Patterned one-dimensional photonic crystals with acidic/alkali vapor responsivity

Abstract

A series of patterning responsive one-dimensional photonic crystals (1DPCs) were developed by using a photolithography technique to etch the template for acidic/alkali vapor sensing by the naked eye through a change in color.

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Chemical odors and volatile compound detection are a topic of great interest in physics and material science owing to concerns about environmental monitoring, industrial quality control and human health care.¹ Ammonia and hydrochloric acid, which are harmful and can cause diseases, are easily found in many laboratories and factories; thus, it is necessary for monitoring these compounds in air.

Fabricating suitable sensing devices with efficient responsivity is the key point in such efforts. Responsive photonic crystals (RPCs), which use stimulus-responsive materials as elements that can alter diffraction wavelength or intensities upon exposure to physical² or chemical stimuli³ have been a fast and flexible approach for stimuli sensing.⁴ Recently some research advances in several groups have led to a burst of activities on responsive photonic crystals to detect arrange of odors including water-vapor,⁵ CO₂,⁶ volatile organic compounds⁷ vapor and specific gas mixture.⁸ The inherent responsivity of RPCs can be monitored by measuring the shift of the stopband position or observed by the remarkable color change. In this case, patterning photonic crystals can be treated as an effective option for the detection signals read-out.⁹ Patterning photonic crystals can be fabricated by self-assembly,¹⁰ inkjet-printing,¹¹ and artificial lithography.^12,13 Some of these strategies are limited by the structure, stability, high cost, and difficulties associated with template fabrication together with the complex response detection means of the external stimuli.¹⁴ Therefore, it is highly desirable to carry out easy fabrication of responsive photonic crystals without using intricate fabrication and detection methods.

Currently, a few studies point to new strategies to make responsive pattering 1DPCs, which show their advantages in convenient preparing process.¹⁵ 1DPCs are special kinds of crystal multilayer structures, which consist of alternating layers of high and low-refractive-index materials with planar interfaces between each pair of layers.¹⁶ In particular, most of the responsive 1DPCs are focused on manipulating layer thickness,^5a,17 while conducting polymers focus on tuning the refractive index.¹⁸ Odors can be adsorbed strongly on conducting polymer photonic crystals because of the conducting polymers' doping/de-doping process, leading to effective reflective index change at room temperature,¹⁹ whereas some sensors need complicated features to monitor the changes.²⁰ By making use of the simple ordered titanium dioxide (TiO₂) layer, 1DPCs are easily processed to achieve patterning with the help of state-of-the-art photolithography techniques.²¹

In this article, an artificial patterning TiO₂/polyaniline (PANI) 1DPCs is fabricated and employed to detect environmental response to NH₃ and HCl vapors (see Fig. 1a). The color of the as-prepared 1DPCs changed reversibly between green and red when they were placed in HCl and NH₃ atmospheres alternatively, which was caused by the shift of PC's stopband because of the variation of PANI's refractive index in different acidic and alkali vapor environment (Fig. 1b). Compared with other traditional studies,²² this method uses a combination of chemistry with material design and has a better responsiveness. The remarkable color change in the photolithography pattern makes the naked-eye sensing function a reality, which would be helpful for chemical and biological sensor applications for real-time monitoring of acidic and alkali vapors of optical signal changes (Fig. 1c).

Fig. 1 (a) Schematic of organic/inorganic hybrid 1DPCs by alternated spin-coating of TiO₂ and PANI. (b) The dedoping/doping transformation from emeraldine salt (ES) to emeraldine base (EB) form of PANI. (c) Schematic of the gas responsive 1DPC and the spectra measurement system.

In this study, highly uniform, mechanically stable TiO₂/PANI 1DPCs with alternate layers of TiO₂ and PANI have been realized using the spin-coating method. TiO₂ can form a uniform film easily and has a higher refractive index compared with PANI layer.²³ As the low refractive index layer, we use the porous conducting polymer EB form of PANI, which allows the stimuli gases to flow through it and does not corrupt the ordered structures, allowing us to demonstrate the platform for developing colorimetric gas sensors. Note that the photonic bandwidth becomes narrower and the intensity of the peak grows with increasing numbers of layers. This is in good agreement with a previous theory of the thickness dependence of the optical response of photonic crystal slabs (Fig. 2a). The angle dependence results were determined by spectroscopic ellipsometry. When the incident angles are 75°, 60°, 45°, 30° and 15°, respectively, the corresponding Bragg peaks' positions are 470 nm, 500 nm, 550 nm, 580 nm and 615 nm (Fig. S1†). The structural colors get blue-shifted as the incident angle increases. A change of photonic crystals’ period also influences the optical properties of the 1DPCs. By increasing photonic crystals’ period only, the Bragg peak position will be red-shifted (Fig. 2b). Note that the Bragg peak can be manipulated in the full visible range from blue to red by choosing proper photonic crystals’ periods. The obvious photonic stopband and the vivid structure color can be easily obtained in several bilayers’ structure. The structure of film is very uniform over a large area, see in Fig. 2b. The cross-sectional SEM image of a stacked film, from which we can see that it possesses an ordered multilayer structure in a large area, is shown in Fig. 2c.

Fig. 2 (a) Reflection intensity of the 1DPCs with increasing number of bilayers; (b) Digital pictures of different structural color films and their reflection spectrum of the 1DPCs with different thicknesses; (c) cross-sectional SEM images of a 1DPC film and its enlarged view image in the top; (d) SEM image of PANI's porous structure.

For a stimulus vapor response, PANI/TiO₂ 1DPCs were fabricated with their photonic band gaps in the visible spectrum region. When a stimuli gas molecule enters a 1DPC structure, an obvious refractive index change of the conducting polymer can occur and the photonic crystals' optical period changes at the same time. These cooperative, fast responsive structure properties variety leads to a significant gas sensing affinity. The responsive photonic crystals reported the gas sensitive event through a gradual shift of the position of the diffraction peak to long wavelength with an increase of the refractive index. This color change phenomenon can be explained by the formula, see ESI formula†.

During the process of stimuli gas sensing, the acidic vapor of HCl binds to the porous PANI layers of the prepared 1DPCs. The obvious transformation from EB to ES is triggered by a reorganization of the electronic structure, which induces the change of the refractive index (Fig. 1b). As the refractive index of the ES form of PANI is larger than that of EB form, the average refractive index increases, which presents a clear red shift of the Bragg diffraction peak at normal incidence. However, in Fig. 3a, we observed a shift maximum of 60 nm, which is larger than that caused by only a change in refractive index (see the calculation portion in ESI†). Thus, the additional shift should be attributed to the increase of the photonic crystals’ period, which is caused by the porous structure swelling of the PANI layers (see ESI†).

Fig. 3 (a) The reflection spectrum of PANI/TiO₂ 1DPCs exposed to HCl and then irradiated by UV light. (b) The changing reflection spectrum of an HCl-doped PANI/TiO₂ 1DPCs irradiated by UV light for 20 min. (c) Digital photographs of the letter pattern taken after the HCl-doping process by UV light-irradiated photolithography.

The porous structural PANI layers of the prepared 1DPC show excellent gas carrying capacity as the responsive vapor infiltrates into pores of the 1DPCs (Fig. 2d). In different sensing environments, TiO₂ layers keep their thicknesses and refractive index invariable. To summarize, the specific response of acidic vapor in the PANI/TiO₂ 1DPCs mainly depends on the increase of the average refractive index, and the swelling of the porous PANI layers also contributes a little, both of which are reported by layer-by-layer 1DPCs through a red shift of the Bragg diffraction peak.

A corresponding “SEU” pattern can be produced with color distinct from that of the substrate as the 1DPCs are irradiated directly by UV light with a predesigned photo mask (Fig. 3c). This colorful 1DPC exhibited a stopband in the green color region with its stopband position located at 531 nm (Fig. 3a). After this 1DPC had been treated with HCl, the stopband was red-shifted to the red color region (Fig. 3a). Correspondingly, the red color 1DPC changed its appearance to green color after it had been treated with NH₃. Fig. 3b shows 1DPCs covered by a designed hollow pattern mask that was exposed to ultraviolet (UV) light with wavelengths below 380 nm for 20 min. After the HCl-doped 1DPC film has been irradiated, 1DPCs shows a pattern image as the yellow-green color area, a demonstration of one time use writing pattern was achieved. An evident shift of the photonic stopband is also observed simultaneously. As the patterned films show a “SEU” pattern in the middle area, the pattern boundary is extremely clear. The photocatalytic process for degradation of PANI with TiO₂ was directly observed using UV light irradiation. This transformation is owing to the reorganization of PANI's electronic structure, which is different from the doping and dedoping process.^19a The interface interaction between PANI and TiO₂ was caused by UV irradiation. In FT-IR (see ESI Fig. S2†), the main characteristic peaks of PANI can be clearly seen. Compared with the main bands of the EB form of PANI, it shifted to lower wavenumbers after being doped with HCl. However, after irradiation, all bands of the doped ES form of PANI showed another shift to lower wavenumbers. Therefore, the effective refractive index of the pattern area changes as the irradiation process carries on.

To investigate the oscillation of this gas sensing 1DPCs, alkali vapor (NH₃) was introduced to test the 1DPCs after acidic response (Fig. 4a). In Fig. 4b, it can be observed that the 1DPCs specifically recognize the alkali vapor NH₃ and display a remarkable response. Note that the color comes back to the original state immediately upon exposing the 1DPCs to saturated pressure of concentrated NH₃. Fig. 4c shows the reversible conversion of the stopband position of the film when alternately exposed to NH₃ and HCl vapor for five cycles. Fig. 4d presents the color difference of the patterning film ‘S’ over the period of a response cycle, including the initial state, response under acid and alkali atmosphere. Fig. S2† shows photographs of the as-prepared 1DPCs without an artificial pattern corresponding times of exposure to HCl and NH₃ vapors. The initial color of the film was green before being exposed to HCl, and the color underwent a change from green to nearly red with an exposure time of 15 seconds; however, its color returned back to green when placed in a NH₃ environment. The patterned films of letter ‘E’ and ‘U’ were reported in Fig. S3.† It was clear that the change in the peak position was repeatable and reversible, although there was a small fluctuation in the peak position, which is attributable to the reversible transformation between the ES and EB forms of PANI during the dedoping and doping process.

Fig. 4 The reflectance spectra of a PANI/TiO₂ 1DPCs exposed to HCl (a) and NH₃ (b) vapors, respectively. (c) Reversible changes of the stopband of the PANI/TiO₂ 1DPCs with HCl and NH₃ vapors. (d) Photographs of the as-prepared patterned 1DPCs corresponding to different times of exposure to HCl and NH₃ vapors.

In conclusion, we have demonstrated a facile method to elucidate response to acidic/alkali vapor by the naked eye through color change based on patterning responsive 1DPCs. The distinct ‘SEU’ patterns were etched by the photolithography technique because of TiO₂. The responsive process is very fast and the repeatability is perfect. Considering the visible read-out, low-cost fabrication approach and easy packing without the aid of sophisticated instrumentation, our 1DPCs would be promising as economically colorful sensors in chemical and biological fields, and environmental monitoring.

Acknowledgements

This work was supported by the Fundamental Research Funds for the Central Universities and the Precious Apparatus Funds of Southeast University.

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Footnote

† Electronic supplementary information (ESI) available: I. Methods and characterization; II. Bragg's formula of 1DPCs; III. the performance of angle dependence; IV. photograph of 1DPCs without pattern; V. the sensor performance with pattern ‘E & U’. See DOI: 10.1039/c4ra02468k

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