Anal. Chem. 2009, 81, 7844–7848
Perovskite-Type Oxide Thin Film Integrated Fiber Optic Sensor for High-Temperature Hydrogen Measurement Xiling Tang,† Kurtis Remmel,† Xinwei Lan,‡ Jiangdong Deng,§ Hai Xiao,‡ and Junhang Dong*,† Department of Chemical and Materials Engineering, University of Cincinnati, Cincinnati, Ohio 45221, Department of Electrical and Computer Engineering, Missouri University of Science and Technology, Rolla, Michigan 65409, and Center for Nanoscale Science, Harvard University, Cambridge, Massachusetts 01238 Small size fiber optic devices integrated with chemically sensitive photonic materials are emerging as a new class of high-performance optical chemical sensor that have the potential to meet many analytical challenges in future clean energy systems and environmental management. Here, we report the integration of a proton conducting perovskite oxide thin film with a long-period fiber grating (LPFG) device for high-temperature in situ measurement of bulk hydrogen in fossil- and biomass-derived syngas. The perovskite-type Sr(Ce0.8Zr0.1)Y0.1O2.95 (SCZY) nanocrystalline thin film is coated on the 125 µm diameter LPFG by a facile polymeric precursor route. This fiber optic sensor (FOS) operates by monitoring the LPFG resonant wavelength (λR), which is a function of the refractive index of the perovskite oxide overcoat. At high temperature, the types and population of the ionic and electronic defects in the SCZY structure depend on the surrounding hydrogen partial pressure. Thus, varying the H2 concentration changes the SCZY film refractive index and light absorbing characteristics that in turn shifts the λR of the LPFG. The SCZY-coated LPFG sensor has been demonstrated for bulk hydrogen measurement at 500 °C for its sensitivity, stability/ reversibility, and H2-selectivity over other relevant small gases including CO, CH4, CO2, H2O, and H2S, etc. The production of hydrogen and syngas (mainly H2 + CO) from biomass and coal gasification and shift reactions is anticipated to play a major role in future power generation by advanced turbine and fuel cell technologies. The ability of in situ measurement of the gas composition is needed for intelligent process control to improve the energy efficiency, system reliability, and emission reduction. However, current electrochemical hydrogen sensors are incapable of direct deployment and safe operation in the high temperature and corrosive environments involved in the hydrogen/syngas production and utilizations.1 New sensors are thus under strong * Corresponding author. E-mail:
[email protected]. † University of Cincinnati. ‡ Missouri University of Science and Technology. § Harvard University. (1) Korotcenkov, G.; Han, S. D.; Stetter, J. R. Chem. Rev. 2009, 109, 1402.
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demand to bridge the technical gaps of in situ gas measurement in the related harsh conditions.2 The fiber optic sensors (FOS) have the general advantages of chemical and thermal stabilities, small size, passive sensing mechanisms, remote operability, ease to construct multiplexed or distributed sensors, and insusceptibility to electromagnetic fields. The long-period fiber grating (LPFG) is an inline fiber device with its core refractive index changing periodically to promote coupling between the core mode and the copropagating cladding modes.3-5 The LPFG transmission spectrum is characterized by a series of attenuation bands displaying resonant peaks at specific wavelengths (λR) given by4,6 λR ) (neff - nclad)Λ
(1)
where λR is the resonant wavelength, neff is the effective index of the propagating cladding mode, nclad is the index of the cladding mode, and Λ is the grating period ranging from 100 to 1000 µm. The effective refractive index of the cladding mode is extremely sensitive to the index of the medium surrounding the LPFG.6-10 Thus, LPFG-based refractive index chemical sensors can be established by coating thin films of chemically sensitive photonic materials on the surface as illustrated in Figure 1.11-13 Here we demonstrate a new LPFG optic device coated with a proton conducting perovskite oxide thin film for in situ measurement of bulk hydrogen at high temperature. The perovskite-type proton-electron/hole mixed conducting oxides have the general chemical formula of AB1-xMxO3-δ, where A and B are metal ions (2) Chorpening, B. T.; Tucker, D.; Maley, S. M. IEEE Sensors, Vienna, Austria, October 24-27, 2004. (3) Vengasarkar, A. M.; Lemaire, P. J.; Judkins, J. B.; Bhatia, V.; Erdogan, T.; Sipe, J. E. J. Lightwave Technol. 1996, 14, 58. (4) Erdogan, T. J. Lightwave Technol. 1997, 15, 12774. (5) Li, Y.; Wei, T.; Montoya, J. A.; Saini, S. V.; Lan, X.; Tang, X.; Dong, J.; Xiao, H. Appl. Opt. 2008, 47, 5296. (6) Rees, N. D.; James, S. W.; Tatam, R. P. Opt. Lett. 2002, 27, 686. (7) Rao, Y. J. Opt. Laser Eng. 1999, 31, 297. (8) Bhatia, V. Opt. Express 1999, 4, 457. (9) Hou, R.; Hassemlooy, Z.; Hassan, A.; Lu, C.; Dowker, K. P. Meas. Sci. Technol. 2001, 12, 1709. (10) Shu, X.; Zhang, L. J. Lightwave Technol. 2002, 20, 255. (11) Cusano, A.; Pilla, P.; Contessa, L.; Iadicicco, A.; Campopiano, S.; Cutolo, A.; Giordano, M.; Guerra, G. Appl. Phys. Lett. 2005, 87, 234105. (12) Gu, Z.; Xu, Y.; Gao, K. Opt. Lett. 2006, 31, 2405. (13) Zhang, J.; Tang, X.; Dong, J.; Wei, T.; Xiao, H. Opt. Express 2008, 16, 8317. 10.1021/ac9012754 CCC: $40.75 2009 American Chemical Society Published on Web 08/14/2009
Figure 1. Schematic illustration of the film-coated LPFG and the transformation of the light transmission spectrum.
commonly from the group of Ca, Sr, and Ba and the group of Ce, Tb, Zr, and Th, respectively.14 M is a dopant with a variety of choices for elements and doping level (normally x ) 0-0.2) that significantly influences the oxygen deficiency (δ), conductivity, and material chemical stability.14,15 At high temperatures, exposing these perovskite oxides to oxygen- and hydrogencontaining gases causes the following reactions in the solid phase:16-18 k1 1 •• X in oxygen: VO + O2 798 OO + 2h• 2
k2
X in hydrogen: H2 + 2h• + OO 798 2OH•
(2)
(3)
where the hydroxyl ion OH• is formed by associating a proton with a lattice oxygen ion (OXO) and k1 and k2 are the equilibrium constants. These reactions change the types and populations of the ionic and electronic defects, band gap energy, lattice parameters, and density of the oxide. Consequently the electric conductivity as well as the optical refractive index and light absorbance vary as a function of the surrounding oxygen or hydrogen partial pressure. The proton conducting cerate- and zirconate-based perovskite oxides commonly have their B-site Ce4+ or Zr4+ partially substituted by various aliovalent ion dopants, e.g., Yb3+, Sc3+, Y3+, In3+, Tb3+, etc.14 In general, the doped-cerates have higher conductivity than the doped-zirconates. However, the zirconatebased materials (e.g., SrZrO3 or CaZrO3) possess better chemical stability than the cerates (e.g., SrCeO3 and BaCeO3) in high temperature CO2-containing atmospheres. The substitution of Ce4+ with Zr4+ in the perovskite-type doped-cerates increases the CO2-resistance but compromises on the proton conductivity.19,20 In this study, the perovskite-type Sr(Ce0.8Zr0.1)Y0.1O2.95 (SCZY) was selected as the sensing material under the consideration for both H2-sensitivity and stability in CO2. EXPERIMENTAL SECTION Preparation of LPFG and Polymeric Precursor. The LPFG used in this research was fabricated using point-by-point CO2Liu, Y.; Tan, X.; Li, K. Catal. Rev. 2006, 48, 145. Phair, J. W.; Badwal, S. P. S. Ionics 2006, 12, 103. Uchida, H.; Maeda, N.; Iwahara, H. Solid State Ionics 1983, 11, 117. Hibino, T.; Mizutani, K.; Yajima, T.; Iwahara, H. Solid State Ionics 1992, 58, 85. (18) Guan, J.; Dorris, S. E.; Balachandran, U.; Liu, M. J. Electrochem. Soc. 1998, 145, 1780. (19) Kreuer, K. D. Solid State Ionics 1999, 125, 285. (20) Katahira, K.; Kohchi, Y.; Shimura, T.; Iwahara, H. Solid State Ionics 2000, 138, 91. (14) (15) (16) (17)
laser irradiation in singlemode silica fibers (Corning SMF-28), which have a 9 µm diameter Ge-doped core and a 125 µm diameter fused silica cladding.5 The grating segment has a total length of ∼50 mm and a Λ of 520 µm. This LPFG exhibited long-term stability at 550 °C with only a small thermal drifting of -1.9 nm over 200 h and could survive at 800 °C for 1 h to a few hours before the resonant peak diminished. The thermal instability at high temperature is caused by annealing of the laser-inscribed gratings in the core that leads to the disappearance of the periodical variation of the refractive index. The SCZY nanocrystalline thin films were synthesized on the fiber surfaces by a polymeric precursor route which includes precursor film coating and thermal treatment steps at temperatures tolerable to the LPFG. The precursor is a polyester-metal chelate prepared by acid-catalyzed polymerization21 of a metal ioncontaining ethylene glycol aqueous solution via the following procedure. An aqueous solution was obtained by dissolving 0.02 mol of strontium nitrate (Sigma-Aldrich, 99+%), 0.002 mol of yttrium nitrate hexahydrate (Aldrich, 99.9%), 0.016 mol of cerium nitrate hexahydrate (Aldrich, 99%), 0.002 mol of zirconyl chloride octahydrate (Sigma-Aldrich, 98%) in 20 mL of DI water. This metal ion solution was mixed with 40 mL of ethylene glycol (SigmaAldrich, 99%) and 0.02 mol of glycine (Sigma, 99%) to form the starting solution for polymerization. The polymerization process was conducted at 80 °C for 48-72 h in a 125 m flask with its mouth open for removal of water. The flask was then sealed and cooled naturally to room temperature. The resultant polymeric solution was aged for 3-6 days before use and is good for film coating within 5 days thereafter. Fabrication of SCZY Thin Film. The polymeric precursor film was brush-coated on the horizontally suspended fiber. This polymeric film was dried at ∼100 °C and then converted to a SCZY oxide film by firing at 550 °C for 3 h. The SCZY film thickness can be controlled by repeating the coating-firing process. The LPFG coated with a SCZY film of desired thickness was further treated at 700 °C for 0.5 h in air. The purpose of the brief treatment at 700 °C is to inhibit further grain growth in the nanocrystalline film during long-term operation at a relatively lower temperature (i.e., 500 °C).22,23 Gas Sensing Experiments. The high-temperature gas sensing tests were performed using an apparatus similar to that described in a previous publication.13 The SCZY-coated LPFG (21) Anderson, H. U.; Nasrallah, M. M.; Chen, C. C. Method of Coating a Substrate with a Metal Oxide Film from an Aqueous Solution Comprising a Metal Cation and a Polymerizable Organic Solvent. U.S. Patent 5,494,700, February 27, 1996. (22) Kulkarni, A.; Bourandas, A.; Dong, J.; Fuierer, P. A.; Xiao, H. J. Mater. Res. 2006, 21, 500. (23) Dong, J.; Hu, M. Z.; Payzant, E. A.; Armstrong, T. R.; Becher, P. F. J. Nanosci. Nanotechnol. 2002, 21, 61.
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segment was hosted in a 1/4 in. i.d. stainless steel tube, which is placed horizontally in a tubular furnace (±1.0 °C). The two ends of the tube were connected to the sample gas supply unit and ventilation, respectively. The LPFG transmission spectrum was obtained in a near IR wavelength range from 1510 to 1640 nm by sweeping the wavelength of a tunable laser (Agilent 81640A) and detecting the light using an optical power detector (Agilent 8164A) coordinated by a computer data acquisition system. In all measurements, a low flow rate of the sample gas (10 cm3 (STP)/min) was used to ensure that the gas reached the set temperature when contacting the sensing element. The sensor was examined at 500 °C under atmospheric pressure for its responses to H2, CO, CO2, H2O, CH4, and H2S, which are the key components in the biomass and fossil-derived syngas gases.24 For safety assurance, a household CO detector was installed near the sensor testing apparatus and a H2S absorber was used in the vent line. RESULTS AND DISCUSSION SCZY Thin Film Characterization. The refractive index of the SCZY nanocrystalline thin film was measured by an ellipsometer in the same wavelength range 1510-1640 nm. The film used for ellipsometry test was synthesized on silicon wafer by spincoating of the polymeric precursor and subsequent thermal treatments under conditions identical to the SCZY-LPFG sensor fabrication. Figure 2a shows the surface AFM and the crosssection SEM images of a Si(001)-supported SCZY film obtained by five times of coating. These images show that the film has a smooth surface and a uniform thickness (∼150 nm) necessary for the ellipsometry test. The refractive index of the SCZY nanocrystalline film is presented in Figure 2b, which varies from 1.682 to 1.675 as the wavelength increases from 1510 to 1640 nm. The stability of the SCZY nanocrystalline material in CO2 and H2 was evaluated by X-ray diffraction (XRD) examination using particulate samples prepared in parallel to the film synthesis. The SCZY particle sample was treated in H2, CO2, and air flow at 500 °C for multiple cycles. During the thermal treatment in each gas, the sample was kept at 500 °C for 3 h and then cooled down to room temperature with the testing gas kept flowing. The resultant XRD patterns showed no sign of changes in the perovskite phase after being treated with each gas. Sensor Structure and Thermal Stability. The SCZY film used in this study was obtained with coating five times and a final 0.5 h firing step at 700 °C. Figure 3 shows the microscopic images of the cross section of a SCZY-coated LPFG fiber. The SCZY film thickness was ∼ 500 nm. This overcoat thickness is appropriate for achieving high sensitivity for the specific nov (∼1.68) of the SCZY film according to the theoretical modeling results in the literature.12 To ensure the preservation of the optical function of the SCZYLPFG device, the transmission spectrum was measured in different stages of the film synthesis. Figure 4 presents the typical transmission spectra of the bare LPFG and the five-time coated SCZY-LPFG after firing at 550 and 700 °C, respectively. The final SCZY-LPFG exhibits a strong resonant peak which is necessary for quantitative measurement of ∆λR. The stability of the SCZY(24) NETL Test Protocol. Testing of Hydrogen Separation Membranes; U.S. Department of Energy, DOE/NETL-2008/1335, October 2008.
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Figure 2. The SCZY thin film on silicon wafer: (a) AFM image of the surface (insert, cross-section SEM image) and (b) refractive index (n) and extinction coefficient (k).
LPFG at 500 °C in terms of λR thermal drift was ∼-6.5 × 10-3 nm/h as measured over a period of 240 h. The experimental values of λR presented later in this technical note were compensated with this thermal drifting rate. The TEM examination of the unsupported SCZY samples (inserts in Figure 4) showed that the SCZY grain size increased from 5 to 10 nm after firing at 550 °C to 10-15 nm after firing at 700 °C. The grain size of the SCZY fired at 700 °C showed no appreciable change after further calcination at 500 °C for more than 100 h. The unsupported sample fired at 700 °C was also ground and used in the XRD tests mentioned above. Sensor Selectivity Evaluation by Single Gases. The SCZYLPFG sensor was tested for its responses to H2, CO, CH4, CO2, H2O, and H2S in binary mixtures containing a target gas and inert N2. The results are shown in Figure 5. All tests were performed at 500 °C and atmospheric pressure. In the oxygen-free atmosphere of H2/N2, H2 reacts with the lattice oxygen ions by25 (25) Kosacki, I.; Anderson, H. U. Sens. Actuators, B 1998, 48, 263.
k3
X H2 + 2OO 798 2OH• + 2e-
(4)
where the concentrations of electronic (n) and ionic [OH•] defects are related to the hydrogen partial pressure (PH2) by the equilibrium constant (k3): X ] [OH•]n ) √k3PH21/2[OO
(5)
Figure 5. SCZY-LPFG sensor responses to H2, CO, CO2, CH4, H2O, and H2S single gases carried by N2: (a) ∆λR as a function of gas concentration for H2, CO, CO2, and CH4; (b) ∆λR responses to pulse feed of 3% H2O and 3% H2S.
Figure 3. SEM pictures of (a) the cross-section of the SCZY-coated fiber and (b) the high magnification image of the SCZY nanocrystalline thin film on fiber.
As shown in Figure 5a, the sensor exhibited a strong red shift (∆λR > 0) in response to increasing the H2 concentration (xH2) from 0 to 80%. With forced interception at the origin, a simple linear relation of ∆λR ) 16.54xH2(nm) is obtained (R2 ) 0.9978), namely, a ∆λR of 0.1654 nm for a 1 mol % increase in H2 concentration at 500 °C. The present instrument resolution of λR measurement is 0.001 nm. Therefore, the sensitivity of the SCZY-LPFG is sufficient for detecting a 1% change in H2 concentration at atmospheric pressure, which can meet the industrial needs for in situ bulk monitoring with error 800 °C. However, XRD examination of the SCZY treated in CO2 showed no sign of carbonate formation likely because of the relatively lower temperature (500 °C) and the material stabilization by the zirconia dopant. The sensor’s small response to CO2 may be caused by CO2 chemisorption at the basic SCZY surface because of the very large grain boundary areas in the nanocrystalline structure. This may also explain its insensitivity to CO2 concentration Analytical Chemistry, Vol. 81, No. 18, September 15, 2009
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Figure 6. ∆λR as a function of H2 concentration in multicomponent gas (0) and in H2/N2 binary (O) at 500 °C and 1 bar (insert: single wavelength temporal response to switching between air and 3%H2 in N2).
because the surface chemisorption is often saturated at low partial pressures of adsorbate. The influences of H2O and H2S on the sensor response were evaluated by pulse feeding with 3% H2O and 3% H2S, respectively, into a continuous flow of N2. As shown in parts b and c of Figure 5, the sensor had a very small response to H2O (
