Porous Anodic Aluminum Oxide Bragg Stacks as Chemical Sensors

Porous anodic aluminum oxide (AAO) Bragg stacks were fabricated by a modified two-step anodization, and the optical transmittance spectra of AAO Bragg...
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17952

J. Phys. Chem. C 2008, 112, 17952–17956

Porous Anodic Aluminum Oxide Bragg Stacks as Chemical Sensors Dong-Lai Guo, Li-Xia Fan, Feng-Hua Wang, Sheng-You Huang, and Xian-Wu Zou* Department of Physics, Wuhan UniVersity, Wuhan 430072, P.R.China ReceiVed: August 4, 2008; ReVised Manuscript ReceiVed: September 15, 2008

Porous anodic aluminum oxide (AAO) Bragg stacks were fabricated by a modified two-step anodization, and the optical transmittance spectra of AAO Bragg stacks soaking in varied analytes (air, series of alcohols and alkanes) were collected. The results show that both the wavelength and the intensity of the reflected light are sensitive to the analyte’s refractive index and infiltration; e.g., for any two adjacent analytes in a series of alcohols or alkanes, the difference of transmittance in dip is greater than 3%. This phenomenon enables us to fabricate chemical and biological sensors and in situ monitor the organic chemical reaction just by analyzing the intensity of the reflected light without a spectrophotometer. Introduction Porous materials are widely used as adsorbent and catalyst carrier because of their large surface area and specific surface properties. They absorb watery molecules and thus significantly change their effective refractive index, so chemical and biological sensors were invented to take advantage of this property.1 By analyzing the Fabry-Pe´rot interference from a thin porous silicon film, optical thickness was deduced and reflected the effective refractive index comprising a porosity-weighted average of the refractive indices of Si and of the medium filling the pores.2,3 Furthermore, the porous silicon film shows a highly selective response to different individual materials and brings to light the detection of DNA, proteins, and toxins.2-8 Recently, multilayer structures of nanoporous materials have garnered significant interest in chemical and biological sensing.9-15 Particularly, Bragg stacks with periodic refractive index have high reflectivity for a certain wavelength of light. The strong response to the refractive index of analytes could be useful for simple, rapid, and in situ monitoring of various chemical/ biological species through a spectrophotometer. The principle of the structural color of a Bragg stack can be obtained directly from the Bragg condition λ ) 2nd, where d is the thickness of a pair of layers and n is its effective refractive index. Thus, we can say that the wavelength of the structural color of a Bragg stack is determined by the effective refractive index, which is a composite of the refractive indices of the Bragg stack and analyte. Very recently, Choi et al. reported that the mesoporous Bragg stack composed of spin-coated multilayer stacks of mesoporous TiO2 and mesoporous SiO2 showed a tunable color sensitivity (∆λ/∆n) to different series of analytes by adjusting the layer thickness of different compositions.15 It implies that the surface properties of porous Bragg stacks play an important role in color sensitivity to the different analytes. Anodic aluminum oxide (AAO) membranes with nanoporous structure have been widely used as templates in the fabrication of various nanomaterials because of the cheap equipment, easy technology, and high controllability of the process.16-18 It was confirmed that the pore density of an AAO template was inversely proportional to the square of the anodizing potential in the equilibrium condition.19,20 So the pore density can be * Corresponding author. Tel: +86-27-68752989-8423. Fax: +86-2768752989-8423. E-mail: [email protected].

modulated by changing the anodizing potential. Using this method, three-dimensional structures of AAO templates were obtained.21-23 A significant challenge faced in fabricating AAO template with periodic structure (Bragg stack) is that the pore density in AAO template could not be altered abruptly. If ones just reduced the voltage in steps, the stem pore divided into branched pores gradually and the branches appeared at different depths. To solve this problem, a process to thin the barrier layer before changing the anodizing potential was added.22 However, another problem comes after. While thinning the barrier layer, the formed structure of AAO template is also corroded. Therefore, fabricating complex three-dimensional structures of AAO template is limited. As an amelioration of the voltage modulation method, we developed a current-control method by which the pore density of AAO template can be suddenly changed without an additional barrier layer thinning process.24 So the AAO Bragg stack with periodical pore density and refractivity can be fabricated by the periodical modulation of current density based on the AAO template technology. Experimental Methods AAO Bragg stacks were prepared by using a two-step anodization process with platinum foil as cathode. The aluminum foils (99.95%) were degreased and electropolished to a mirror finish. As the first step, the clean aluminum foils were anodized in a 0.3 M/L sulfuric acid solution at 20 V and at 5 °C for 10 h. After that, the formed alumina layers were removed by a mixture of phosphoric acid (6 wt %) and chromic acid (1.8 wt %) at 60 °C for 3 h. Then we carried out the second step of anodization. In this step, the anodization was performed at 20 V and at 22 °C in the same electrolyte for 1 h to create primary stem pores. By the end of the hour, the current density arrived at a stable value of 5.6 mA/cm2. To form periodical pore density, the operation, which make a stem pore divide into several branching pores, was used. This operation was as follows: in a period, a current density of 5.6 mA/cm2 was applied for 36 s and followed by another 1.4 mA/cm2 for 144 s. When this operation was repeated, the AAO Bragg stacks were obtained. In this way, the AAO Bragg stacks with 30, 60, or 90 periods were fabricated and named sample 1, 2, or 3, respectively. Sample 4 is an improved multilayer structure created by using the thinning procedure. In the similitude of sample 3, sample 4 possessed 90 periods of pore density, but

10.1021/jp806926f CCC: $40.75  2008 American Chemical Society Published on Web 10/29/2008

Porous AAO Bragg Stacks as Chemical Sensors the thickness of layers was decreased gradually. To do this, the anodization duration of high and low current densities descended 0.07 and 0.28 s layer after layer; i.e., in the first period, the high and low anodization current lasted 36 and 144 s, and at the last period, they lasted 29.7 and 118.8 s, respectively. Sample 4 was annealed in argon atmosphere at 400 °C for 4 h. The annealed sample was named sample 5. For all samples, at the end of the electrolysis, the aluminum substrates were ridded up by immersion in saturated mercury chloride aqueous solution. After rinsing with deionized water and drying in air, selfsustained AAO Bragg stacks with periodical porosity were obtained. The optical transmittance spectra were completed by a UV-Vis-NIR spectrophotometer (Cary 5000). The observation of morphology was handled by SEM (FEI SIRION field emission gun). The AAO membrane used in photoluminescence (PL) analysis was produced in 0.3 M/L oxalic acid aqueous solution at 40 V and at 22 °C for 2 h, and the PL analysis was operated by fluorescence spectrometer (Hitachi F-4500) with the incident light of 350 nm. The analytes used in this investigation are air (1, n ) 1.000, ε ) 1.000 536 4); a series of alcohols, including anhydrous ethanol (2, n ) 1.360, ε ) 25.3), anhydrous 2-propanol (3, n ) 1.377, ε ) 20.18), anhydrous 1-butanol (4, n ) 1.399, ε ) 17.84), and anhydrous 1-hexanol (5, n ) 1.418, ε ) 13.03); and a series of alkanes, including anhydrous n-hexane (6, n ) 1.375, ε ) 1.890), anhydrous n-octane (7, n ) 1.398, ε ) 1.948), and anhydrous n-decane (8, n ) 1.411, ε ) 1.991). The organic analytes’ refractive index and dielectric constants shown above were measured at 20 °C. In order to eliminate the influence of local variation in the spectral measurement process, the AAO Bragg stack was stuck to the inwall of a glass cuvette and the cuvette was fixed with an aperture mask by a self-made fixture. The diameter of the limiting aperture was 8 mm. To reduce the measuring uncertainty, the AAO stack was soaked in the analyte for 15 min, and then the spectral measurement was initiated. Measurements were performed according to the analyte’s sequence and the processes repeated twice to obtain the mean value as the final results. The error of color shift for the same analyte is no more than 0.5 nm. Results and Discussion As shown in Figure 1, the AAO Bragg stacks possess obvious structural color and have strong light mirroring in a certain wavelength. Figure 2 shows the SEM images of an AAO Bragg stack. Figure 2a is cutaway view of sample 3 (90 periods). The membrane is composed of upper and low parts. The upper part is a straight hole array, which is generated by constant voltage, and the low part is the periodical branched structure, which is formed by the current density modulation, and each period of aluminum oxide layer is 160 nm in thick. Figure 2b is an enlarged image of the branch-beginning area, which is marked by b in Figure 2a. In this area, the periodical branched structure is immersed in electrolyte more than 4 h, the outside of pores are corroded seriously, and the fractured surface of the membrane is hackly. On the contrary, the periodical branched structure, which is formed in later stage, is almost not corroded by the electrolyte. Figure 2c shows an enlarged image in such an area. This area is nearby the interface of the oxide layer and aluminum substrate and marked by c in Figure 2a. Since the structure is almost not corroded, Figure 2c displays an alternative porous structure. To make a distinction between the stem pore half-period and branched pore half-period, in the rectangular portion of Figure 2c, the light gray part corresponds to a

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Figure 1. Photographs of AAO Bragg stacks taken by transmission (a) and reflection (b), with a fluorescent lamp as light source.

branched pore half-period (porosity 25%) and the deep gray part expresses stem pore half-period (porosity 12%). These AAO Bragg stacks possess strong selectivity of transmittance. Figure 3a plots the transmittance spectra of samples 1, 2 and 3, which have 30, 60, and 90 periods of pore density, respectively. It can be seen from Figure 3a that the transmittance dip is 38.8% at 515 nm, 4.2% at 511 nm, and 1.5% at 450 nm for samples 1-3 and the corresponding peak width at half-height (PWHH) is 18, 39, and 116 nm, respectively. This shows that when the number of layers (periods of pore density) varies from 60 to 90, the transmittance dip deepens; besides, the dip position shifts to the shorter wavelength and PWHH increases. The reason for decreasing transmittance is that the increment of layers enhances reflection. As for the dip shifting and PWHH widening, these are attributed to the inhomogeneous corrosion of the pores in different layers by the electrolyte solution. As the number of layers increases, anodization duration is prolonged and the pores in the earlier layers get more corroded. Thus, for the sample with more layers, the earlier layers have larger porosity and smaller effective refractive index than the sample with fewer layers. According to the Bragg law λ ) 2nd, the transmittance dip (to be exact, the valley) should extend for a large range and shift toward the shorter wavelength. To fabricate AAO Bragg stacks with a deep and narrow transmittance dip, a constant λ for all layers is needed. In consideration of a successive decrease of corrosion quantity layer by layer, which causes a successive increase of effective refractive index, we adopt a simple compensatory technique: making the thickness decrease successively from the first to the 90th layer by reducing the growth duration of each layer. Figure 3b shows the transmittance spectra of samples 3, 4, and 5, which all possess 90 layers of pore density but differences in detail. Sample 3 has the same thickness in every layer, and sample 4 has successively decreasing thickness from the first to the 90th layer. It can be seen from Figure 3b that the transmittance dip

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Figure 2. SEM images of the AAO Bragg stacks. (a) Cutaway view of sample 3. (b) Enlarging image in the interface area between straight hole structure and periodical branched structure. This area is marked by b in part a. (c) Enlarged image of area c indicated in part a. It shows an almost perfect periodical branched structure. The light gray part corresponds to a branched pore half-period fabricated by a low current density, and the deep gray part expresses stem pore half-period fabricated by high current density.

of sample 4 is much narrower (PWHH ) 45nm) and deeper (3 × 10-5at 444 nm) than that of sample 3. It proves the effectiveness of this compensatory method. For both samples 3 and 4, the immersing duration in electrolyte solution is more than 5 h, so the upper part of the templates is corroded seriously. The rough surface will cause light scattering and decreasing transparency. To smooth the interface between the alumina and air, annealing at 400 °C for 4 h is used on sample 4. The annealed sample 4 is denoted as sample 5. It can be seen from Figure 3b that the transparency of sample 5 has overall improvement by comparison with sample 4. 1. Color Sensitivity of AAO Bragg Stacks. Among samples 1-3, which have the same thickness in every layer, sample 2 has an obvious transmittance dip in its transmittance spectra. Meanwhile, the dip shifting of sample 2 is negligible in comparison with that of sample 1, since its structure was not inhomogeneously seriously corroded by the electrolyte. Thus, sample 2 was selected to accomplish the chemical sensing measurement. Figure 4 shows the photographs of sample 2. In the case of reflection, the Bragg stack displays a blue color (see Figure 4a). When the AAO Bragg stack soaks in ethanol, the stack exhibits visibly longer wavelength and less intensity in comparison with

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Figure 3. (a) Effect of the number of periods on the transmittance spectra. Samples 1, 2, and 3 have 30, 60, and 90 periods, respectively. (b) Effect of additional procedures on the transmittance spectra. By comparison with sample 3, sample 4 is fabricated with successively decreasing anodizing duration. Sample 5 is obtained from sample 4 after annealing.

when the stack is exposed in air (see Figure 4b). Obvious variation of wavelength and intensity of reflected light with soaking shows that the AAO Bragg stack has high color sensitivity and transmittance sensitivity. To investigate the color sensitivity of AAO Bragg stacks, we collected the transmittance spectra of the AAO Bragg stack when the stack was exposed to air and a series of alcohols and alkanes, respectively. Figure 4c shows the transmittance spectra of the AAO Bragg stack in air and in ethanol. It can be seen from Figure 4c that filling the nanopores of the AAO Bragg stack with ethanol leads to a red shift of the transmittance dip. This can be explained as follows. Essentially, the AAO Bragg stack is a periodic structure in porosity. When the stack is exposed to air, the refractive index of the stack is the average of that of air and bulk aluminum oxide. When the stack soaks in ethanol, the refractive index of the stack increases, because ethanol’s refractive index is larger than that of air. Therefore, the wavelength of transmittance dip for the stack soaking in ethanol shifts to a longer wavelength in comparison with that for the stack in air. The color sensitivity of AAO Bragg stacks can be used to identify the soaked-in analyte. Figure 4d plots the wavelength of transmittance dip as a function of the refractive index of analyte for air and the series of alcohols and alkanes. The wavelength of transmittance dip of the AAO Bragg stack exhibits respective linear correlation with the refractive index

Porous AAO Bragg Stacks as Chemical Sensors

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Figure 5. Photoluminescence spectra of AAO membrane, which is exposed in air (1) and respectively soaks in a series of alcohols and alkanes. The alcohols include anhydrous ethanol (2), 2-propanol (3), 1-butanol (4), and 1-hexanol (5). The alkanes include n-hexane (6), n-octane (7), and n-decane (8). The inset is the dielectric constant for each analyte.

Figure 4. Photographs of AAO Bragg stack (sample 2) taken by reflection (a, b). (a) the stack is exposed in air. (b) the stack soaks in ethanol. (c) The transmittance spectra of the stack that is exposed to air and soaks in ethanol, respectively. (d) The wavelength of the transmittance dip of AAO Bragg stack as a function of the refractive index of the analyte, when the stack is exposed to air (1) and soaks in a series of alcohols and alkanes, respectively. The alcohols include anhydrous ethanol (2), 2-propanol (3), 1-butanol (4), and 1-hexanol (5). The alkanes include n-hexane (6), n-octane (7), and n-decane (8).

of the analyte for the series of alcohols and alkanes. The slope of the wavelength-refractive index curve (∆λ/∆nanalyte) represents the variation of the wavelength of the transmittance dip with the refractive index deviation of the soaked-in analyte from air. Therefore, ∆λ/∆nanalyte can indicate the sensitivity. ∆λ/ ∆nanalyte is 71.4 for the series of alcohols and 61.9 for the series of alkanes. In the case of the AAO Bragg stack, n of the Bragg equation λ ) 2nd represents the composite refractive index of the stack. n ) (nair2a + nalumina2b + nanalyte2f) for effective dielectric constant approximation.25,26 nair, nalumina, and nanalyte are the refractive indices of the air, alumina, and the analyte, respectively, and a, b, and f are their volume ratio in the composite system. Therefore, we write the sensitivity∆λ/∆nanalyte ) 2d(nanalyte/n)f. As to the factor nanalyte/n, it varies not much and approaches to 1 slowly with f increasing. This result indicates that the color sensitivity depends on the thickness of a pair of layers and the volume ratio of the ambient analyte in the AAO Bragg stack. Since the hydrophilic AAO membrane is prone to be infiltrated by the analyte with larger solvent polarity, analyte with larger solvent polarity takes up more space in the AAO Bragg stack. Thus, we attribute the disparity in the slope of the wavelength-refractive index curve for different series of analytes to the different volume ratio of the analytes in the AAO nanostructure. To confirm the assumption that the volume ratio of the analytes in the porous AAO Bragg stack is positively correlated with their solvent polarities, we designed a simple experiment to investigate the infiltration of analytes in porous AAO nanostructure by the photoluminescence of AAO membrane. The AAO membrane is a one-dimensional porous array. This membrane prepared by electrochemically anodic oxidation in

oxalic acid solution has strong blue emission band. This blue emission originates from optical transition in oxygen-deficient defect centers, F (oxygen vacancy with two electrons) and F+ (oxygen vacancy with only one electron) centers.27 As the AAO membrane is immersed in polarity solvent, the oxygen vacancies react with the polar groups in the analyte. Thus, the electrons in F and F+ centers associate with the polar groups and the oxygen vacancies decrease.28 As a result, the blue emission band weakens. In general, the larger infiltration capacity of the analyte in AAO membrane leads to a larger opportunity for the polar groups to remove the oxygen vacancy; thus, the intensity of the blue emission band could be used to indicate the volume ratio of analyte in the AAO membrane. The less the intensity is, the larger the volume ratio is. Figure 5 displays the blue emission band of AAO membrane soaking in various analytes and the dielectric constant of each analyte. Usually, the dielectric constant is used to represent the polarity of the analyte. It can be seen from Figure 5 that the dielectric constants are close together for each analyte series, as is the blue emission band. Even though there is little difference in the dielectric constant and the blue emission for the analytes of the alkane series, they are much different from those of the analytes of the other series. Besides, we note that the compactness of porous AAO membrane fabricated by electrochemical anodization is poor and the free surface is very large in the porous nanostructure. Thus, we come to the conclusion that the solvent polarity affects the infiltration of the analyte, which then has influence on the volume ratio and at last leads the behavior of ∆λ/∆nanalyte in Figure 4d. 2. Transmittance Sensitivity of AAO Bragg Stacks. Figure 4c also shows that filling the nanopores of AAO Bragg stack with ethanol not only leads to a red shift of transmittance dip, but also makes the transmittance increase. The relative change of transmittance can be expressed by the ratio of the difference between the transmittance of the AAO Bragg stack soaking in the analyte and that soaking in another analyte, i.e., ∆T ) (T - T0)/T0. ∆T can indicate the transmittance sensitivity. When the AAO Bragg stack soaks in a series of alcohols and alkanes, the relative change of transmittance is illustrated in Figure 6. The insets give the corresponding transmittance spectra. Figure 6 shows that with an increase of the analyte’s refractive index, the dip in transmittance increases, corresponding to weakening of the reflected light. This phenomenon can be attribute to the

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Guo et al. to a change of refractive index of the analyte. Besides, the depth of the transmittance dip exhibits obvious sensitivity to a change of refractive index of the analyte, resulting from the change of refractive index contrast in a pair of layers of the AAO Bragg stack. On the basis of these phenomena, AAO Bragg stacks not only have potential applications in Bragg reflectors but also could lead to a simple method to in situ monitor an organic chemical reaction by measuring the intensity of the reflected light with a photodiode. Acknowledgment. This work was supported by FANEDD of China (No. 200525) and the Science and Technology Program of Wuhan City (No. 20067003111-07). References and Notes

Figure 6. Relative change of the transmittance (∆T) as a function of wavelength when the AAO Bragg stack soaks in series of alcohols (a) and alkanes (b), respectively. The insets correspond to transmittance spectra. ∆T ) (T/T0 - 1), where T is the transmittance and T0 is the value corresponding to ethanol (a) and n-hexane (b), respectively.

decreasing contrast of refractive index between a pair of layers in the AAO Bragg stacks, where the contrast is the key to generate the dip (photonic band gap).29,30 By checking the transmittance spectra of any two adjacent analytes in a series of alcohols or alkanes, the red shift of transmittance dip is less than 2 nm and the change of the transmittance dip is greater than 3%. This implies that the intensity of reflected light could be a better sensitivity index for use in chemical and biological sensing. Conclusion By periodically modulating the current density, we obtain a nanostructure with periodical porosity and effective refractivity in an AAO template. These AAO Bragg stacks have an obvious transmittance dip in the visible light waveband. This method can precisely control the porosity and thickness of each layer by adjusting the current density and duration. The free-standing AAO membrane, which could be produced cheaply and rapidly, possesses good optical performance in the range of visible and near-infrared waveband; thus, the AAO Bragg stacks have potential applications as Bragg reflectors. The AAO Bragg stack also exhibits significant optical response to the organic analytes. By introducing the anayltes into the nanopores of the AAO Bragg stack, the transmittance dip is shifted to longer wavelength according to the Bragg equation. For a series of alcohols and alkanes, the color sensitivity of the AAO Bragg stack is different. It is attributed to the distinct polarity of the two types of analytes. In comparison with alkanes, alcohols are more infiltrative with the AAO stack, so they take up more space and have larger volume ratio, thus yielding enhanced sensitivity

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