    (2004)
  • G.F. Fine et al.

    Metal oxide semi-conductor gas sensors in environmental monitoring

    Sensors

    (2010)
  • J. Pan et al.

    Plasma-modified SnO2 nanowires for enhanced gas sensing

    J. Phys. Chem. C

    (2010)
  • S. Mathur et al.

    Plasma-assisted modulation of morphology and composition in tin oxide nanostructures for sensing applications

    Adv. Eng. Mater.

    (2007)
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    Erin P. Stuckert is a doctoral student at Colorado State University. She received the B.S. degree from University of Wisconsin Eau Claire in 2011 in Chemistry and Mathematics. Her research interests include nanomaterials synthesis and modification for improved gas sensors. In addition she is using analytical characterization techniques to develop new sensor materials and architectures.

    Ellen R. Fisher is a Professor of Analytical, Physical, and Materials Chemistry at Colorado State University. She received a B.S. degree in Chemistry and Mathematics from Texas Lutheran University (formerly College) in 1986 and the Ph.D. in physical-analytical chemistry from the University of Utah in 1991. Her research interests span diverse topics in topics in the molecular-level understanding of the chemistry of vapor deposition processes, plasma science, radical-surface interactions, laser spectroscopy, materials for sensors, optical diagnostics, nanostructured materials, chemistry education and the responsible conduct of research.

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