Optical and Chemi-Resistive Sensing in Extreme ... - ACS Publications

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Optical and Chemi-Resistive Sensing in Extreme Environments: LaDoped SrTiO3 Films for Hydrogen Sensing at High Temperatures Andrew M. Schultz, Thomas D. Brown, and Paul R. Ohodnicki, Jr.* National Energy Technology Laboratory, 626 Cochrans Mill Road, Pittsburgh, Pennsylvania 15236, United States ABSTRACT: For efficient operation of next-generation fossil fuel technologies, development of sensors capable of withstanding harsh environments is required. Optical waveguide based sensing platforms have become increasingly important, but a need exists for materials that exhibit useful changes in optical properties in response to changing gas atmospheres at high temperatures. In this manuscript, the onset of a near-IR absorption associated with an increase in free carrier density in doped metal oxide films to form so-called conducting metal oxides is discussed in the context of results obtained for undoped and La-doped SrTiO3 films. Film characterization results are presented along with measured changes in optical absorption resulting from various high temperature treatments in a range of gas atmospheres. Optical property changes are also discussed in the context of a simple model for optical absorption in conducting metal oxide thin films. The combination of experimental results and theoretical modeling presented here suggests that such materials have potential for high temperature optical gas sensing applications. Simulated sensing experiments were performed at 600−800 °C, and a useful, rapid, and reproducible near-IR optical sensing response to H2 confirms that this class of materials shows great promise for optical gas sensing.

1. INTRODUCTION For efficient operation of next-generation fossil fuel energy technologies such as lower emission fossil-fueled power plants including oxy-fuel combustion processes for carbon capture and sequestration and coal gasification to produce syngas which can be converted to electrical power using solid-oxide fuel cells (SOFCs) or gas turbines, improved sensors are needed.1−3 The applications require sensors to operate in harsh environments, including high temperatures, high pressures, and corrosive, highly reducing, and highly oxidizing gas streams. Improved harsh environment sensors and controls would also enable significant gains in energy efficiency for the existing fleet of coal-fired power plants and a number of major domestic manufacturing industries.3−7 Sensor technologies capable of withstanding extreme environments can be embedded in situ at locations that allow for collecting the highest value information. Traditional chemical sensors rely on conductometric elements, employing metal oxide thin films as the sensing material. A large number of studies have been targeted toward understanding the fundamental mechanisms governing changes in electrical resistance in response to external gas atmospheres. The optical response of metal oxide materials to changing gas atmospheres has only recently begun to be understood.4,8−11 Optical gas sensors allow for monitoring of multiple different optical properties (transmission, reflection, luminescence, etc.)12−14 and compatibility with optical fiber based remote sensing approaches. Depending on the material system under investigation, a relationship may exist between the optical properties of a metal oxide thin film and the measured electrical conductance, but optical properties are also dictated by localized surface effects and electronic transitions between © 2015 American Chemical Society

bound states of the electronic structure. As a result, the existing chemi-resistive literature only provides a starting point to understanding the mechanisms responsible for the optical gas response of these materials. In support of advanced optical sensor development, systematic studies of the measurable optical and electronic properties of metal oxide systems in relevant atmospheric conditions are required.6,7,10,12−22 Previous efforts have examined optical sensing behavior of base oxides such as SnO2 films,7 doped transition metal oxides such as Al-doped ZnO,22,23 and Au nanoparticle incorporated oxides.21,24 These results show strong potential for optical hydrogen sensing, though instabilities in microstructure at high temperatures under reducing atmospheres limit the applicability for monitoring hydrogen gas streams such as those in SOFC anodes. This study measures the optical and resistive behavior of sol−gel derived undoped and La-doped SrTiO3 films at elevated temperatures under relevant atmospheric conditions. SrTiO3, a cubic perovskite oxide, is a well-studied material exhibiting high temperature stability. A-site substituted La acts as a donor impurity at high temperature under reducing conditions, increasing the carrier concentration.25−28 This increase in carrier concentration is expected to result in a near-infrared (NIR) optical response, similar to previously studied Al-doped ZnO22,23 films and Nb-doped TiO2.22 Reported changes in the NIR absorption spectrum of SrTiO3 upon exposure to reducing atmospheres provide additional Received: December 12, 2014 Revised: February 23, 2015 Published: February 25, 2015 6211

DOI: 10.1021/jp512391f J. Phys. Chem. C 2015, 119, 6211−6220

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The Journal of Physical Chemistry C

Figure 1. (a) Empirically derived optical constants of 30% La-doped STO before and after a reduction treatment at 800 °C as measured at room temperature. (b) Experimentally measured (thin solid lines) and theoretically modeled (thick dashed lines) transmittance spectra for the same films using the empirically derived optical constants showing excellent agreement. The estimated parameters for the free carrier contribution in the reduced films are summarized in the inset of part b. For the undoped films and the doped films in the as-calcined or oxidized state, a free carrier contribution did not improve the quality of the fit over the wavelength range investigated. Details of structure and composition of experimental films can be found in Figures 3−5.

evidence to support the use of SrTiO3 films as optical gas sensors.29 In this manuscript, we present theoretical and experimental results for the near-IR optical response of La-doped SrTiO3 films. First, a theoretical model is developed using experimentally determined optical constants from the literature and standard optical modeling approaches. We also illustrate the expected changes in optical spectra for such materials due to modifications in free carrier density through high temperature gas atmosphere exposure. Following the theoretical discussion, we present experimental results for three films of varying La concentration (0, 15, and 30% La-doped SrTiO3). The optical spectra were measured over a broad range of wavelengths (200−2500 nm) before and after high temperature (800 °C) treatments in various gas atmospheres. Simulated gas sensing experiments were then performed at 600−800 °C to demonstrate the NIR optical sensing response at elevated temperatures. The simulated gas sensing experiments were repeated while monitoring film resistance to correlate observed optical effects with well-understood chemi-resistive sensing effects.

can be modeled in the long-wavelength visible and nearinfrared range as indicated in eq 1 below as a function of the excitation frequency (ω), DC electrical conductivity (σω=0), relaxation time of free carriers (τ), and permittivity of free space (e0). The real (n) and imaginary (k) parts of the complex optical constant (ñ) can then be calculated from the dielectric constants as a function of wavelength according to eq 2. In our prior work, we expressed the Drude contribution in terms of the free carrier density (N) and the damping frequency of free carriers (Γ), but here we express it in terms of conductivity because experimental measurements of electrical conductivity using four-point probe techniques are presented in subsequent sections. Using the effective optical constants of the glass substrate and the oxide thin film of thickness d, the optical transmittance and reflectance of the film/substrate system can be calculated as a function of wavelength using standard optical models as described in our previous work.22,23 εTotal = εInterband + εFree ≈ ε∞ + εDrude ≈ ε∞ +

2. THEORETICAL RESULTS AND DISCUSSION In the visible and near-infrared wavelength ranges, the dielectric constant of most oxide based thin films can be modeled as a sum of contributions associated with interband electronic transitions (εInterband) and the excitation of free electrons (εFree). The contribution due to the interband electronic transitions is typically modeled as a sum of oscillators which account for allowed optical transitions that are associated with the electronic band structure. Examples of simple and common oscillator models include Lorentz, Gaussian, and Tauc−Lorentz functions, although more sophisticated models are also employed routinely to more accurately reflect detailed features of the electronic structure. In contrast, the contribution associated with the excitation of free electrons can be approximated using the Drude model (εDrude) which is essentially a simple oscillator function without a resonance at a finite frequency.22−24 In the near-infrared region and at even longer wavelengths, εInterband can be approximated as a constant value of εInterband = ε∞. According to these approximations, εTotal

σω0 ε0(τω 2 + iω)

n ̃ = (εTotal)1/2 = n + ik

(1) (2)

Similarly, the effective optical constants of a film deposited on a substrate can be extracted from empirically measured optical transmittance and reflectance data. To empirically estimate the effective optical constants of the 30% La-doped STO films synthesized here before and after a reduction treatment at 800 °C over the wavelength range of λ = 300−2500 nm, the optical transmittance and reflectance spectra were measured with light incident from the film side. The errors between simulated spectra and measured spectra were minimized by using a standard Levenberg−Marquardt algorithm with adjustable parameters including σω=0, τ, d, and a number of additional parameters that are associated with the interband electronic transitions modeled using a single Tauc−Lorentz oscillator.22,23,30,31 The empirically derived optical constants of the La-doped STO film obtained in this way, n and k, are presented in Figure 1a along with results of the experimental trans6212


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yellow mixture was allowed to stir for 5 min. A solution of 2.00 mmol of citric acid in 1.000 mL of deionized water was quickly added to the Ti-acac mixture. Any resulting precipitate quickly redissolved with stirring. After 5 min, a solution containing a total of 1.00 mmol of Sr(NO3)2 and La(NO3)3·6H2O in 1.000 mL of deionized water was added to the solution. Finally, 0.500 mL of glacial acetic acid was added to promote wetting of the substrate. The resulting solution served as a stable stock solution for all subsequent film depositions. Films were deposited on fused quartz substrates. Substrates were cleaned ultrasonically first in ethanol and then in acetone for 15 min each. They were then submerged in a solution of dilute HCl (1:3 in deionized water) for at least 1 h. Before deposition, substrates were rinsed in deionized water and dried with forced air. Films were spin coated onto substrates with 0.200 mL of precursor solution. Spin rates were 1 krpm for 5 s followed by 2.2 krpm for 30 s to achieve final layer thickness. Before additional layers, the film was cured for 45 min on a hot plate preheated to 120 °C. Subsequent layers were deposited following the same procedure. Cured films were calcined by heating to 950 °C over 10 h, holding at 950 °C for 6 h, and cooling to room temperature over 3 h. Simulated sensing tests were carried out using a custom built automated gas flow system allowing for varying compositions of N2, H2, and O2. Flow rate was constant at 100 SCCM at atmospheric pressure. Specular optical transmission measurements were carried out using an ARCoptix compact Fourier transform near-IR spectrometer. The light source was a high power halogen lamp for NIR illumination and a balanced deuterium lamp for UV−visible illumination. Conductivity measurements were acquired with optical transmission measurements using an automated van der Pauw system and Pt-wire leads.

mittance spectra measurements and the theoretically modeled spectrum showing excellent agreement in Figure 1b. As can be observed in Figure 1a, the onset of a significant free carrier contribution to the dielectric constant and the associated optical constants is apparent after the reduction treatment for the 30% La-doped STO film which is reversible after a subsequent high temperature oxidation step. For the oxidized and as-calcined films, the optical constants can be appropriately fitted and modeled even without assuming a significant contribution associated with the finite electrical conductivity. Fits were also performed for corresponding undoped and 15% La-doped STO films, resulting in similar agreement with experimentally measured spectra but with significantly suppressed contributions associated with the free carrier contribution in the as-reduced state. For the undoped STO films, the addition of a free carrier contribution was not observed to improve the fits even in the reduced state, suggesting a dramatically reduced optical signature of the change in free carrier density for the undoped films. A summary of the fitted values of DC electronic conductivity (σω=0) and free carrier relaxation time (t) is presented in the inset of Figure 1b for the various films after 800 °C reduction. Although the values of conductivity for the 15 and 30% La-doped films are significantly higher than would be expected on the basis of experimentally measured results presented in subsequent sections, the relative trends are in agreement. The trends are also consistent with expectations that are based upon optical sensing responses in subsequent sections. A significantly higher “optical DC conductivity” as compared to the measured “electronic DC conductivity” has been reported in prior works for films with nm-scale grain sizes and was claimed to be associated with the decreased relative importance of grain boundary scattering at optical frequencies.22 Similarly, a significant crack density of synthesized films too small to introduce significant light scattering could potentially result in a discrepancy between optical and electronic measurements of DC electronic conductivity due to the relatively large associated path length of free carriers for the latter not accounted for in standard van der Pauw techniques.

4. EXPERIMENTAL RESULTS 4.1. Physical Characterization of As-Calcined Films. In Figure 3, scanning electron microscope (SEM) results are presented depicting the microstructure of the as-prepared films.

3. EXPERIMENTAL DETAILS Thin films of La-SrTiO3 were prepared through a sol−gel process modified from Liu et al.32 (see Figure 2). A 1.00 mmol portion of Ti-isopropoxide and 1.00 mmol of acetylacetone were combined with magnetic stirring. The resulting pale

Figure 3. Scanning electron microscope images of the (a) SrTiO3, (b) 15% La-SrTiO3, and (c) 30% La-SrTiO3 film microstructure. Magnification is identical for each image.

Figure 2. Route to prepare stable stock solutions for thin film deposition. 6213


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Figure 4. Angularly integrated optical spectra as collected using a spectrophotometer with integrating sphere. (a) Measured transmission spectra for undoped and doped La-SrTiO3 films. (b) Measured reflection spectra for undoped and doped films. (c) Absorption spectra calculated from measured transmission and reflection spectra in parts a and b using the relation 1 = T + R + A. (d) Tauc plot showing the indirect band gap for all three films.

Films are fine grains (grain size on the order of 100 nm), flat, and continuous. Small crack networks can be observed, particularly in the case of the 15 and 30% La-doped films. The crack network is discontinuous, with some grains in direct contact across the crack and providing a continuous SrTiO3 network for resistivity measurements presented later in this manuscript. Optical transmission and reflection spectra (measured with the film side facing the source) were measured for each film immediately following film preparation. Measured spectra along with calculated absorption spectra (from 1 = T + R + A) are presented in Figure 4a−c. Absorption spectra for films are similar regardless of the level of La doping. All films show an onset of band edge absorption at roughly 350 nm and minimal absorption in the visible or near-IR wavelengths. From transmission interference fringes, film thicknesses were estimated to be 50−60 nm thick in all cases. Figure 4d shows a Tauc plot for the doped and undoped films. Plots form a linear region when plotted versus ahν1/2, reflecting an indirect band gap of 3.2 eV agreeing with previously reported values for the SrTiO3 band gap.33−35 Minimal variation is observed in the value of the band gap for the doped compared to undoped films in the as-calcined state. In order to characterize the long-term stability of films after long-term exposure to highly reducing atmospheres, two additional films of each composition were prepared. One set was exposed to alternating cycles of 100% H2 and N2 in 1 h increments over 72 h. After exposure, the exposed films and the as-prepared films were compared using X-ray diffraction (XRD). Results for long-term structural characterization are presented in Figure 5. Diffraction patterns for each film composition as-prepared and after the long-term exposure test are plotted. All films show clear (110) and (200) cubic perovskite peaks, with barely distinguishable (211) peaks also present. The lack of other peaks in the X-ray patterns is attributed to low signal as a result of the small film thicknesses rather than to any preferred film texture. No evidence for second phase development, structural evolution, or coarsening is observed after hydrogen exposure. The broad peak centered at 21.5° is attributable to the fused quartz substrate. Further structural characterization was performed on the 30% doped film after completion of all gas sensing tests described in the remainder of this document. A cross-section transmission electron microscopy (TEM) sample was prepared using focused ion beam (FIB) milling. Low and high resolution TEM micrographs are presented in Figure 5b and c, respectively. Figure 5b shows the substrate, film, and protective

Figure 5. (a) X-ray diffraction of films as-calcined and post-H2 exposure. All films before and after show peaks attributable to the pure cubic perovskite phase, with a broad substrate peak centered at 21.5°. (b) Cross section TEM micrograph of 30% La-doped film showing the substrate, film, and protective Pt coating deposited prior to focused ion beam milling. Cross sectioning and microscopy were performed subsequent to gas sensing tests detailed in the following sections of this manuscript. (c) High resolution TEM image of Ladoped SrTiO3 film, with calculated FFT (inset). From the FFT, the planes that can be resolved correspond to (001) type with the appropriate d-spacings. The brighter dots are the (002)-type plane dspacings.

Pt layer applied prior to FIB milling. Optically determined film thicknesses can be verified from this micrograph, and the observed in-plane grain diameters ranging from ∼40 to 100 nm are consistent with the SEM images presented in Figure 3. The high resolution micrograph presented in Figure 5c shows clear atomic contrast with a La-SrTiO3 grain. The planes that can be resolved correspond to (001) type with the appropriate d6214


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Figure 6. Calculated absorption spectra for the undoped (a), 15% La-doped (b), and 30% La-doped (c) SrTiO3 films as-calcined, after 800 °C hydrogen treatment, and after subsequent reoxidation in air. The doped samples show an absorption band in the near-IR wavelengths after hydrogen treatment. The band disappears after reoxidation, and no NIR absorption is resolved over the wavelength range of interest.

time is zeroed to the beginning of the gas flow cycle for each of the three examined temperatures. The plots represent the percent transmission observed at λ = 1800 nm. Transmission in the figures is normalized to the transmission at the start of the sensing test, and plots are rigidly shifted to increase figure readability. Figure 7a shows simulated sensing test results for the undoped SrTiO3 film. The transmission through the undoped sample is largely independent of gas composition at all temperatures. Small shifts in transmission, particularly for the test at 600 °C, are small, within the measurement error. Only a 100% hydrogen flow at 800 °C resulted in a resolvable 0.7% shift in transmission through the film. Parts b and c of Figure 7 show the results for the 15 and 30% doped films, respectively. In contrast to the undoped films, the La-doped films show a significant response to hydrogen at elevated temperatures. Minimal shift in the transmission is observed for either film at 600 °C, but at 700 °C, both films demonstrate a decrease in transmission as a result of hydrogen exposure. At 700 °C, the 15% doped sample begins to respond measurably to hydrogen at levels as low as 5%. At 800 °C, increasing hydrogen composition in the atmosphere is associated with a monotonically decreasing transmission through the film. The decrease in transmission at 800 °C ranged from 1.4% for a 1% H2 flow to 4.5% for 100% H2. The 30% doped sample shows a higher sensitivity to hydrogen at 700 °C, with measurable responses to hydrogen compositions as low as 2%. The magnitude to the transmission decrease increased monotonically to hydrogen at 700 °C, with the highest decrease of 3.7% observed for 100% hydrogen. Within the experimental error, the magnitude of the transmission decrease increased across all gas compositions at 800 °C and was monotonically increasing across all atmospheric conditions. The maximum decrease in transmission was 5.4%, observed at 100% H2. In all cases where a transmission response was observed, it was characterized by a rapid onset in the transmission decrease. Even at the lowest hydrogen concentrations, the response saturated on the order of minutes after the introduction of hydrogen into the gas stream. This rate of response increased with increasing temperature and gas composition, with responses to high levels of hydrogen occurring on the order of seconds. After the initial rapid response, the magnitude of the transmission decrease is relatively stable, within the experimental error. Recovery in UHP N2 was slower than

spacings. The brighter dots are the (002)-type plane d-spacings. Some of the obtained zone axis patterns could not successfully be indexed to the cubic STO structure, but in all cases, dspacings were consistent with expected values. Angular distortions in zone axis diffraction patterns could potentially result from lattice distortions as compared to the assumed simple cubic structure, particularly in the case of high levels of La incorporation into the lattice. 4.2. Optical Response to Hydrogen Atmospheres. To characterize the optical response to different gas atmospheres across different wavelengths, the films were held at 800 °C in a 100% H2 atmosphere for 30 min and rapidly cooled. Transmission and reflection spectra were acquired for each film following the same procedure as that for the as-calcined films. After spectra were acquired, films were reheated to 800 °C in static laboratory air. After cooling, additional transmission and reflection spectra were acquired. Calculated absorption spectra for the as-calcined, hydrogen reduced, and air reoxidized films are presented in Figure 6. All three absorption spectra for the undoped SrTiO3 film show negligible absorption in the depicted range (near-IR, 860−2500 nm). In contrast, the doped films both show a significant NIR absorption band. The magnitude of the absorption increases with increasing wavelength. Increased La content is associated with increased NIR absorption. At 2500 nm, the magnitude of absorption is 2 and 8%, respectively, for the 15 and 30% Ladoped films. The onset of absorption moves to shorter wavelengths with increasing La content. The experimental results presented in Figure 6 correspond well with the calculated results for highly doped SrTiO3 films presented in Figure 1b, where a decrease in transmission is observed with increasing carrier content in the films. Further studies of the optical response of La-doped SrTiO3 films was carried out by monitoring the real time spectral optical transmission at elevated temperatures under a cycle of gas flows. Each film was exposed to a sequence of 1, 2, 5, 10, 20, 50, and 100% H2 in a balance of N2 alternating with 100% N2. Each hydrogen exposure was 1 h, and each nitrogen recovery period was 1 h. The exposure sequence was performed at 600, 700, and 800 °C. Broad spectrum specular NIR transmission was monitored using a FTIR spectrometer. Figure 7 shows the compiled results for simulated sensing tests on each of the three examined films. Each film composition is plotted separately, and for each composition, 6215


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higher temperatures, transmission returned to the initial baseline value faster than for lower temperatures. In all cases, the recovery is characterized by an initial rapid increase in transmission that slows as the transmission level nears the initial state. Similar multistep recovery profiles were observed for Au-incorporated TiO2 films.20,21 Calibration curves for each film are plotted in Figure 8 using the data from Figure 7. Values for the magnitude of the sensing response are calculated using the median value for transmission during gas flow compared to the value immediately before onset of the sensing response. Representative values for these points are identified as arrows in Figure 7c for the 800 °C 100% hydrogen sensing response. The colors for the plots in Figure 8 correspond to the colors of each plot in Figure 7. This summarization of optical simulated sensing results elucidates the overall sensing response for the various samples and temperatures. As stated when describing Figure 7, the undoped sample (Figure 8a) shows no response within experimental error to hydrogen at 600 or 700 °C. Only at 800 °C and 100% H2 is any resolvable response in transmission observed. Similarly, for all three film compositions at 600 °C, no response is observed that can be definitively attributed to changing gas atmospheres. At elevated temperatures, the responses for 15% La (Figure 8b) and 30% La (Figure 8c) demonstrate a generally monotonic response to increasing hydrogen composition in the gas stream. Of particular note, the optical transmission response to hydrogen is nearly linear with respect to the log of hydrogen gas composition for both the 15% La-doped film at 800 °C and the 30% La-doped film at 700 °C. These results suggest promise for further optimization of La-doping level to achieve appropriate signal response at specified operating temperatures. Potential also exists for improved sensing response through microstructural engineering. Film deposition and processing were not optimized for these experiments, and future work to study effects of processing and microstructure on sensing behavior would be expected to result in further improvement. 4.3. Chemi-Resistive Response to Hydrogen Atmospheres. After characterization of the near-IR optical transmission sensing response to hydrogen, the chemi-resistive response was examined. The resistance of the films was measured using an automated four-point van der Pauw system. Sheet resistance of the film sample was continuously monitored under varying gas compositions at 800 °C. The sample was exposed to gas compositions of 1, 2, and 5% hydrogen in an ultrapure nitrogen balance. Pure UHP nitrogen was flowed in between each hydrogen composition. Figure 9a plots the film conductivity calculated from measured sheet resistance and film thicknesses as determined from Figure 4, following the relation σ = (RS·t)−1, where σ is the DC conductivity, RS is the experimentally measured sheet resistance, and t is the film thickness as determined from optical measurements. At 800 °C, the baseline four-point DC conductivity for all three film compositions is relatively insensitive to doping level. All three films have an estimated baseline conductivity of about 0.02 S/cm, regardless of doping level. This is attributed to an ionic, rather than electronic, compensation of La atoms as discussed further in the discussion section of this report. Each film shows a large magnitude response to the hydrogen flows. The undoped film shows an order of magnitude change in conductivity during hydrogen flow to 0.18 S/cm, with monotonically increasing conductivity at increasing levels of hydrogen. The magnitude of the

Figure 7. Results of gas sensing tests for (a) undoped SrTiO3, (b) 15% La-doped SrTiO3, and (c) 30% La-doped SrTiO3 films at 600, 700, and 800 °C. Transmission through the sample is normalized to the value at the beginning of each test, and plots are rigidly shifted to improve figure readability. Hydrogen composition in the gas stream is also plotted, and in all cases, UHP nitrogen forms the balance of the gas atmosphere. In some cases, longer term drifts in the signal are observed. One possible explanation is incomplete recovery of the film in the UHP nitrogen recovery atmosphere.

sensing onset, with recovery back to at least 95% of initial transmission within less than 1 h observed at 800 °C. At 700 °C for both the 15 and 30% doped samples, transmission through the film does not reach full recovery within the nitrogen flow period. Similar to the rate of sensing onset, the recovery appears dependent on gas composition and temperature. At 6216


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Figure 8. Compiled visible sensing results for La-doped SrTiO3 films. The magnitude of the sensing response is plotted versus the log of gas composition. The color of each plot corresponds to the plots in Figure 7. For all three films, increasing the temperature increased the magnitude of the sensing response. Increasing the La composition resulted in increased sensing responses at lower temperatures. At 600 °C, none of the films showed a measurable response to hydrogen. At 800 °C, all three films showed some response, with the 30% doped sample having the greatest response.

5. DISCUSSION AND NEED FOR FUTURE WORK The optical response for previously studied metal oxide film materials such as Al-ZnO or Nb-TiO2 in the near-infrared range was attributed to an increase in the free carrier concentration upon exposure to reducing gas streams.22,23 The near-IR sensing response for the films in these experiments is expected to follow a similar mechanism. Many studies have focused on explaining the defect structure of SrTiO3 and La-doped SrTiO3 depending on processing conditions, temperature, doping composition, and gas atmosphere.25,38−44 These studies generally focus on the relationship between defects introduced through intentional doping, intrinsic defects, and oxygen partial pressure in the ambient atmosphere. In these cases, the partial pressure of hydrogen is generally low, as even very small amounts of hydrogen can significantly reduce the partial pressure of oxygen in the gas stream. Other research has focused on the presence of hydrogen defects in oxide lattices;34,45−52 however, these reports generally do not consider the relation between hydrogen defects and the more traditional defects previously mentioned. In relevant applications, these sensors experience high hydrogen partial pressures and low oxygen partial pressures. Thus, both categories of defects may be expected to potentially contribute to the overall defect structure of the films. First, we consider the general equilibrium oxygen vacancies and oxygen partial pressure in the ambient atmosphere in metal oxides, given by eq 3.39,40

conductivity change increases with increasing doping levels to 0.40 S/cm. For the 30% La doped film, the magnitude of the resistivity change reaches 2 orders of magnitude to 2.2 S/cm at 5% H2. The values for the 15% doped sample fall in between the undoped and 30% doped films. The results from Figure 9a are collected into calibration curves in Figure 9b. These curves show that each of the three films had a monotonically increasing response to the hydrogen compositions measured in these tests. The highly doped sample had the largest overall magnitude of conductivity change for each of the samples, as well as the highest difference in response for the varying hydrogen levels. Compared to the optical results in Figure 8, the magnitude of the resistive sensing response is much larger than the optical response by multiple orders of magnitude. The details of the interrelationship between the electrical resistance response and the optical response require further investigation and will be the subject of future efforts. The experimental values for sample conductivity differ significantly as compared to the best fit conductivity values derived from theoretical fitting of optical properties in Figure 1. Discrepancies between experimental and theoretical conductivity values can be potentially explained by various differences between assumptions for an ideal film in the model versus actual film structure and experimental measurement techniques. As the grain size in the film decreases, the deviation of the effective conductivity at optical frequencies as compared to dc conductivity increases as a result of electron scattering at the grain boundary. Carrier concentration and mobility as estimated by the model at optical frequencies only considers intragranular scattering due to the short mean path length of carriers and so ignores increased resistivity as a result of higher concentrations of grain boundary defects.36 Additionally, the observed microstructure of the sol−gel films includes significant intergranular porosity and finescale cracking, increasing the path length for DC-resistivity measurements. Finally, the van der Pauw method assumes a continuous, homogeneous film without any holes or thickness deviations.37 These ideal conditions differ from the films used in this experiment, contributing to the decreased conductivity compared to the theoretical estimations.

red

× OO HooI ox

1 O2(g) + V •• O + 2e′ 2

(3)

Increasing hydrogen levels in the gas stream are associated with decreased partial pressure of oxygen and, according to eq 1, increased carrier concentration in the film. This is expected to be the primary contributor to the resistive and optical response for the undoped SrTiO3 film, where intentional extrinsic doping is not a factor. The impurity concentration of oxygen in ultra high purity nitrogen is on the order of 10−6 atm, which represents the highest O2 concentration seen during testing. Second, we take into account the compensation of donor defects through ionic and electronic compensation. Previous reports suggest that the compensation mechanism of donor 6217


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increase the concentration of free carriers in the material. This explains the similar conductivity measurements for the baseline tests in Figure 9. Under reducing conditions, this equilibrium is driven toward the right, releasing oxygen into the ambient atmosphere and generating mobile charge carriers in the material. As the level of hydrogen in the gas stream is increased, the portion of donor atoms resulting in free charge carriers increases. The increase in free charge carriers results in increased conductivity and increased free carrier optical absorption in the NIR range. Lastly, we consider uptake of hydrogen into the SrTiO3 lattice. Experimental and theoretical reports have found evidence for formation of hydroxide ions on oxygen sites. This is represented by eq 5. × OO +

6. CONCLUSIONS The results presented in this report demonstrate the suitability of La-doped SrTiO3 films toward application in optical sensing devices. With increasing La content, the optical sensing response for sol−gel SrTiO3 films increased. Sensing response increased with increasing temperature for all compositions. At elevated temperatures, the sensing response was rapid and monotonic with respect to hydrogen composition in test gas streams. Recovery was complete with acceptable recovery times. Results suggest that further examination of La content allows for development of sensors optimized for desired operating temperature. Films are stable even after long-term hydrogen exposure, with no resolvable difference in the X-ray diffraction patterns. The results presented here suggest that incorporation of La-doped SrTiO3 films into optical sensing devices shows strong promise for the development of hydrogen sensors capable of withstanding the harsh environments in next-generation energy generation technologies.

dopants in SrTiO 3 depends on external gas atmospheres.25,27,53,54 In reducing conditions, La acts as a standard donor impurity, occupying a Sr site in the lattice and donating a free electron. Under oxidizing conditions, La donors have been reported to be compensated via Sr vacancies and the formation of SrO enriched phases, either in the form of Ruddlesden− Popper phases or SrO second phase.55 The equilibrium between these two compensation mechanisms is represented by eq 4. red 1 1 1 × 1 (SrO) + V″Sr HooI La•Sr + Sr Sr + e′ + O2 2 2 2 4 ox

Ionic compensation

(5)

Calculations of hydrogen defects in transition metal oxides have suggested that hydrogen incorporated into the SrTiO3 lattice forms shallow donor states.47 As hydrogen is absorbed into the lattice, it releases its electrons and incorporates with oxygen atoms neighboring strontium vacancy sites. Additionally, hydroxide ions are believed to be energetically stabilized within strontium vacancies. 45 The concentration of strontium vacancies is directly increased with increasing La content in the film following eq 4. Higher doping levels increase the concentration of strontium vacancies, and thus the number of available sites for incorporation of hydrogen into the lattice. Higher concentrations of hydrogen in the gas atmosphere would correspond to an increased potential for the incorporation of hydrogen into the lattice. The combined effects of oxygen vacancies generated because of the extremely low oxygen partial pressures during hydrogen gas flow and direct hydrogen incorporation into the lattice are believed to contribute to the observed increase free carrier absorption in the film. Future work is needed to definitively characterize defect structure under various hydrogen containing atmospheres and identify the source of the NIR free carrier optical absorption.

Figure 9. Conductivity response at 800 °C for H2 atmospheres of 1, 2, and 5%. (a) Full conductivity measurements for each of the three films. All three films show a strong change in conductivity after the onset of hydrogen flow. The magnitude of the change increases with increasing hydrogen composition in the gas stream and with increasing doping composition. (b) Calibration curves calculated from the data in part a. The magnitude of the conductivity change was calculated by comparing values prior to and during hydrogen flow (shown by arrows in Figure part a for the 1% test). All three samples show a monotonic response to increasing hydrogen levels, with the 30% showing the greatest difference in response for differing hydrogen compositions.

La•Sr +

1 H 2(g) ⇌ (OH)•O + e′ 2

Electronic compensation

(4)

For the case of the films in this experiment, the high oxygen partial pressure during initial annealing likely results in purely ionically compensated films. This ionic donor compensation mechanism results in low conductivity samples, regardless of doping composition. In the ionic regime, donors do not

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The Journal of Physical Chemistry C Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the U.S. DOE Advanced Research/ Crosscutting Technologies program at the National Energy Technology Laboratory. This research was supported in part by an appointment to the National Energy Technology Laboratory Research Participation Program, sponsored by the U.S. Department of Energy and administered by the Oak Ridge Institute for Science and Education. This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

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