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The potential to detect hydrogen concentration gradients with palladium infused mesoporous-titania on D-shaped optical fiber Zsolt L. Poole, Paul R Ohodnicki, Aidong Yan, Yuankun Lin, and Kevin P. Chen ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.6b00583 • Publication Date (Web): 08 Dec 2016 Downloaded from http://pubs.acs.org on December 9, 2016
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ACS Sensors
The potential to detect hydrogen concentration gradients with palladium infused mesoporous-titania on D-shaped optical fiber Zsolt L. Poole1,*,†, Paul R. Ohodnicki2, Aidong yan1, Yuankun Lin3, and Kevin P. Chen1. 1
Department of Electrical and Computer Engineering, University of Pittsburgh, Pittsburgh, PA 15261, USA National Energy Technology Laboratory, 626 Cochrans Mill Road, Pittsburgh, PA 15236, USA 3 Department of Physics, University of North Texas, Denton, TX 76203, USA 2
ABSTRACT: A distributed sensing capable high temperature Dshaped optical fiber modified with a palladium nanoparticle sensitized mesoporous (~5nm) TiO2 film, is demonstrated. The refractive index of the TiO2 film was reduced using block-copolymer templating in order to realize a mesoporous matrix, accommodating integration with optical fiber. The constructed sensor was analyzed by performing direct transmission loss measurements, and by analyzing the behavior of an integrated fiber Bragg grating. The inscribed grating should reveal whether the refractive index of the composite film experiences changes upon exposure to hydrogen. In addition, with frequency domain reflectometry the distributed sensing potential of the developed sensor for hydrogen concentrations of up to 10% is examined. The results show the possibility of detecting chemical gradients with sub-cm resolution at temperatures greater than 500°C.
ploration. By these techniques, a length of fiber can be turned into a linear combination of millions of sensors with µm spatial resolution, presenting an exciting opportunity for probing chemical gradients with high spatial resolution, with a potential to provide an unprecedented level of insight into a number of critical processes such as in chemical processing plants, fuel cell, etc.
KEYWORDS: Rayleigh backscatter, mesoporous film, palladium nanoparticles, refractive index engineering, optical fiber sensing, hydrogen sensing, gradient sensing, distributed sensing. Optical fiber is widely explored sensor platform due to its small form and its ability to withstand harsh conditions such as high temperatures and corrosive environments1-2. A number of sensing applications for optical fiber already exist1-5, but the inert nature of silica limits its detection capability. Incorporating sensory materials with optical fiber is a practical way to extend its inherent sensory limitations. Explored materials include thin palladium films for hydrogen sensing4, 6-7 and films of various oxides and other materials for the detection of a variety of chemical species813 . Response types from absorptive based11 to refractive index based4, 7-10, 13, to surface plasmon resonance based5-6, 14 have been observed. It is also known that structuring on the scale that is comparable with the sensory modulated depth (~1-10nm, proportional to the Debye length) can provide significant sensory enhancements due to a surface area increase and by fully modulating the sensory material15-19. Sensitization with noble metals such as palladium can further enhance the sensory mechanism by influencing the rate of response, the type of the response, the magnitude of the response, along with addressing the selectivity issues of oxides16, 19-20. Methods such as Rayleigh and Brillouin scattering allow the probing of local light variations in a section of an optical fiber, with micrometer resolution. Instrumentation for measuring spatially resolved variations along an optical fiber is well developed for its use in the communication industry for loss characterization. Unfortunately, the use of these techniques in chemical sensing is not well explored, possibly due to the added difficulty associated with the multidisciplinary nature of optical fiber chemical sensor ex-
Figure 1. An illustration of the proposed device configuration for evanescent tunneling based sensing comprising of modified optical D-fiber, modified with a Pd-nanoparticle infused mesoporousTiO2 which relays changes to the light in the fiber through evanescent tunneling. In order to integrate TiO2 with optical fiber its refractive index must be reduced from its nominal value of ~2.4 to be below the refractive index of silica (~1.45). In a prior work we have shown that substantial reductions in the refractive indices of oxides are possible, by converting the oxide into a 3D mesoporous matrix on the
