Opt Quant Electron (2018) 50:237 https://doi.org/10.1007/s11082-018-1507-z
Metal layers with subwavelength texturing for broadband enhancement of photocatalytic processes in microreactors M. Rašljić1 · M. Obradov1 · Ž. Lazić1 · D. Vasiljević Radović1 · Ž. Čupić2 · D. Stanisavljev3
Received: 26 October 2017 / Accepted: 18 May 2018 © Springer Science+Business Media, LLC, part of Springer Nature 2018
Abstract In this paper we joined plasmonics and microreactors for photocatalytic optofluidic devices. To this purpose we consider the use of subwavelength texturing of plasmonic films applied to microreactor channel bottom to ensure SPP localization and field enhancement. The small volume of the microchannel is ideal for the enhancement of photocatalytic processes using localized evanescent fields. A great advantage of our approach is that it is highly compatible with the standard chemical bulk micromachining techniques which are commonly used in fabrication of the microreactors i.e. standard photolithography can be used to define microchannels, and radiofrequent sputtering to deposit a gold film as a plasmonic material over the roughened surface. Subwavelength surface texturing can be obtained by varying etching techniques and parameters and the microreactor building materials. We show using ab initio FEM modeling that the stochastic surface profile ensures broadband coupling of visible light as well as enables us to merge plasmonic sensors and microreactors into a single device. Keywords Plasmonic · Microreactors · Thin films · Photocatalysis
This article is part of the Topical Collection on Focus on Optics and Bio-photonics, Photonica 2017. Guest Edited by Jelena Radovanovic, Aleksandar Krmpot, Marina Lekic, Trevor Benson, Mauro Pereira, Marian Marciniak. * M. Rašljić
[email protected] 1
Centre of Microelectronic Technologies, Institute of Chemistry, Technology and Metallurgy, University of Belgrade, Njegoševa 12, 11000 Belgrade, Serbia
2
Centre of Catalysis and Chemical Engineering, Institute of Chemistry, Technology and Metallurgy, University of Belgrade, Njegoševa 12, 11000 Belgrade, Serbia
3
Faculty of Physical Chemistry, University of Belgrade, Studentski trg 12‑16, 11000 Belgrade, Serbia
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1 Introduction Electromagnetic waves bound to an interface between conductor with free electron plasma and dielectric are called surface plasmon polaritons (SPP). SPP ensures extreme localization of electromagnetics near fields in subwavelength volumes (Schuller et al. 2010). Nanoparticles of good plasmonic materials, usually gold or silver, are often used as couplers and field concentrators due to enhanced light scattering caused by localized surface plasmon resonance (LSPR) (Maier 2007). Such extreme localizations are useful for numerous practical applications including ultrasensitive chemical sensing, enhancement of photodetectors and many others (Barnes et al. 2003; Obradov et al. 2014). Microreactors are devices in which chemical processes occur in cells or channels of micrometer dimensions. A typical microreactor consists of a channel system with a width and a depth of several to several hundred micrometers. Microreactors allow for excellent control of the conditions under which the reaction takes place due to their small size, most notable is the temperature control, i.e. temperature can be quickly changed and uniformly maintained across the entire device. In addition, they are very suitable when handling toxic or explosive substances, since microliter quantities are used and the danger is minimized. Also, due to the small amount of materials used in the experiment, they are very suitable for the synthesis of expensive materials. Microreactors have a wide range of application, in synthesis of organic and inorganic materials, nanoparticles, biosynthesis (Yao et al. 2015), in environmental application (Das and Srivastava 2016) and photocatalysis (Gorges et al. 2004) to name a few. A wide class of chemical reactions can be enhanced using solar radiation (Zhang et al. 2013). This is useful for wide range of applications like waste treatment, air purification, self-cleaning surfaces, water splitting, CO2 reduction, etc. (Herrmann 1999; Wu et al. 2008; Chen et al. 2011). In order to convert as much optical radiation as possible into electrochemical energy plasmonic enhancement is used in vast majority of photocatalytic systems. The most common approach to plasmonic enhancement of photocatalytic reactions is the use of metal nanoparticles. The benefits of LSPR in photocatalytic reactions are numerous and include enhancement of local electric field, enhanced UV/ VIS absorption, local heating effect, molecule polarization effects, etc. However the use of nanoparticlses can be problematic in applications that have a constant microfluidic flow. An alternative approach readily applicable for the microreactors is the use of rough metallic films (Tan et al. 2016). In our work we will analyze the use of rough metallic surfaces as broadband couplers for plasmonic enhancement of photocatalytic reactions in microfluidic devices–microreactors.
2 Theory We connect plasmonic photocatalysis and microreactors with the rough metallic surface of microchannels. An illustration of our microreactor is shown on Fig. 1. The main material of microreactors can be silicon, glass, polycarbonate, ceramics, etc. The geometric properties of microchannels are defined using the standard photolithography processes. Rough surfaces can be obtained by several methods. The main groups are wet chemical etching (micromachining), dry etching, mechanical abrasion (mechanical machining,
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Fig. 1 Illustration of microreactor with gold-coated rough bottom surface of microchannel
Fig. 2 Illustration of different random profiles of rough surfaces in microchannels
polishing, grinding) and beaming/irradiation (e.g. laser ablation, UV illumination). The bottom of a microchannel is then covered with plasmonic material, e.g. using RF sputtering. An illustration of some possible surface profiles is shown in Fig. 2. Since the wave vector of SPP is much larger than the wave vector of a propagating mode, in order to ensure coupling it is necessary to employ either a refractive or a diffractive structure (Maier 2007). The surface of a microchannel can be approximated as a superposition of diffractive gratings with different spatial periodicities. Because of that the efficiency of SPP-to-propagating coupling would be directly related with the properties of the surface relief. Since a stochastic profile will have a large number of components, it will enable coupling for different angles and at different wavelengths. The relation between the lattice constant of the grating a and the wavevector of the diffracted mode Δk is given with Eq. (1).
Δk = ± n
2𝜋 a
(1)
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where n is an integer. The matching between SPP and propagating mode is given with (2) and propagating mode is defined with (3) where ω is the angular frequency, c is the speed of light in the medium above the plasmonic surface and θ is the incident angle of the propagating mode.
k⃗spp = k⃗prop + Δk⃗ kprop =
𝜔 sin 𝜃 c
(2) (3)
Surface roughness is modeled using Eq. (4). The stochastic nature of the relief is achieved via a random Gaussian function g(n) (amplitude modulation) and a random uniform function u(n) (phase modulation). Roughness is defined by the total number of gratings N, as illustrated in Fig. 3. ∑N g(n) sin (nΔkx + u(n)) (4) ,n ≠ 0 f (x) = n=−N 2N An increase of optical energy density due to plasmonic localization in the microreactor channels polarizes the reactant molecules in the fluid and enhances the adsorption to the metal surface. In addition to that, heating up the local environment due to the absorptive losses in metal increases the mass transfer of the molecules and enhances the reaction rates.
3 Results and discussion Utilizing Comsol Multiphysics RF module we can numerically model the optical properties of our rough gold film. Since we modeled our surface profile as a series of periodic functions we can represent the entire channel using a unit cell with the length of the largest grating period (a = 1 μm). For simplicity we use a 2D model while the width of the channel is considered to be infinite (much larger than the grating period). Electric permittivity of gold is very well described by lossy Drude model (Zeman and Schatz 1987; Maier 2007) and the dielectric in the microreactor channel is assumed to be air. A plane wave enters the simulation domain through an active port and a pair of periodic boundary conditions are
Fig. 3 Surface roughness modeled by (4) left to right: N = 1; N = 10; N = 30; N = 50
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applied to the edges of the unit cell. Normal incidence is assumed. We calculated total optical energy in a 1 μm deep channel for different values of surface roughness and compared it to a situation with a flat channel surface. Increases in density of optical states within the channel of microreactor depending on stochastic surface roughness are shown in Fig. 4. What can readily be observed is that the rough film allows for multiple resonant wavelengths where incident light couples to plasmonic modes increasing the total optical energy within the channel. The dispersive properties of the surface profiles with a lower number of superimposed gratings tend towards multitude of sharp resonant peaks while increasing the number of gratings tends to merge resonant peaks into a smooth bandlike dispersion. However in both cases rough metal films offer an enhancement of density of optical states across the visible spectrum. This opens up two possible approaches to utilize rough metal films in enhancing photocatalytic processes.
Fig. 4 Relative optical energy increase in 1 µm channel depth for different levels of surface roughness: a N = 8; b N = 9; c N = 10; d N = 11; e N = 12
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The first is under white light illumination where a significant part of the spectrum is plasmonically enhanced, this can be viewed as sort of a passive regime. Second approach is based on properties of plasmonic biochemical sensors. Biochemical sensors function on a principle of spectral shifting of dispersion characteristic in presence of a substance of interest i.e. light couples to surface modes at a specific wavelength only in a presence of a desired substance. Same can be applied to microreactors with rough films and under coherent illumination, device “activates” only in the presence of the desired substance i.e. active regime of operation. Electric field spatial distribution for different levels of surface roughness are presented in Fig. 5. It can be seen that the plasmonic modes are localized around
Fig. 5 Electric field intensity spatial distribution for different levels of surface roughness: top) N = 8 at 350 nm wavelength; middle) N = 10 at 500 nm wavelength; bottom) N = 12 at 640 nm wavelength
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subwavelegnth details on the surface (edges, grooves, etc.). Coupling between plasmonic modes can be observed when two edges are close to each other (Fig. 5 middle) in a similar fashion to arrays of nanoparticles further enhancing density of optical states. Near field plasmonic enhancement significantly increases the density of optical states but rapidly decays with the distance from the surface. The scattered far field components further enhance the density of optical states by increasing the optical path through the channel. Reducing the channel depth maximizes the influence of near field enhancement offering an additional degree of freedom when designing microreactors for photocatalytic reactions.
4 Conclusion In this paper we examined how rough surface of microchannels enhances density of optical states in microfluidic devises for photocatalytic reactions and we showed that a rough surface can be considered as a series of different sinusoidal diffractive gratings. Stochastic surface profiles behave similarly to a collection of metal nanoparticles with different sizes ensuring a broadband coupling. This approach can be used for different photocatalytic processes and devices. A useful trait of this method is that SPP-based chemical sensors can be conveniently integrated into such microreactors, being based on the same mechanism, thus ensuring a possibility for in situ characterization and realtime control of the reactor products. Acknowledgements This work was supported by the Serbian Ministry of Education, Science and Technological Development under Projects No. TR32008.
References Barnes, W.L., Dereux, A., Ebbesen, T.W.: Surface plasmon subwavelength optics. Nature 424(6950), 824–830 (2003) Chen, H.M., Chen, C.K., Lin, C.C., Liu, R.-S., Yang, H., Chang, W.-S., Chen, K.-H., Chan, T.-S., Lee, J.-F., Tsai, D.P.: Multi-bandgap-sensitized ZnO nanorod photoelectrode arrays for water splitting: an X-ray absorption spectroscopy approach for the electronic evolution under solar illumination. J. Phys. Chem. C 115(44), 21971–21980 (2011) Das, S., Srivastava, V.C.: Microfluidic-based photocatalytic microreactor for environmental application: a review of fabrication substrates and techniques, and operating parameters. Photochem. Photobiol. Sci. 15(6), 714–730 (2016) Gorges, R., Meyer, S., Kreisel, G.: Photocatalysis in microreactors. J. Photochem. Photobiol. A Chem. 167(2), 95–99 (2004) Herrmann, J.-M.: Heterogeneous photocatalysis: fundamentals and applications to the removal of various types of aqueous pollutants. Catal. Today 53(1), 115–129 (1999) Maier, S.A.: Plasmonics: Fundamentals and Applications. Springer, New York (2007) Obradov, M., Jakšić, Z., Vasiljević-Radović, D.: Suppression of noise in semiconductor infrared detectors using plasmonics. J. Opt. 16(12), 125011 (2014) Schuller, J.A., Barnard, E.S., Cai, W., Jun, Y.C., White, J.S., Brongersma, M.L.: Plasmonics for extreme light concentration and manipulation. Nat. Mater. 9(3), 193–204 (2010) Tan, F., Li, T., Wang, N., Lai, S.K., Tsoi, C.C., Yu, W., Zhang, X.: Rough gold films as broadband absorbers for plasmonic enhancement of T iO2 photocurrent over 400–800 nm. Sci. Rep. 6, 33049 (2016)
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Wu, J.C., Wu, T.-H., Chu, T., Huang, H., Tsai, D.: Application of optical-fiber photoreactor for CO2 photocatalytic reduction. Top. Catal. 47(3–4), 131–136 (2008) Yao, X., Zhang, Y., Du, L., Liu, J., Yao, J.: Review of the applications of microreactors. Renew. Sustain. Energy Rev. 47, 519–539 (2015) Zeman, E.J., Schatz, G.C.: An accurate electromagnetic theory study of surface enhancement factors for silver, gold, copper, lithium, sodium, aluminum, gallium, indium, zinc, and cadmium. J. Phys. Chem. 91(3), 634–643 (1987) Zhang, X., Chen, Y.L., Liu, R.-S., Tsai, D.P.: Plasmonic photocatalysis. Rep. Prog. Phys. 76(4), 046401 (2013)
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