Zinc Phthalocyanine Thin Film-Based Optical Waveguide H2S Gas Sensor

The detection of hydrogen sulfide (H2S) is essential because of its toxicity and abundance in the environment. Hence, there is an urgent requisite to develop a highly sensitive and economical H2S detection system. Herein, a zinc phthalocyanine (ZnPc) thin film-based K+-exchanged optical waveguide (OWG) gas sensor was developed for H2S detection by using spin coating. The sensor showed excellent H2S sensing performance at room temperature with a wide linear range (0.1 ppm–500 ppm), reproducibility, stability, and a low detection limit of 0.1 ppm. The developed sensor showed a significant prospect in the development of cost-effective and highly sensitive H2S gas sensors.


Introduction
Hydrogen sulfide (H 2 S) is a colorless gas with pungent odor and is usually produced during the decomposition of organic matter, smelting, waste-water treatment, and landfill designing. As an air pollutant, H 2 S seriously affects human health and has been classified as a toxic and dangerous chemical by occupational safety and health standards. Low concentrations of H 2 S gas can damage the human respiratory and nervous systems, while long-term or high-level exposure can cause dizziness, vomiting, and even death [1]. Therefore, the detection of H 2 S gas has gained immense scientific attention and it is imperative to develop a H 2 S gas sensor with high-efficiency, low detection limit, and quick response.
The waveguide (WG) technology is well developed and can be utilized in gas sensors. Huang et al. [11] proposed an evanescent field-absorption gas sensor based on the silicon-on-sapphire WG in the mid-IR (infrared) wavelength region to detect fluctuations in CO 2 concentration at the atmospheric base level.
Kazanskiy et al. [12] proposed a novel polarizationinsensitive hybrid plasmonic WG design, which corresponded to the absorption line of the deadly methane gas. Khonina et al. [13] presented a more sensitive dual hybrid plasmonic WG structure for the detection of methane gas. Ranacher et al. [14] presented a photonic gas sensor concept based on a silicon WG by using infrared evanescent field absorption, and quantitative measurements of CO 2 were conducted for concentrations as low as 500 ppm CO 2 .
WG sensors have numerous attractive features, such as high sensitivity, fast response, and room temperature operation, as listed in Table 1. The OWG sensor is based on the principle of evanescent waves. An optical waveguide is generally composed of a cladding layer (refractive index, n c ), a waveguide layer (n f ), and a substrate (n s ). In the waveguide structure, when n f > n s > n c , the light undergoes total internal reflection during propagation in the wave guide layer. The light wave penetrates into a very thin layer of the second medium and undergoes a Goos-Hänchen shift before returning to the first medium layer. The light wave that penetrates the second medium is called an evanescent wave. Phthalocyanines (Pcs), especially metal Pcs show high photo-and thermal stability due to their 18-π conjugated electron system composed of carbon-nitrogen conjugated double bonds. As a class of macrocyclic planar aromatic compounds, MPcs are promising organic compounds with a huge potential for the application in electro-optical devices [19], photodynamic therapy [20,21], catalysts [22], nonlinear optics [23], and gas sensors [24][25][26].
Zinc phthalocyanine (ZnPc) has received significant attention in the optical field due to its excellent nonlinear optical properties, such as high light absorption coefficient in the visible region, high triplet quantum yield, and high lifetime [27,28]. Gas sensors based on ZnPc organic semiconductors have recently become noticeable because of their inherent advantages [29,30]. Yet, studies on ZnPc-based materials such as OWG sensors remain limited. In this work, we reported that a highly sensitive and cost-effective ZnPc thin film-based K+-exchanged OWG gas sensor was developed for H 2 S detection. In this study, we fabricated ZnPc thin films by using spin coating with different rotation speeds and different mass fractions of ZnPc. The influence of coating conditions on the gas sensing properties of the films were examined. The principles of the sensing and stability of the film-based sensors were discussed.

Reagents and instrumentation
ZnPc was purchased from Shanghai Bailingwei Chemical Technology Co., Ltd. Polyvinyl pyrrolidone (PVP) was purchased from Tianjin Bodi Chemical Technology Co., Ltd. A DHG-9023A vacuum drying chamber (Shanghai-Hengke), along with a KW-4A spin coater (Shanghai Kaimeite Artificial China Technology), was used for film coating. A gas detection tube was also employed (working range: 2 ppm to 200 ppm, Gastec, Beijing Municipal Institute, Beijing, China), together with a homemade OWG testing system.

Preparation of volatile organic compounds and H 2 S
Volatile organic compounds (VOCs) were obtained by using a natural volatilization method. A specific amount of organic solvent was added to a 600 mL container with a micro syringe and left to volatilize naturally for 3 h after sealing.
H 2 S was obtained by the reaction of ferrous sulfide (FeS) and concentrated hydrochloric acid (HCl). A specific amount of FeS was weighted into a 600 mL container, followed by the addition of a controlled volume of HCl.
The different concentrations of gases were obtained by using the dilution method. The concentrations of the prepared gases were determined by using a gas detection tube, and the results were consistent with the calculated values.

Preparation of the K + -exchanged OWG component
The K + ion exchanged OWGs were prepared by using a thermo-ionic diffusion method. Firstly, potassium nitrate (KNO 3 ) powder was fused in a muffle furnace at 400 ℃. A clean glass slide was then immersed in the molten KNO 3 to induce ion exchange. After 40 min, the Na + ions on the glass surface were exchanged with the K + ions and cooled to room temperature. The cooled glass slide was then washed several times with distilled water and absolute ethanol, and reserved for later use.

Fabrication of ZnPc thin film-based OWG sensitive element
ZnPc (sensitive material) and PVP (film former) were dissolved (in the desired amounts) in 10 mL N,N-dimethylformamide (DMF) to obtain a ZnPc solution. This solution was coated on the surface of a K + -exchanged glass slide by spin coating. The as-fabricated sensor was dried under vacuum for 24 h at room temperature. The fabricated film was characterized by using ultraviolet spectrophotometry (UV-1780 ultraviolet spectrophotometer, SHIMADZU, Japan), field emission scanning electron microscopy (FE-SEM, SU-8019, Japan Hitachi), and Fourier-transform infrared spectrometry (FT-IR, Bruker Co., Germany).

OWG testing system
The homemade OWG gas detection system used in this study consisted of a laser, a reflector, chamber (2 cm × 1 cm × 1 cm), a ZnPc thin film-based OWG gas-sensing element, a prism, photomultiplier, a personal computer, and a carrier (air) (Fig. 1). In general, the ZnPc thin film-based sensitive element was fixed in the OWG detection system (Fig. 1) and a prism coupling method was used to excite guided light. In the gas detection process, dry air with a flow rate of 30 cm 3 /min was used as a carrier gas which was injected into the flow cell before the target gas to ensure the complete contact of the sensitive film with the target gas. A few drops of diiodomethane [CH 2 I 2 , (refractive index) n = 1.74] were added to adhere the prism to the surface of the glass optical waveguide element. When a semiconductor laser beam with a wavelength of 670 nm entered the surface of the OWG sensitive element through the first prism, the light entered the sensitive layer in the form of evanescent wave [31]. When the sensitive element interacted with the measured gas, it caused a change in the optical performance of the sensitive element and the form of propagation of the evanescent wave, thereby causing a change in the output light intensity. The light output by the second prism was transmitted to the photomultiplier tube and converted into an electrical signal. The changes in the output light intensity with time were recorded by the computer.

UV-vis measurements
In general, Pcs have two main characteristic peaks in the ultraviolet-visible (UV-vis) spectrum, which are the B band of 300 nm -400 nm and the Q band of 600 nm -700 nm [32]. These two absorption peaks are caused by the transition of π electrons on the phthalocyanine ring, and the Q band is the main concern in practical applications [33]. Figure 2 shows the UV-vis spectra of the ZnPc solution in DMF and the ZnPc thin film. The Q-bands of the ZnPc solution and ZnPc thin film were observed at 668 nm and 678 nm, respectively. Compared with the ZnPc solution, the ZnPc thin film Q-band showed a red-shift for approximately 10 nm. This phenomenon could be attributed to the formation of J-aggregates in the thin film [34]. Pc molecules formed two types of aggregates, H-and J-aggregates.
J-aggregates exhibited better photoelectric properties than H-aggregates because of their higher dipole moment [35].

SEM measurements
Generally, the gas sensitivity of the film is closely related to its surface morphology. Thus, the surface morphology of the ZnPc thin film was examined at different magnifications by field emission scanning electron microscopy (FE-SEM). It is evident from Fig. 3(a) that ZnPc molecules existed as small particles and the average particle size of the particles was 10 nm. Moreover, from Fig.  3(b), it could be seen that the ZnPc thin film had a smooth and dense structure.

FT-IR spectroscopy
The FT-IR spectra of ZnPc before and after contacting with H 2 S were measured. The results are shown in Fig. 4. It is evident from the figure that after contacting with H 2 S, the characteristic absorption peaks of ZnPc had a tendency to move to high wave numbers. This phenomenon could be attributed to the higher symmetry of ZnPC after contacting with H 2 S.

Gas sensing performance
To choose a suitable light source for the OWG testing system, the ZnPc thin film was exposed to 1 000 ppm of measured gases and the changes in absorbance were monitored. The maximum changes in absorbance were caused by H 2 S (Fig. 5). Figure 5 shows that after contacting with H 2 S gas, the absorbance of the film was greatly reduced and the Q band had a significant split and a red shift. This trend was most significant in the range of 600 nm -800 nm, therefore a 670 nm semiconductor laser was used as the light source.  Figure 6 shows results for the optimization of the fabrication conditions. The screening results for the rotation speed, ZnPc mass fraction, and PVP content are listed in Table 2.
In Fig. 6, ΔI denotes the change in the output light intensity and is calculated using the following formula [36]: where I gas is the output light intensity of the gas and I air is the output light intensity of the surrounding air. From Fig. 6, it can be seen that the ZnPc thin film OWG prepared with a 1 600 rpm of rotating speed (Table 3), 0.07% ZnPc mass fraction (Table 4), and 1.0% PVP content (Table 5) exhibited the greatest sensing response to H 2 S. Therefore, it was confirmed to be the optimum condition for the fabrication of the ZnPc thin film. Moreover, it is       Figure 7 shows the selective response of the sensor prepared under optimal conditions to 1 000 ppm of VOCs (a) and 100 ppm of H 2 S (b). The response of the sensor to VOCs was very small [ Fig. 7(a)], and the sensor exhibited the largest response to H 2 S because of the protonation interactions [ Fig. 7(b)]. The protonation process could be used to explain the absorption change (Fig. 8). As the pyrrolic and azomethine (meso) nitrogen atoms in Pc exhibited acidic and basic characteristics, respectively, it was an amphoteric molecule. Unprotonated MPcs exhibited D 4h symmetry. However, the symmetry decreases after protonation, causing the splitting and red-shifting of the Q band [37]. In addition, upon exposure to ethylenediamine (EDA)-saturated vapor, the absorption spectrum of the film returned to its initial unprotonated state. These results indicated that the ZnPc thin film could be protonated by H 2 S (acidic substance) and deprotonated by EDA (basic substance). From Fig. 7(b), it is evident that after contacting with H 2 S, the response time was 3 s, but the response value did not recover automatically to the baseline because of the irreversible protonation process.
According to Fig. 9, when the sensor is in contacting with H 2 S, the response value increased rapidly. After the response value attained the maximum value, it began to recover. However, owing to irreversible protonation, the response value could not return to the baseline in a short time. However, the response value could be returned to the baseline by contacting with saturated EDA. By using this technique, every H 2 S response-recovery process is repeatable. The relative standard deviation (RSD) value was 2.6%.  Figure 10 shows a low concentration of H 2 S detection by using the ZnPc thin film-based sensor. As observed from Fig. 10, the output light intensity increased with an increase in the H 2 S concentration. The sensor could detect as low as 0.1 ppm of H 2 S (S/N = 3.6). Figure 11 shows a scatter plot for different H 2 S concentrations and the corresponding ΔI values. The linear relationship was y = 532x + 4 424, R 2 = 0.982 3.
The long-term stability of the ZnPc thin film-based sensor was studied by exposure to 100 ppm of H 2 S after 1 day, 7 days, 14 days, 21 days, and 28 days of its fabrication. The results are shown in Fig. 12. A small change was observed in the gas sensing performance of the sensor during this period. The RSD value of the sensor was 2.6%. These results demonstrated the long-term stability and reproducibility of the sensor. The stability against humidity variation was also studied. Different saturated salt solutions were prepared to obtain different humidities ( Table 6). The results are shown in Fig. 13. From the effect of different humidities on the sensor, it could be considered that the change in humidity had little effect on the sensor.   Fig. 13 Effect of the relative humidity on sensor.

Conclusions
In this study, a novel ZnPc thin film-based OWG gas sensor was fabricated and its gas sensing performance was studied. The fabricated OWG sensor showed a high selectivity to H 2 S due to protonation. The ZnPc thin film-based OWG sensor was able to detect H 2 S at a level of 100 ppb. The sensor also showed the good reproducibility, stability, and fast response. The low cost, easy implementation, and high sensitivity of this sensor make it suitable for many environmental and chemical applications.